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					                       Collection Technique ..........................................................................

                      Cahier technique n° 167

                      Energy-based discrimination for
                      low-voltage protective devices

M. Serpinet
R. Morel

       s Merlin Gerin s Modicon s Square D s Telemecanique
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n° 167
Energy-based discrimination
for low-voltage protective


Joined Merlin Gerin in 1972 and worked until 1975 in the low-voltage
equipment design offices, in charge of designing electrical cabinets
for various installation layouts. Since 1975, he has managed research
and development testing for low-voltage circuit-breakers. Graduated
in 1981 from the ENSIEG engineering school in Grenoble.
In 1991, after managing a Compact circuit-breaker project from the
preliminary studies on through to production, he was appointed head
of the electromechanical design office in charge of «anticipating»
future developments.

Robert MOREL

Graduated with an engineering degree from ENSMM in Besançon and
joined Merlin Gerin in 1971. Specialised in designing low voltage
switchgear and participated in designing the Sellim system.
In 1980, took over development of Compact circuit-breakers and
Interpact switches.
In 1985, became manager of the Low-Voltage Current Interruption
design office in the Low-voltage Power Components division.

ECT167 first issued, march 1998

                           Eb                                                     High-set instantaneous release (HIN)
                           Energy let through by the protective device            Instantaneous release used to limit thermal
                           during breaking. This energy is characterised by       stress during a short-circuit.
                           ∫ ib dt ≈ I t b
                              2        2

                                                                                  Instantaneous release (INS)
                           ib                                                     Release without an intentional time delay
                           Limited short-circuit current actually flowing         system. It trips at a low multiple of In to ensure
                           through the circuit-breaker (the break current is      short-circuit protection.
                           less than Ip).
                                                                                  Long-time release (LT)
                           Ip                                                     Release with an intentional time delay system
                           Prospective short-circuit current that would           (several seconds) for overload protection.
                           develop in the absence of protective devices
                           (rms value).
                                                                                  Partial discrimination
                                                                                  Discrimination is said to be partial when it is
                           Ir                                                     ensured only up to a certain level of the
                           Corresponds to the overload protection setting.        prospective current (Ip).

                           tb                                                     Selective circuit-breaker
                           The actual breaking time (arc extinction).             Circuit-breaker with an intentional time delay
                                                                                  system (time discrimination).
                           Electronic processing unit.                            Short-time release (ST)
                                                                                  Release with an intentional time delay system
                           Actuator                                               ranging from ten to several hundred
                           Device capable of producing a mechanical               milliseconds. If the time delay is reduced as Ip
                           action.                                                increases, the system is referred to as
                                                                                  dependent short-time (DST).
                           Circuit-breaker rating
                           Corresponds to the models of the range                 Total discrimination
                           (ex. 160 A, 250 A, 630 A, 800 A, etc.).                Discrimination is said to be total when it is ensured
                                                                                  for all values of the prospective fault current.
                           Current-limiting circuit-breaker
                           Circuit-breaker which, when interrupting a short-      Trip-unit rating
                           circuit current, limits the current to a value         Corresponds to the maximum current setting of
                           considerably less than the prospective current (Ip).   the trip unit.

Cahier Technique Schneider n° 167/ p.2
                                                  Energy-based discrimination for
                                                  low-voltage protective devices

                                                  The purpose of this “Cahier Technique” publication is to present the new
                                                  energy-based discrimination technique that ensures tripping discrimination
                                                  between protective devices during a short-circuit. Both simpler and more
                                                  effective than standard discrimination techniques, it has been implemented
                                                  on the Compact NS range of circuit-breakers used in low-voltage power
                                                  distribution networks. Discrimination is ensured for all prospective fault
                                                  currents on the condition that upstream and downstream circuit-breakers
                                                  have different current ratings (ratio u 2.5) with a trip-unit rating ratio u 1.6.
                                                  Following a brief review of standard discrimination techniques, the authors
                                                  examine the behaviour of circuit-breakers and various trip units from the
                                                  energy standpoint.
                                                  They then demonstrate that total discrimination is possible up to the
                                                  circuit-breaker breaking capacity, over several levels, without using time
                                                  discrimination techniques.

1. Discrimination in low-voltage                  1.1 Definition                                                             p. 4
protective devices                                1.2 Enhanced safety and availability                                       p. 5
                                                  1.3 Discrimination zones                                                   p. 5
2. Discrimination techniques for short-circuits   2.1 Current discrimination                                                 p. 7
                                                  2.2 Time discrimination                                                    p. 7
                                                  2.3 “SELLIM” discrimination                                                p. 8
                                                  2.4 Zone selective interlocking                                            p. 9
                                                  2.5 Combining the different types of discrimination                        p. 9
3. Energy-based discrimination                    3.1 Choice of operating curves                                             p. 10
                                                  3.2 Characterisation of a Compact NS circuit-breaker                       p. 11
                                                  3.3 Characterisation of the trip units                                     p. 13
4. Advantages and implementation of               4.1 Current-limiting circuit-breaker fitted with a pressure trip system    p. 16
energy-based discrimination                       4.2 Discrimination with Compact NS circuit-breakers                        p. 18
                                                  4.3 Combination with traditional protective devices                        p. 19
5. Conclusion                                                                                                                p. 21
6. Appendix - indications concerning breaking with current limiting                                                          p. 22

                                                                                            Cahier Technique Schneider n° 167 / p.3
1 Discrimination in low-voltage protective devices

1.1 Definition
                            In an electrical installation, loads are connected            Several types of overcurrents may be
                            to sources via a succession of protection,                    encountered in an installation:
                            isolation and control devices. This “Cahier                   c overloads,
                            Technique” publication deals essentially with the
                                                                                          c short-circuits,
                            protection function using circuit-breakers.
                                                                                          c inrush currents,
                            In a radial feeder layout (see fig. 1 ), the purpose
                            of discrimination is to disconnect only the faulty            as well as:
                            load or feeder from the network and no others,                c earth faults,
                            thus ensuring maximum continuity of service.                  c transient currents due to voltage dips or
                            If discrimination studies are not or are incorrectly          momentary loss of supply.
                            carried out, an electrical fault may cause several            To ensure maximum continuity of service, there
                            protective devices to trip, thus provoking an                 must be coordination between protective
                            interruption in the supply of power to a large part           devices.
                            of the network. That constitutes an abnormal loss             Note that voltage dips may provoke unnecessary
                            in the availability of electrical power for those             opening of circuit-breakers by actuating
                            parts of the network where no fault occurred.                 undervoltage releases.






                                          If passes through CB1, CB2, CB3, CB4.

                            Fig. 1: several circuit-breakers are concerned by the fault If.

Cahier Technique Schneider n° 167 / p.4
1.2 Enhanced safety and availability
                  A specific type of protective device exists for        Studies begin with an analysis of requirements
                  each type of fault (overloads, short-circuits, earth   concerning protective devices needed for
                  faults, undervoltages, etc.). However, a fault may     each type of fault. The next step is an evaluation
                  simultaneously bring several types of protective       of coordination possibilities between the
                  devices into play, either directly or indirectly.      protective devices concerned by a given fault.
                                                                         The result is improved continuity of service while
                  Examples                                               still guaranteeing protection of life and property.
                  c A high short-circuit current creates an              The following chapter will deal exclusively with
                                                                         discrimination in the event of overcurrents
                  undervoltage and may trip the undervoltage
                                                                         (overloads and short-circuits).
                  protective device.
                                                                         In this context, the existence of discrimination
                  c An insulation fault may be interpreted as a          between circuit-breakers is determined quite
                  zero-phase sequence fault by an earth-leakage          simply by whether several circuit-breakers open
                  protective device and as an overcurrent by the         or not (see fig. 2 ).
                  short-circuit protective device (applicable for TN
                  and IT earthing systems).                              Total discrimination
                                                                         Discrimination is said to be total if and only if,
                  c A high short-circuit current may trip the earth-
                                                                         among the circuit-breakers potentially concerned
                  leakage protective device (in TT earthing
                                                                         by a fault, only the most downstream circuit-
                  systems) due to local saturation of the
                                                                         breaker trips and remains open, for all fault
                  summation toroid which creates a false zero-
                                                                         current values.
                  phase sequence current.
                  For a given network, discrimination studies and        Partial discrimination
                  the evaluation of the protection system in general     Discrimination is said to be partial if the above
                  are based on the protective device                     condition is no longer valid for fault currents
                  characteristics published by the manufacturers.        exceeding a certain level.

                                                CB1                                                     CB1

                                                            CB2                                                     CB2

                  a) CB1 and CB2 open.                                   b) CB1 opens CB2, remains closed.
                  ⇒ discrimination is not ensured, i.e. power is not      ⇒ discrimination is ensured, i.e. power is available for the
                  available for the feeders where no fault occurred.     feeders where no fault occurred (continuity of service).

                  Fig. 2: circuit-breaker behaviour during a fault.

1.3 Discrimination zones
                  Two types of overcurrent faults may be                 Overcurrents with higher values are short-circuits
                  encountered in an electrical distribution network:     that must be cleared as rapidly as possible by
                  c overloads,                                           instantaneous (INS) or short-time (ST) releases
                  c short-circuits.                                      on the circuit-breaker.
                  Overcurrents ranging from 1.1 to 10 times the ser-     Discrimination studies are different for each type
                  vice current are generally considered as overloads.    of fault.

                                                                                        Cahier Technique Schneider n° 167 / p.5
                            Overload zone
                            This zone starts at the ILT operating threshold of
                            the long-time (LT) release. The tripping (or time-
                            current) curve tb = f (Ip) is generally of the
                            inverse-time type to remain below the                         tb   Overload discri-
                            permissible thermal stress curve of the cables.                    mination zone
                                                                                                CB2 CB1
                            Using the most common method, the curves of
                            the LT releases concerned by the fault are plotted
                            in a system of log-log coordinates (see fig. 3 ).
                            For a given overcurrent value, discrimination is
                            ensured during an overload if the non-tripping
                            time of the upstream circuit-breaker CB1 is
                            greater than the maximum breaking time
                            (including the arcing time) of circuit-breaker CB2
                            Practically speaking, this condition is met if the
                            ratio ILT1/ILT2 is greater than 1.6.

                            Short-circuit zone                                                 Overloads               Short-circuits
                            Discrimination is analysed by comparing the                                                            Ip
                            curves of the upstream and downstream circuit-
                                                                                                 I LT2   I LT1       I ins2
                            The techniques that make discrimination
                            possible between two circuit-breakers during a
                            short-circuit are based on combinations of            Fig. 3: overload discrimination.
                            circuit-breakers and/or releases of different types
                            or with different settings designed to ensure that
                            the tripping curves never cross.
                            A number of such techniques exist and are
                            presented in the next chapter.

Cahier Technique Schneider n° 167 / p.6
2 Discrimination techniques for short-circuits

                  Several techniques can be used to ensure                 c “SELLIM” discrimination,
                  discrimination between two circuit-breakers              c zone selective interlocking,
                  during a short-circuit:
                                                                           c energy-based discrimination (see chapters 3
                  c current discrimination,                                and 4).
                  c time discrimination,

2.1 Current discrimination
                  This type of discrimination is the result of the
                  difference between the thresholds of the
                  instantaneous or ST releases of the successive
                                                                                   tb   CB2 CB1             Short-circuit
                  Applied primarily in final distribution systems, it is
                                                                                                            discrimination zone
                  implemented using rapid circuit-breakers not
                  including an intentional tripping time-delay system.
                  It protects against short-circuits and generally
                  results in only partial discrimination.
                  This form of discrimination is all the more
                  effective when the fault currents are different,
                  depending on where they occur in the network,
                  due to the non-negligible resistance of
                  conductors with small cross-sectional areas
                  (see fig. 4 ).                                                                                          Ip
                  The discrimination zone increases with the                                              Iins2 Iins1
                  difference between the thresholds of the                         Short-circuit
                  instantaneous releases on circuit-breakers CB1                   discrimination limit
                  and CB2 and with the distance of the fault from
                  CB2 (low Isc < Iins of CB1).
                  The minimum ratio between Iins1 and Iins2 must           Fig. 4: current discrimination.
                  be 1.5 to take into account threshold accuracies.

2.2 Time discrimination
                  To ensure total discrimination, the time-current         Use of selective circuit-breakers
                  curves of the two circuit-breakers must never            The term selective means that:
                  cross, whatever the value of the prospective             c the circuit-breaker trip unit has a fixed or
                  short-circuit current. For high fault currents, total    adjustable time-delay system;
                  discrimination is guaranteed if the horizontal
                                                                           c the installation and the circuit-breaker can
                  sections of the curves to the right of Iins1 are not
                                                                           withstand the fault current for the duration of the
                  one on top of another.
                                                                           intentional time delay (sufficient thermal and
                  Several solutions may be implemented to                  electrodynamic withstand capacities).
                  achieve total discrimination:                            A selective circuit-breaker is generally
                  c the most common involves installing selective          preceded in the network by another selective
                  circuit-breakers including an intentional time-          circuit-breaker that has a longer intentional time
                  delay system,                                            delay.
                  c the second applies only to the last distribution       Use of this type of circuit-breaker, corresponding
                  stage and involves using current-limiting circuit-       to time discrimination solutions, results in total
                  breakers.                                                breaking times greater than 20 ms (one period)

                                                                                          Cahier Technique Schneider n° 167 / p.7
                                  tb                          CB2 : rapid                        tb   CB2 CB1          CB2 : rapid current limiting
                                                              CB1 : selective with                                     CB1 : rapid
                                          CB2      CB1
                                                              1-2-3 ST settings
                                                       Installation and/or circuit-
                                                       breaker thermal withstand
                                                       capacity limit

                                                                                     Ip                                                    Ip
                                                               Iins1        IDIN1
                            Note: use of a high-sed instantaneous release                    Note: use of dependent ST releases (dotted line) on
                            determines the discrimination limit.                             CB1 improves discrimination.
                            Fig. 5: time discrimination.                                     Fig. 6: pseudo-time discrimination.

                            in the event of a fault. This figure may run up to a             c they severely limit short-circuit currents due to
                            few hundred milliseconds (see fig. 5 ).                          fast opening times and high arcing voltages,
                            When the installation (and perhaps even the                      c the higher the prospective short-circuit current,
                            circuit-breaker) cannot withstand a high short-                  the faster they act.
                            circuit current (Isc) for the entire time delay,                 Use of a current-limiting circuit-breaker
                            circuit-breaker CB1 must be equipped with a
                                                                                             downstream thus makes it possible to ensure
                            high-set instantaneous release (HIN).
                                                                                             “pseudo-time” discrimination between two
                            In this case, the discrimination zone is limited to              protection levels. This solution, due to the
                            the high-set threshold of the upstream circuit-                  current limiting effect and rapid clearing of the
                            breaker (see fig. 5 ).                                           fault, limits thermal and electrodynamic stresses
                            Use of current-limiting circuit-breakers and                     in the installation (see fig. 6 ).
                            “pseudo-time” discrimination
                            These circuit-breakers have two main

2.3 “SELLIM” discrimination

                                                                       Fault at B                              Fault at A

                                                                       i1                                      i1
                                                                       u1                                      u1

                                                                       i2                                      i2
                                                                       u2                                      u2


                                                                       i3            26 k                     i3                  34 kÂ
                                                                       u3                                      u3

                                                                                                              3.5 ms
                                                                                    2.5 ms                                  12 ms
                            Fig. 7: “SELLIM” discrimination.
                            (CB1 - Compact C250 L SB
                            CB2 - Compact C125 N).

Cahier Technique Schneider n° 167 / p.8
                  The “SELLIM” system offers a number of                       trip threshold and clears the fault in less than a
                  advantages: discrimination, cascading, reduced               half-period. CB1 detects only a single current
                  stresses in the installation.                                wave and does not trip. The fault current
                  Upstream from a rapid circuit-breaker CB2, the               nonetheless causes contact repulsion, thus
                  system requires an ultra current-limiting circuit-           limiting the current and the resulting stresses.
                  breaker CB1 fitted with a special release that               This limiting of the fault current means that
                  does not trip during the first half-wave of the fault        downstream circuit-breakers may have breaking
                  current (see fig. 7 ).                                       capacities less than the prospective fault current.
                  A major fault at B is detected by both circuit-              A fault at A causes repulsion of the contacts of the
                  breakers.                                                    current-limiting circuit-breaker, thus limiting the
                  CB2, equipped with an instantaneous release,                 stresses produced by the fault current. CB1 opens
                  opens as soon as the fault current exceeds its               after the second half-wave of limited current.

2.5 Zone selective interlocking
                  This technique requires data transmission
                  between the trip units of the circuit-breakers at
                  the various levels in a radial feeder network.
                  The operating principle is simple (see fig. 8 ):
                  c each trip unit that detects a current greater                    CB1                  Logic
                  than its tripping threshold sends a logic wait                                          relay
                  order to the next trip unit upstream,
                  c the trip unit of the circuit-breaker located just                                                       Logic
                  upstream of the short-circuit does not receive a                                                          wait
                  wait order and reacts immediately.                                                                        order
                  With this system, fault clearing times remain low
                  at all levels in the network.
                  Zone selective interlocking is a technique used                             CB2                      Logic
                  with high-amp selective LV circuit-breakers,                                                         relay
                  though its main application remains HV industrial
                  networks. For further information, refer to “Cahier
                  Technique” Publication Number 2, entitled
                  “Protection of electrical distribution networks by           Fig. 8: zone selective interlocking.
                  the logic selectivity system”.

2.6 Combining the different types of discrimination
                  The different types of discrimination presented              The costs of non-discrimination and of the
                  above are generally combined to ensure the                   various devices selected are taken into account.
                  highest degree of availability of electrical power.          The energy-based discrimination technique
                  See figure 9 for an example.                                 presented in the next chapter constitutes a true
                  Discrimination studies are still carried out using the       innovation that will considerably simplify LV
                  tables supplied by manufacturers. The tables indi-           discrimination studies and make possible total
                  cate the discrimination limits for each combination          discrimination over several levels at minimum
                  of circuit-breakers and for the various trip units.          cost.

                           Circuits         Type of discrimination                                             Type of
                           concerned        Zone selective Time               “SELLIM”       Pseudo-time       circuit-breaker
                           Head of                                                                             Selective
                           LV network                                                                          logic
                           Power                                                                               Selective
                           distribution                                                                        Rapid/current
                                                                                                               limiting SELLIM
                           Final                                                                               Rapid/current
                           distribution                                                                        limiting

                  Fig. 9: example of uses for different types of discrimination.

                                                                                              Cahier Technique Schneider n° 167 / p.9
3 Energy-based discrimination

                            Energy-based discrimination is an improved and          c increases in installed power, which lead to
                            generalised version of the pseudo-time                  higher short-circuit currents and correspondingly
                            technique described in the preceding chapter.           higher breaking capacities;
                            Discrimination is total if, for all values of Ip, the
                                                                                    c the need to limit stresses in the installation as
                            energy that the downstream circuit-breaker lets
                                                                                    well as the level and duration of fault currents.
                            through is less than that required to actuate the
                            trip unit of the upstream circuit-breaker.              When reasoning in terms of energy and in order
                            The actual implementation of the energy-based dis-      to understand energy-based discrimination, the
                            crimination principle is covered by a Merlin Gerin      choice of the means of presenting the operating
                            patent and has been incorporated in the design of       curves is essential and the subject of the next
                            the new Compact NS range of circuit-breakers.           section.
                            These rapid and highly current-limiting circuit-        Following that discussion is an analysis of the
                            breakers meet the rapidly evolving criteria of the      behaviour in terms of energy for current-limiting
                            market concerning:                                      circuit-breakers and the various trip units.

3.1 Choice of operating curves
                            The tb = f (Ip) curves commonly used for
                            discrimination studies are of no use with current-
                                                                                          I2 t
                            limiting circuit-breakers when currents exceed
                                                                                       (A2 s)
                            25 In (breaking times are less than 10 ms at a
                            frequency of 50 Hz).
                            Discrimination studies may no longer be carried
                            out on the basis of periodic phenomena, but
                            rather require analysis of transient phenomena.
                            An understanding of energy-based
                            discrimination requires that the following
                            elements be characterised:
                            c the current wave that the circuit-breaker lets
                            through during breaking, which is characterised
                            by its Joule integral ∫ i2 dt (often expressed as                                                        Ip(A)
                            I2 t ), and corresponds to the breaking energy Eb,
                                                                                                                    10 In
                            c the sensitivity of the releases to the energy
                            corresponding to the current pulse.                           t(s)
                            Thus, quite logically, the above characteristics
                            are represented using I2 t = f (Ip) curves
                            instead of tb = f (Ip) curves (see fig. 10 ).
                            It should be noted that standard IEC 947-2
                            specifies characterisation of circuit-breakers
                            using such curves.
                            For practical reasons the I2 t = f (Ip) curve is
                            presented in a system of log-log coordinates.
                            For discrimination studies, the limits of the
                            breaking I2 t value (Eb for circuit-breakers) are
                            between 104 and 107 A2 s for prospective                                                     ST1
                            currents ranging from 1 to 100 kA. Three powers                                                    ST2
                            of ten are therefore used for Eb and two for the                                                         INS
                            Assuming that the half-wave of the interrupted
                            current is equivalent to half of a sine-wave with                                      10 In 15 In 30 In
                            the same initial slope as the prospective current,                              I2
                                                                                    Fig. 10: tb = f (Ip) and t = f (Ip) curves for a circuit-
                            the breaking energy Eb may be expressed as a            breaker equipped with an electronic trip unit.
                            function of Ip using the following expressions

Cahier Technique Schneider n° 167 / p.10
                  (see the appendix on breaking with current                 instantaneous thresholds and less than the
                  limiting):                                                 contact repulsion threshold:
                  v for t u 10 ms                                            (2) ⇒ Eb = Ip2 x 2 x 10-2.
                  (2) ⇒ Eb = Ip2 t                                           c t = 10 ms is the breaking time at the current-
                  v for t < 10 ms                                            limiting threshold:
                  (3) ⇒ Eb = 4 f2 Ip2 tvb3                                   (2) ⇒ Eb = Ip2 x 10-2.
                  or                                                         c t = 9 to 4 ms which indicate circuit-breaker
                  (4) ⇒                                                      behaviour when current limiting:
                          4 2 f Ip                                           (3) ⇒ Eb = Ip2 tvb3 x 104.
                  On the basis of these equations, the Ip2 t / Ip
                  system can be improved, thus providing further             Peak-current lines
                  information on the virtual breaking time (tvb) and         Similarly, on the basis of equation (4)
                  the limited peak current value (îb).                                  îb
                                                                             Eb =
                                                                                    4 2 f Ip
                  Time lines (see fig. 11 )
                                                                             a series of lines corresponding to constant,
                  A series of lines representing constant breaking           limited peak currents can be included in the
                  times can be included in the log-log                       representation (see fig. 11 ).
                  representation for a given frequency.
                                                                             It should be noted that this method of
                  For example, when f = 50 Hz, the line for:                 representation makes it possible to characterise
                  c t = 20 ms corresponds to the most common                 circuit-breakers and trip units at 50 Hz for three-
                  breaking time when Ip is greater than the                  pole, two-pole and single-pole faults.

                                            I2 t                            40 ms
                                                                            20 ms
                                                                            10 ms

                                                                                        7 ms

                                                                                                5 ms
                                         (A s)

                                                                                                       î = 40 kA
                                                                                                       2.5 ms

                                                                                                       î = 20 kA


                                                                                                       î = 10 kA

                                                                                      î = 5 kA
                                             10                                                            Ip (kA)
                                                       1   3   5       10        30       50 100
                  Fig. 11: graph representing energies.

3.2 Characterisation of a Compact NS circuit-breaker
                  Display of the breaking I2 t value                         The curves presented here correspond to three-
                  The  I2 t values that a circuit-breaker lets               phase faults at 400V/50Hz.
                  through are determined by standardised type                The same curves may be generated for other
                  tests or by computer models run for a given                voltages and other frequencies. The indicated
                  voltage and frequency.                                     values are the maximum values obtained

                                                                                               Cahier Technique Schneider n° 167 / p.11
                            irrespective of the moment at which the fault                        c Point D: when the fault current reaches
                            occurs (upper limits) (see fig. 12 ).                                approximately 1.7 times the contact repulsion
                                                                                                 level, the energy is sufficient to totally open the
                            Curve analysis                                                       contacts. At that point, the breaking time is
                            A great deal of information is available from the                    typically 10 ms.
                            graph in figure 12 which corresponds to a                            This reflex-type breaking is autonomous and a
                            250 A Compact NS circuit-breaker, equipped                           trip unit is required only to confirm the tripped
                            with a dependent ST (DST) electro-mechanical                         status of the circuit-breaker and avoid untimely
                            release with a 10 In threshold.                                      reclosing of the contacts.
                                                                                                 c Zone E: when the fault currents runs beyond
                            The information characterises the different
                                                                                                 2 times the contact-repulsion level, current
                            phases in the breaking behaviour of the current-
                                                                                                 limiting is increasingly effective and results in
                            limiting circuit-breaker depending on the value of
                                                                                                 increasingly short breaking times.
                            the prospective short-circuit current Ip.
                                                                                                 c Point F: the end of the curve represents the
                            c Point A: when the fault current reaches the trip
                                                                                                 breaking capacity limit of the circuit-breaker.
                            threshold of the release, the breaking time is
                            typically 50 ms for an INS or DST release.                           The curve provides a great deal of information:
                            c Point B: when the fault current is greater than                    c tripping threshold (I threshold, point A);
                            the trip threshold of the release, the breaking                      c breaking I2 t value as a function of the
                            time drops and stablises at 20 ms beginning                          prospective current;
                            at 16 In.                                                            c contact-repulsion level (Ir, point C);
                            c Point C: when the fault current reaches the                        c breaking capacity (point F);
                            contact repulsion level, current limiting starts due                 c breaking time (tvb) as a function of the prospec-
                            to the insertion of an arc voltage in the circuit.                   tive current;
                            Current limiting results in the return to in-phase                   c limited peak current (îb) as a function of the
                            conditions for the voltage and the current and                       prospective current;
                            consequently a drop in fault clearing times                          c current value above which tvb < 10 ms
                            from 20 ms to 10 ms as Ip increases.                                 (beginning of current limiting).

                                                        Ι2 t
                                                                                                40 ms
                                                                                                20 ms
                                                                                                10 ms

                                                                                                           7 ms

                                                                                                                  5 ms

                                                    (A s)

                                                                                                                         î = 40 kA
                                                                                                                         2.5 ms
                                                                             (B)                                         î = 20 kA


                                                                                                                         î = 10 kA

                                                                                                         î = 5 kA
                                                        10                                                                   Ip (kA)
                                                                   1       3     5         10       30       50 100
                                                                       (10 In)

                            Fig. 12: breaking curve for a current-limiting circuit-breaker.

Cahier Technique Schneider n° 167 / p.12
3.3 Characterisation of the trip units
                   Trip units are characterised by their response                     The high-set release is an instantaneous unit
                   time to a given current (full-wave, half-wave,                     with a threshold of 15 to 50 In.
                   etc.).                                                             The release may be either electro-mechanical or
                   By modifying the duration and the peak value of                    electronic.
                   the current, which corresponds to the various                      c Constant time-delay release
                   currents limited by a circuit-breaker, a number of                 This is an instantaneous release fitted with a
                   tests can be run to obtain a series of points                      “clock-type” time delay system intended to make
                   which may be plotted on the previously                             tripping selective with respect to the downstream
                   described graph, thus producing the curve                          circuit-breaker.
                   characterising a trip unit.                                        The time delay may range from 10 to 500 ms
                                                                                      and is generally set using notched dials.
                   Magnetic trip units
                                                                                      Figure 13 shows the curve (20 ms setting) for a
                   c Instantaneous release                                            short-time delay.
                   Generally made up of a magnetic U and a blade,                     If the thermal stess (I2 t) resulting from a long
                   it ensures short-circuit protection. The response                  time delay must be limited, the high-set release
                   time is under 50 ms at its operating threshold                     enters into play (see fig. 13 ).
                   (between 5 and 10 times the rated current), then                   c Dependent time delay release (function of Ip,
                   drops rapidly to below 10 ms when the current                      dependent short-time - DST).
                   increases (see fig. 13 ).
                                                                                      The time delay results from the inertia of a mass
                   c High-set release                                                 and is therefore inversely proportional to Ip
                   As indicated in the time discrimination section,                   (see fig. 13 ).
                   the role of high-set releases in time
                   discrimination systems is to limit thermal                         Electronic trip units
                   stresses (see fig. 5 ) in the installation and the                 The instantaneous thresholds in electronic trip
                   circuit-breaker.                                                   units are sensitive to the rms value or the peak

                                              Ι2 t
                                                                                     40 ms
                                                                                     20 ms
                                                                                     10 ms

                                                                                                  7 ms

                                                                                                           5 ms

                                           ( A s)

                                                                                                                  î = 40 kA
                                                                                                                  2.5 ms
                                                                    High set )


                                                                            y (S
                                                                 Tim s fixe

                                                                                                  nt    T)

                                                                                              nde    DS
                                                                                          epe elay (


                                                                                                           î = 20 kA
                                                                                                 ous (INS)


                                                                                                                  î = 10 kA

                                                                                                î = 5 kA
                                               10                                                                     Ip (kA)
                                                        1       3     5        10          30       50 100
                                                            (10 In)

                   Fig. 13: curves for various magnetic releases.

                                                                                                         Cahier Technique Schneider n° 167 / p.13
                            current value. For high fault currents, their I2 t                        with those in figure 10 for the breaking
                            characteristic is theoretically a straight line                           energies of the circuit-breaker.
                            (îb = constant).
                                                                                                      Trip units with arc detection
                            In fact, the above is true for current pulse
                            durations greater than the response time of the                           Generally combined with electronic trip units,
                            actuating elements of the trip unit (generally                            arc detectors may be used to provide protection
                            4 ms). Below this value, the inertia of the                               for:
                            mechanical elements of the trip unit produces,                            c a cubicle: if an arc occurs in a cubicle, the
                            for high Ip values, a characteristic similar to                           detector orders opening of the incoming circuit-
                            that of an instantaneous electro-mechanical                               breaker,
                            release.                                                                  c a selective circuit-breaker: positioned in the
                            The trip unit must therefore be characterised by                          breaking unit, the detector provokes via the
                            its Eb = f (Ip) curve by carrying out tests                               electronic trip unit the instantaneous tripping of
                            identical to those for magnetic trip units.                               the circuit-breaker.
                                                                                                      The circuit-breaker is thus self-protected and can
                            These trip units may be of either the
                                                                                                      therefore be used up to the limit of its
                            instantaneous or time delay type.
                                                                                                      electrodynamic withstand capacity.
                            It is possible to combine several types of
                            electronic trip units, for example:                                       Pressure trip system
                            c 10 to 15 In - ST (40 ms),                                               The pressure that develops in the breaking unit
                                                                                                      of a circuit-breaker is a result of the energy
                            c 15 to 30 In - ST (10 ms),                                               produced by the arc.
                            c > 30 In - INS.                                                          Above a certain fault current level, this pressure
                            Figure 14 is an illustration of this example. The                         may be used for detection and tripping.
                            curves for this combination should be compared                            This is possible by directing the expanding gases

                                                       I2 t
                                                                                                     40 ms
                                                                                                     20 ms
                                                                                                     10 ms

                                                                                                                    7 ms

                                                                                                                           5 ms

                                                    (A s)

                                                                                                                                  î = 40 kA



                                                                                                                                  2.5 ms




                                                                                                     (INS)                        î = 20 kA


                                                                                                                                  î = 10 kA

                                                                                                                  î = 5 kA
                                                           10                                                                         Ip (kA)
                                                                    1       3      5           10            30       50 100
                                                                        (10 In)

                            Fig. 14: examples of combinations of electronic trip-unit curves.

Cahier Technique Schneider n° 167 / p.14
in the unit toward a piston that trips the circuit-     ensured between circuit-breakers with different
breaker (see fig. 15 ).                                 ratings for all overcurrents greater than 20 In.
Pressure trip systems may be used to:                   It is this energy-based trip system (constant I2 t
c ensure self-protection of a selective circuit-        value) that makes possible the energy-based
breaker (similar to the arc detector),                  discrimination technique employed in the
c improve breaking and operating reliability of a       Compact NS current-limiting circuit-breakers.
rapid current-limiting circuit-breaker.
If each circuit-breaker is fitted with a correctly
designed pressure trip system, discrimination is

                                  P1               P2          P3              Breaking units

                                                                               Flap valves

           Fault on phase 1
           pressure P1 pressure P2 and P3

Fig. 15: operation of the pressure trip system.

                                                                    Cahier Technique Schneider n° 167 / p.15
4 Advantages and implementation
of energy-based discrimination

                            Note that the circuit-breaker trip-unit system,                       c tripping dependability (safety),
                            whether electromechanical, electronic or a                            c minimum disturbance for correctly functioning
                            combination of the two, must offer the following                      circuits (voltage dips),
                                                                                                  c ease of discrimination studies.
                            c minimum stresses in the installation (limited î
                            and I2 t values),

4.1 Current-limiting circuit-breaker fitted with a pressure trip system
                            The above requirements may best be met with a                         circuit current, the shorter the response time, which
                            pressure trip system, combined with either an                         leads to a virtually constant tripping time at I2 t.
                            electromechanical or electronic trip unit.                            The energy let through by the current-limiting
                            Figure 16 indicates the “energy sensitivity” of this                  circuit-breaker during a break follows the same
                            combination. The higher the prospective short-                        curve, but with a slight shift.

                                                       I2 t
                                                                                                 40 ms
                                                                                                 20 ms
                                                                                                 10 ms

                                                                                                               7 ms

                                                                                                                      5 ms
                                                    (A s)

                                                                                                                             î = 40 kA
                                                                                                                             2.5 ms

                                                                                                 trip system
                                                                                                                             î = 20 kA





                                                                                                                             î = 10 kA

                                                                                                            î = 5 kA
                                                           10                                                                    Ip (kA)
                                                                    1         3        5    10         30        50 100

                            Fig. 16: trip-unit combination curves (electromagnetic and pressure or electronic and pressure).

Cahier Technique Schneider n° 167 / p.16
Stresses in the installation
Stresses are limited compared to those observed
in current-limiting circuit-breakers of the previous
On the basis of the example in figure 16 , the
figures for a Compact NS 250 A and an Ip of
40 kA are:                                                              Ur
c 4 ms for the breaking time;
c 20 kA for the peak current;                                       i
c 8 x 105 A2 s for the I2 t.                                                     Ua

Tripping dependability
The pressure trip system is a part of the opening
mechanism for short-circuits and therefore
depends on the current rating of the circuit-
The adjustable DST release, whether
electromechanical (see fig. 13 ) or electronic
(see fig. 14 ), is physically independent of the               a) non-limiting circuit-breaker
pressure trip system. Physical independence
enhances operating dependability.

Voltage dips
Voltage dips in an installation can trip
undervoltage releases in circuit-breakers and
contactors.                                                                              i
Unnecessary opening, following a voltage dip                                                                t (ms)
caused by a short-circuit, results in reduced                                                10                 20
continuity of service.                                                                                     Ua
Consequently, discrimination studies must also
take into account the reactions of undervoltage                                                   Ur
releases and contactors during voltage dips.
A voltage dip in a network lasts until the arc
voltage that opposes the source voltage enables
interruption of the current. It follows that the               b) highly limiting circuit-breaker
voltage dip depends on the type of circuit-
breaker and/or trip unit used:
c with non-limiting circuit-breakers, the voltage
dip is more pronounced and can last from 10 to
15 ms (see fig.17 ),
c with current-limiting circuit-breakers, the rapid
development of a high arc voltage reduces the                                       Ua
voltage dip both in duration and in amplitude                            ic
(see fig.17 ).                                                                               Ur             t (ms)
The voltage dip lasts approximately 5 ms and                                    5            10                 20
amounts to 50 % of the rated voltage for currents
close to the level required for contact repulsion.
The voltage dip amounts to 30 % of the rated
voltage for higher currents, but the duration is
reduced to 3 to 4 ms. The higher the Isc, the
shorter the voltage dip.
Any undervoltage releases equipping the circuit-           Fig. 17: the voltage dip on the network depends on
breakers are not affected by such voltage dips.            the type of circuit-breaker.

The severely limited energy let through by the             the upstream circuit-breaker which remains
circuit-breaker is insufficient to trip the trip unit on   closed.

                                                                             Cahier Technique Schneider n° 167 / p.17
4.2 Discrimination with Compact NS circuit-breakers
                            Using the energy-based discrimination technique                      Using Compact NS circuit-breakers,
                            and depending on the ratios between the                              discrimination is total up to 150 kA.
                            upstream and downstream circuit-breaker ratings                      To ensure total discrimination, the energy that a
                            and the trip unit ratings, the Compact NS range                      circuit-breaker lets through must be less than
                            (100, 160, 250, 400 and 630 A) offers either                         that required to trip the upstream circuit-breaker.
                            partial or total discrimination up to the breaking
                                                                                                 General rule
                                                                                                 Discrimination is total and without any
                            Total discrimination                                                 restrictions if:
                                                                                                 c the ratio between the ratings of the
                            Figure 18 provides an example of total                               successive circuit-breakers is equal to or
                            discrimination up to 100 kA over three levels with                   greater than 2.5,
                            100 A, 250 A and 630 A circuit-breakers fitted                       c the ratio between the trip unit ratings is
                            with various trip units.                                             greater than 1.6.

                                                     I2 t

                                                                                                40 ms
                                                                                                20 ms
                                                                                                10 ms

                                                                                                           7 ms

                                                                                                                  5 ms
                                                  (A s)

                                                                                  ST 400
                                                                                  ST 500
                                                                                  ST 630
                                                              100 A

                                                                                                          630 A          Breaking
                                                                                                                         î = 40 kA
                                                                         ST 160
                                                                         ST 200
                                                                         ST 250

                                                                                                          630 A          Non-tripping
                                                                                                                         2,5 ms
                                                              6                                                          2.5 ms
                                                                                                          250 A          Breaking

                                                                                                          250 A          Non-tripping
                                                                                                                         î = 20 kA

                                                                                                          100 A          Breaking
                                                                                                          100 A

                                                                                                                         î = 10 kA

                                                                                                         î = 5 kA
                                                         10                                                                   Ip (kA)
                                                                  1           3     5      10       30       50 100
                            ST 160, ST 200 and ST 250: electronic trip units
                            for 250 A circuit-breakers.
                            ST 400, ST 500 and ST 630: electronic trip units
                            for 630 A circuit-breakers.

                            Fig. 18: total discrimination between 100 A, 160 A and 250 A Compact NS circuit-breakers.

Cahier Technique Schneider n° 167 / p.18
                                         I2 t

                                                                                      40 ms
                                                                                      20 ms
                                                                                      10 ms

                                                                                                  7 ms

                                                                                                         5 ms
                                      (A s)

                                                                   (10 In)
                                                         ( 8 In)
                                                         160 A

                                                                   250 A
                                                                                                                î = 40 kA
                                                                                                                2.5 ms

                                                                                                                Breaking 160 A
                                                                                                                Non-tripping 250 A

                                                                                                                î = 20 kA

                                                                                 Discrimination limit

                                                                                                                î = 10 kA

                                                                                                î = 5 kA
                                            10                                                                       Ip (kA)
                                                     1               3       5   10        30       50 100

                  Fig. 19: partial discrimination between two Compact NS circuit-breakers, 160 and 250 A.

                  Partial discrimination                                                  downstream device to break high short-circuit
                  If the general rule presented above is not                              currents.
                  respected, discrimination is only partial.                              Note that this is detrimental to discrimination
                  Figure 19 indicates that between a 160 A circuit-                       (except with the SELLIM system).
                  breaker and a 250 A circuit-breaker fitted with a
                                                                                          For the Compact NS, cascading in no way modifies
                  250 A trip unit, discrimination is ensured up to a
                                                                                          the total and partial discrimination characteristics
                  prospective short-circuit current of 4 800 A. This
                                                                                          mentioned above.
                  level is higher than that observed, under the same
                  conditions, with standard Compact circuit-breakers.                     A Compact NS circuit-breaker can however always
                                                                                          assist a downstream circuit-breaker of a different
                  Cascading with the Compact NS                                           type and with insufficient breaking capacity.
                  Cascading, covered by standard NF C 15-100,
                  enables the upstream circuit-breaker to help the

4.3 Combination with traditional protective devices
                  Standard circuit-breakers                                               without reducing the previous discrimination limit. On
                  In an existing installation, the highly limiting                        the contrary, if the new circuit-breaker is installed:
                  Compact NS circuit-breakers may be used for                             c downstream, its current-limiting capacity can
                  extensions or to replace existing circuit-breakers                      only improve the discrimination level, possibly to

                                                                                                            Cahier Technique Schneider n° 167 / p.19
                            the point of making discrimination total                           c the energy that flows through the fuse during
                            (see fig. 20 ),                                                    the break.
                            c upstream, the discrimination level is at least                   To ensure discrimination between an upstream
                            equal to the previous level and the high current-                  circuit-breaker and a fuse, the circuit-breaker trip
                            limiting capacity of the Compact NS can be used                    unit must not react to the sum of these two
                            to reinforce cascading.                                            energies.

                            The I2 t = f (Ip) curves (supplied by
                            manufacturers) concern:
                            c the energy required to blow the fuse

                                                     I2 t

                                                                                              40 ms
                                                                                              20 ms
                                                                                              10 ms

                                                                                                          7 ms

                                                                                                                 5 ms
                                                  (A s)
                                                                        Magnetic 630 A
                                                                                                C 250
                                                                                                                        î = 40 kA
                                                                                                      C 250 L           2.5 ms
                                                          6                                                             Non-tripping
                                                                                                      NS 250

                                                                                                                        î = 20 kA


                                                                                                                        î = 10 kA

                                                                                                        î = 5 kA
                                                     10                                                                      Ip (kA)
                                                              1    3      5              10        30       50 100

                            Fig. 20: replacement of a Compact C250 N, H or L by a Compact NS 250 provides improved discrimination. In this
                            example, discrimination becomes total.

Cahier Technique Schneider n° 167 / p.20
5 Conclusion

               Using a few simple rules, highly limiting circuit-
               breakers that operate faster for higher
               prospective short-circuit currents can be
               implemented to provide total discrimination over
               several network levels. They may also
               implement time-discrimination techniques.
               This is a major technical innovation that can be
               used to:
               c considerably simplify discrimination studies,
               c minimize electrodynamic forces, thermal
               stresses and voltage dips resulting from short-
               This new discrimination technique, referred to as
               energy discrimination and based on total control
               over the energy let through by the circuit-
               breakers during breaking and on the sensitivity
               of the trip units to the same energy, is an
               important contribution to improving the
               availability of electrical power.

                                                                    Cahier Technique Schneider n° 167 / p.21
6 Appendix - indications concerning breaking
with current limiting

                            Figure 21 shows the currents and voltages for a        c tvb 10 ms
                            half-period current-limiting phenomenon.               The circuit-breaker limits the fault current.
                            The short-circuit current (ib) obeys the following     ib and ip have the same initial slope, therefore:
                                              di     di                               = ω Ip             2 = ω ′ îb
                            Ur − Ua = r i + L    ≈ L                               dt
                                              dt     dt
                                                                                   where ω ′ =
                            c at the beginning of the short-circuit, Ua is zero,                         t vb
                            ib and ip are equal and have identical slopes,
                            c when Ua is equal to the network voltage Ur, ib       t vb ω Ip        2 = π îb
                            attains its maximum value (îb) because its             hence:
                            derivative is equal to zero,
                                                                                   î b = t vb 2 f Ip             2
                            c when Ua is greater than Ur, ib declines to zero
                            at tb.                                                 or
                            The interrupted current wave is equivalent to a                         îb
                                                                                   t vb =
                            sinusoidal half wave with a period equal to twice               2 f Ip         2
                            the virtual breaking time (tvb).
                            With the above information, it is easy to              If we express equation (1) as:
                            determine the energy dissipated in the                          2 Eb
                            impedances of the concerned circuit.                   îb =
                                                                                             t vb
                            Expressed in other terms, the formula for this
                            energy, called the “breaking energy”, is:              we obtain:

                                                                                                (                          )
                                    t                                              2 Eb                                        2
                            Eb =         2
                                   ∫0vb ib dt                                            = t vb 2 f Ip                 2
                                                                                    t vb
                            where ib is a sinusoidal function:
                                     1 2
                            Eb =        î t (1).                                   Eb = 4 f 2 Ip 2 t vb 3 (3)
                                     2 b vb
                            It is useful to express Eb as a function of Ip and     Again on the basis of (1), but with îb in mind:
                            the duration (tvb) of the break:
                                                                                            2 Eb       îb
                            c tvb u 10 ms                                          t vb =     2
                            For such a duration, the fault current is low, the               îb    2 f Ip                      2
                            circuit-breaker contacts do not repel each other       we obtain:
                            and there is therefore no arcing voltage:
                            ib = ip and î b =     2 Ip ;                           Eb =                         ( 4)
                                                                                            4       2 f Ip
                            and formula 1 may be expressed as:
                                                                                   Formulas (3) and (4) can be used to plot the time
                            Eb = Ip 2 t (2)                                        and peak current curves.

Cahier Technique Schneider n° 167 / p.22



              0   tr        ta   t                          tvb t b                                      T/2

Ua: arcing votlage                                                ˆ : time corresponding to î
                                                                  t                          b
Ur: network voltage                                               ta: time at which the arc appears
ip: prospective current                                           tb: breaking time
ib: break current (limited)                                       tr: time at which contact repulsion occurs
îb: maximum break current                                         tvb: virtual breking time
ir: contact repulsion current                                     ω: angular frequency of the interrupted wave

Fig. 21: breaking with current limitation.

                                                                              Cahier Technique Schneider n° 167 / p.23
Cahier Technique Schneider n° 167 / p.24
                                                                                            © 1998 Schneider

Schneider   Direction Scientifique et Technique,   Real.: Sodipe - Valence
            Service Communication Technique        Edition: SEST Grenoble
                                                   03.98 - 1500 - Printing: Clerc
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