PROBLM UTILITY SUPPLY by FuzorePejal

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									IEE 2nd International Conference on Advances in Power System Control, Operation and Management, December 1993, Hong Kong



        Problems at the Utility Supply/Consumer Interface
                                        J. Burnett         Martin K. T. Wu

        *Department of Building Services Engineering, Hong Kong Polytechnic, Kowloon, HONG KONG

                        Abstract                            utility companies HV overcurrent protection
                                                            and the consumers main incoming circuit
          The interface between the utility                 breaker (MICE) leads to discrimination
 companies' distribution network and low-                   problems on the consumer's side. For a typical
 voltage installations in large buildings in Hong           supply through a 1500 kVA 1 lkV:380/346 V
 Kong is defined by both policy and ordinance.              transformer the MICB is generally specified as
 The logic of requiring LV systems to satisfy               a draw-out type air-circuit breaker to BS 4752
 both TN-S and TT earthing is questionable.                 rated at 24 MVA breaking capacity [ l ] .
 Arguably it is not possible to achieve separation          Overcurrent and earth fault protection are
 of transformer earthing and consumers                      usually provided through current transformers
 earthing, so a TT classification is overly                 connected to a triple-pole overcurrent relay
 simplistic. Adherence to, or confusion resulting           ( O K ) and an earth fault relay (E/F) in a
 from, these earthing regulations can                       residual-current circuit arrangement, Fig. 1 .
 compromise operating reliability and electrical
 safety. It is doubtful that all the requirements of
 Section 1 1 of the Electricity (Wiring)
 Regulations are fulfilled.
 Keywords: Supply/consumer interface. Low
 voltage systems. Earl hing practice. Electrical
 safety. Discrimination

 1. Introduction
           In a modem technological society, it is
 expected that the electricity supply will be
 reliable and as safe as practically possible
                                                                System           I       I'i         Consumers
                                                                earthing
 within reasonable economic limits. In Hong
 Kong the utility companies consistently achieve
                                                                            -
                                                                           =TT                -
                                                                                              -      earthing

 a high level of reliability, wave form quality             Fig. 1. Earthing systems.
 and voltage stability on all four wires of the low
 voltage supply. Provided consumers design and                      The relays employed are usually
 operate their elecl rical installation and                 induction disk IDMT 1.3s relays, although in
 equipment properly there should be few                     some installations 3s relays remain. As can be
 problems.                                                  seen in Fig. 2 the resulting overcurrent tripping
           Nonetheless, operating problems do               characteristic does not fully satisfy the
 occur as a result of the constraints imposed at            maximum permitted settings required by either
 the supply/consumer interface. Largely as a                of the two local utility companies, and use of
 result of the influence that local utility                 such relays leads to lack of full discrimination
 companies exert through their conditions of                with outgoing overcurrent protective devices
 supply, and a lack of an overriding requirement            rated over 400 A (e.g. 400 A BS 88 fuses).
 imposed by government ordinance, policy and                Improved discrimination is achievable by
 practices with respect to the provision of the             employing very inverse characteristics using
f i f t h supply wire, the circuit protective               solid state relays. However, such application is
 conductor (earthing conductor), has not been               not universal, and discrimination problems
 consistent for many years and remains a matter             remain.
 for concern.
           There has not been a clear                       3. Earthing electrical installations
 understanding of the way in which safe and
 reliable operation of consumers low voltage         The electrical installation within a
                                             building or part thereof, together with the
 installations can be enhanced by application of
                                             source of energy (including supply
 a consistent earthing policy and the provision
 of a good earth connection.                 conductors) forms a system [2,3]. BS 7430 [2]
                                             gives guidance on the methods that may be
 2. Discrimination for MICB's                adopted to earth an electrical system for the
                                             purpose of limiting the potential (with respect
        The need to discriminate between the to the general mass of earth) of currentcarrying
                                                          982
            i00




             10




            .01
                  100                  1000                     10000                    100000
                                            Primary current I,, (A)
Fig. 2. Overcurrent and earth fault tripping characteristics

conductors forming part of the system, and            earthing, i.e. "In every installation, the customer
non-current-carrying metalwork associated with        is required to provide his own earthing system
equipment, apparatus, and appliances                  by which the exposed conductive parts of his
connected to the system.                              installation are connected to earth (Under the
         The former object is normally essential      current IEE Regulations this is considered a TT
to the proper operation of the system, and this       system)".
aspect is generally known as 'system earthing'.                The Rules further prescribed that; "All
The latter concerns the safety of human life, of      exposed conductive parts of the installation
animals and of property, and this aspect is           shall be connected to earth electrode(s) via an
sometimes known as 'equipment earthing'. The          earthing conductor". A Main Earthing
term 'earthing' is used in this regard whether or     Terminal had to be provided to connect circuit
not reliance is placed on the earth itself as a low   protective conductors, main equipotential
impedance return path for earth fault currents.       bonding conductors and any required
Low voltage systems are classified according to       functional earthing conductors, to the earthing
the internationally agreed designations TN, TT,       conductor.
etc. [2,3]                                                     Legislation requires the supply
                                                      company to take responsibility for system
4. Earthing policy in Hong Kong                       earthing, to ensure that, in particular, the
                                                      metallic parts of every transformer be
4.1 Supply Rules                                      efficiently connected to earth, and the neutral
                                                      conductor of the three-phase system be earthed
        In Hong Kong, to secure electricity           at one point only, namely at the transformer. A
supply from either of the utility companies, it is    substation earth bar is provided to which is
necessary for consumers to comply with the            solidly bonded the armouring of the HV
Supply Rules issued by the companies. Prior to        supply cable and all metalwork within the
the 1990 Electricity (Wiring) Regulations [4]         substation. Fig. 1 illustrates the so-called TT
the Rules were specific on the requirements for       system.

                                                  983
         A TT system has one or more points of       given that most outgoing (mains, sub-mains)
 the source of energy directly earthed and the       circuits are protected by fuses or moulded case
 exposed and extraneous parts of the installation    circuit breakers without separate earth fault or
 are connected to a local earth electrode or         residual current tripping, this is generally not
 electrodes that are electrically independent of     proven.
 the source earth(s) [ 2 , 3 ] .                              Earth fault loop impedance's even on
         For urban sites supplied at high voltage    congested sites may be of the order of 0.1-
 (transformer on the premises) given the             0.4 R, and even neglecting arcing fault
 proximity of the transformer earth connections,     impedance the resulting earth fault currents of
 the con sumer's earthing electrodes,                500-2000 amps may not be cleared within 5 s.
 miscellaneous metallic structures and the mass
 of steel in the foundations, it is impossible to    5. Consumer's earth electrode system
 achieve independent earth electrodes because
 the resistance areas largely overlap [2,5]. The              The consumer is required to provide an
TT classification in these circumstances is           earth electrode system. Generally speaking,
 in appropriate.                                     earth electrodes satisfy a specified earthing
         Prior to the Electricity (Wiring)            resistance, must be capable of carrying fault
Regulations, this arrangement might be               current without thermal (I2) related problems,
converted to a TN-S system at the discretion of      and take account of safe touch and step
the utility company. In the case where the            voltages [2]. In Hong Kong the design targets
transformer is within the consumers' premises,       are not at all clear.
this is achieved by the provision of a substantial            The supply company's system earth
sized bonding colnductor between the                 value is not declared, but can itself be up to I R
transformer earth bar and the consumer's main         in value, given that compliance with CP 1013
earth bar (Fig. I).                                  (now BS 7430) would be their guideline. Whilst
         This is a welcome safety feature            in practice it is likely to be much lower than
because the rather unpredictable and relatively       1 R, this is unknown at the design stage. Thus
high earth fault loop impedance of a TT system       the target resistance value for the consumer's
is replaced by a known reliable path of low          earth can only be 'as low as economically
impedance that characterises a TN-S system.          possible'
Countries subscribing to CENELEC and IEC                      For a system voltage of 346 or 3 8 0 V ,
principles would reserve the TT classification       200/220 V to earth, an earth fault loop
for situations where a reliable earth connection     impedance of 1 Q would result in an earth fault
cannot be provided, such as on rural overhead        current of around 200 A, even 0.2 R results in
line systems.                                        only 1000 A . On this basis the thermal
         However, stipulated in the Supply Rules     requirements of the consumer's earth electrode
was the requirement that "when measuring earth       system should be easily met since carrying
fault loop impedance's or testing the operation      200-1000 A is not a difficulty, even for a few 2
of protective devices, the said bonding              m earth rods [6]. However, the consequences of
conductor must be disconnected". This means          connecting these electrodes in parallel with the
that the system must function as a TT system.        system earth must also be considered. An earth
                                                     fault on the HV side of the transformer could
4.2 Electricity (Wiring) Regulations                 result in a much higher fault current level, but
                                                     which is very difficult to predict. The point is
        With the 1990 Electricity (Wiring)           that strictly speaking, the designer cannot
Regulations the pirovision of a bonding              ignore the possibility of higher fault currents
connection between I ransformer and consumer         and potential gradients as the result of an HV
earth terminals and operation as a TN-S system       fault, but he has difficulty to design for it.
is prescribed. This is welcomed since it means                Even supposing that a consumer's earth
that system classification and operation is more     electrode system is designed and installed there
consistently applied. Nonetheless, t h e             is the problem of certifying its true resistance
requirement for an installation to function as       value. It is likely that given the complexity of
part of a TT system., i.e. without the bond, is      site conditions (cramped, extensive buried
retained.                                            metalwork, fortuitous contacts with buried
        The Regulations are just as definite         metallic structures, etc.) the impedance cannot
about compliance with the requirements of a          be measured. Certainly t h e simple tests
TT system. Regulation 12(4) states that all the      described in the BS standards [2,3] will not
requirements of Regulation 1 1 must be satisfied     suffice [ 5 ] .
without t h e bond, and Regulation 1 l ( 1 )
specifies that circuil s shall be provided with 5.1 Disconnecting the bond
effective means to prevent the dangerous
persistence of eairth fault currents and                When the bond between the consumer's
dangerous earth leakage currents. However, earth electrode and the transformer earth

                                                 984
electrode is removed, it is possible to achieve a     problem with such sensitive devices is that they
very low resistance, especially when main             are subject to nuisance tripping due to surge
equipotential bonding to structural steelworks        and transient leakage currents. This realisation
and other substantial metalwork are retained.         probably led to the exemption to the use of
Even discounting main equipotential bonding           rccd's for protecting socket outlets supplying
there are likely to be substantial fortuitous         window type air conditioning units [7].
electrical connections in the vicinity of the                 Given the users tendency to abuse
transformer and consumer earths.                      socket outlet use with various adapters this
         It is argued that the multiplicity of        seems a bad compromise. Modern rccd's are
unknown parallel paths is such as to make the         fitted with surge delays and can adapt to such
contribution of the consumer's earth electrode        load characteristics. The use of a two-pole
to the earth fault loop impedance unknown,            switch or spur unit should have been prescribed
and the path of any TT earth fault current as         when the 15th. Edition of the IEE Regulations
unpredictable. Arguably, main equipotential           was adopted i n Hong Kong in the early 80's.
bonding conductors or other fortuitous
connections could carry the majority of the           6.1 Supplementary bonding
earth fault current, which in principle and
intent, is not their function.                                 Another problem of interpreting the
         Given the substantial size and reliability   IEE Regulations given the dual earthing system
of the bond it does not seem logical to insist on     classification is that of equipotential bonding.
'proving' operation as a TT system. It as already     In a real TT system, the earth fault loop is
been mentioned that without knowledge of the          unreliable and touch voltage levels are likely to
system earth impedance the designer cannot            be higher than in a TN-S system [6]. Therefore,
hope to make reasonably accurate calculations.        more extensive supplementary bonding
It remains as a grey area for design and an act       provisions are called for.
of faith for commissioning.                                    In an TN-S system it is argued, and can
                                                      be shown [6] that the shock risk using rccd's,
5.2 MICB earth fault settings                         mcb's and the so-called 'alternative method of
                                                      design with circuits protected by fuses, in
         On account of the TT classification          combination with equipotential bonding', is
earth fault settings tend to be set at 10% (250 A     acceptably low. It is recognised that the overuse
for the typical 2500 A MICB) but also at a low        of supplementary bonding can compromise
time multiplier setting (tms), down to 0.1.           safety.
Fig. 2. illustrates the problem of discrimination              A common example quoted is t h e
with outgoing circuits. In fact, for sub-main         supplementary bonding of window frames in
circuits rated much above 100 A earth fault           kitchens and elsewhere, where the frame is
disconnection is likely by the MICB, provided         simultaneously accessible with a socket outlet
that the earth fault loop impedance is low            or similar part of the electrical installation. For
enough. Earth faults at t h e top of buildings        example, from EMSD's Code of Practice [7];
often trip basement or ground floor MICB's.           "Metalwork which may be required to be
         When the TN-S bond is in place,              bonded includes service pipes or substantial
installation earth fault loop impedance is very       parts which are at a distance not exceeding 2 m
low and earth fault currents are likely to be         from exposed conductive parts. Examples are
higher. This could be used to improve                 water pipes adjacent to electric heater or
discrimination between the MICB and larger            window frame supporting a ventilation fan or
rated overcurrent protective devices by               an air-conditioner adjacent to a socket outlet".
adjusting the plug setting and raising the tms.                It is argued that simultaneous contact
                                                      between the frame and the socket outlet is
6. Shock protection                                   much less likely than contact with the frame
                                                      and earth. A fault on the socket outlet circuit
        Where earth fault currents are below a        would be transferred to the frame, causing a
few hundred amps the tripping time of some            shock risk hand-to-feet. This is more
circuits may be longer than the safe times            vulnerable by a factor of 1:0.4 than a shock
prescribed in t h e IEE Regulations [3]. The          path hand-to-hand [8]. If the frame is earthed
solution for TT systems is the use of residual        through the structure it need not be bonded,
current devices (rcd's) or circuit breakers           because the touch voltage will be within safe
(rccb's). Application of this measure to sub-         limits. If it is isolated from earth it will not
main circuits is expensive for a system that          constitute a shock risk and need not be bonded.
actually operates TN-S, and can lead to other                  Bonding structural elements with
discrimination problems.                              internal electrical installations is fraught with
        One good effect of the TT classification      difficulty and is often unreliable, as t h e
is the requirement to employ 30 mA rccb's for         connections are not always maintained,
protecting socket outlet circuits. This definitely    especially in non-industrial situations. It is
can be a very important life saver [6]. The           better to provide the bond to earth within the

                                                  985
structural design [6].                                         an electrode.
7 Common earth
 .                                                             7.3 Earthing resistance values

        The use of a common earth for both                            BS 7430 gives sufficient guidance to
system (transformer neutral) earthing and                    enable the resistance of concrete encased
equipment earthing (consumer's earth) is                     steelworks to be estimated. The resistance to
proposed'. The common earth connection                       earth of concrete encased steelworks or
should be substantial and this is best achieved              concrete reinforcing bars will vary according to
by using an appropriate connections to a                     the type of soil and its moisture content and the
substantial part of the structural foundations.              design of the foundations. Concrete will have
                                                             resistivity of 30 to 90 !Am, which is lower than
7.1 Use of foundations                                       some soils.
                                                                      The resistance to earth of reinforced
         Guidance in this respect is given in BS             foundation ( R ) in ohms can be estimated by
7430 [2]; "Foundation metalwork in concrete                  assuming only the vertical reinforcing rods are
can provide a ready made and effective earth                 bonded to the building structure or to the
electrode. The total electrode area formed by                earthing system. The effect of other
the underground metalwork of large structures                reinforcement, which may be attached with tie
can provide an earth resistance lower than that              wire only, is neglected. It is assumed that rods
obtainable by other methods. Overall values                  are equally spaced in a symmetrically pattern.
well below 1 !2 are obtainable". Steel in                    This gives the following equation [2]:
foundation work is g,eneralIy protected against
corrosion by the colncrete and can act as a
satisfactory electrode if provision is made to
ensure electrical continuity and adequate
conductivity. Such steel has an electropotential
similar to that of copper and may therefore be    where:
bonded to copper earth electrodes. On the         p is the resistivity of the soil, in Q.m;
contrary, galvanised steel is strongly            pc is the resistivity of the concrete, in am;
electronegative to both copper and concrete       L is the length of the reinforcing below ground
encased steel so that an earth electrode of this  level, in m;
material should not be used [2].
                                                  6 is the thickness of the concrete between the
7.2 Corrosion                                     rods and soil, in m;
                                                  z is the geometric mean distance of the rod
        It is important that consideration be cluster (BS7430 Table 9 [2])
given to the possibility of corrosion of concrete          For example, for 8 rods evenly spaced
encased foundations. Continuous currents, such in a circle:
as those caused by incompatibility of
foundation steelworks with other types of                              2=               11152as7
buried metalwork to which it may be bonded, where:
can be a problem. Table I summarises the a = radius of the reinforcing rod, in m,
situation for typical construction and earthing s = the distance between adjacent rods, in m.
materials.                                                 The combined effect of similar footings
        Alternating current is not expected to (R,)         arranged in an approximately
cause corrosion. Corrosion of Concrete encased rectangular plan, can be determined for [2]:
steelworks, subject to a.c. fault currents within
its carrying capacity, has been found to be                                   (1 + h)
                                                                    R,, = R, .___
negligible. Damage to the concrete due to                                         n
arcing or rapid evaporation of moisture can in which a - P
occur where either long-term or short-duration                  27rR,s
fault currents exceed the carrying capacity of
            Table I: Sziitability of materials for bonding together (7'able 8 BS 7430 [21)
                               surface area Material or item assumed to have smaller surface area
                                            Steel             I GdVan&Cd St-1 I cOp&XW              I   Tinned copper
                                            4                  I4                Id                 I4
                                                                *
Steel i concrete
       n
                                            .
                                            i
                                                                                   I
                                                                                   .                    I
                                                                                                        .
Galvanised steel in concrete                                     J1
                                                                  '                I
                                                                                   .                    I
                                                                                                        .
                                            4
                                                                 41)               I
                                                                                   .                    I
                                                                                                        .




                                                         986
where:                                              company will common its own earth
R,, is the combined resistance of n similar         connections. This terminal is extended to
electrodes, in R;                                   provide an earth bar in the consumer's main LV
                                                    switchroom.
R, is the resistance of an individual electrode,            Such an arrangement will reduce shock
in R;                                               risk associated with earth fault currents, allow
 h is the factor given in BS7430 Table 3            better designs and remove the uncertainties
p is the soil resistivity, in R.m;                  associated with interpreting the rules for TT
n is the number of electrodes formed by             and TN-S systems. There would be no reason
footings;                                           why rccd's should not be prescribed separately
s is the average distance between adjacent          for socket outlet circuits.
electrodes, in m.
7.4 Example
                                                    9. References

         On this basis a building having 16          [ I ] China Light & Power. 'Approval of
numbers of 400 mm diameter footings, 10m            consumer's schematic wiring diagram'. 1984.
deep with 8 numbers 30 mm diameter rods             Revised.
evenly spaced in a circle, the calculated           [2] British Standard BS 7430 : 1991. 'Code of
combined resistance is 1.13 ohms in Hong            practice for Earthing'.
Kong Island where woodland soils, having an         [3] British Standard . IEE Wiring Regulations.
average resistivity of 95 ohm-metres, dominates     [4] Electricity (Wiring) Regulations. Chapter
[9]. The combined resistance is 0.073 ohms          406 Subsidiary Legislation. Hong Kong.
when located in an offshore reclamation land        [5] Tagg G. F. Multiple-driven-rod earth
where the average soil resistivity is about 3.7     connection. IEE Proc. Vol. 127, Pt. C, No. 4,
ohm-metres [9].                                     July 1980, pp 240-247.
         It is important to ensure electrical       [6] Burnett J. Earthing and Bonding in High
continuity between all metalwork forming the        Rise Buildings. Part I1 - Bonding and automatic
earthing electrode. This best affected by           disconnection. HK Engineer. December 1987.
welding reinforcing bars and by attaching           [7] Electrical & Mechanical Services
bonding conductors to bridge structural joints.     Department. 'Code of Practice for the
It is often possible to secure an effective earth   Electricity (Wiring) Regulations'. 1992.
electrode by providing a suitable means of          [8]      IEC Publication 479-1. 2nd. Ed. 1984.
connection to steel reinforcement in concrete       'Effects of current passing through the human
piles or to sheet steel piling [2].                 body'.
         There remains t h e difficulty of          [9] Halfter, N. A. King, S. Y. "Thermal and
confirming earthing resistance by test.             Electrical Properties of Hong Kong Soils'. HKU
However, for earthing electrodes formed by a        Press. 1970.
number of footings it may be feasible to
measure one footing before it is connected to
the others. The measurements will not be
accurate but will provide sufficient
quantification of the overall resistance.

8. Conclusions
        The dual classification of TN-S and TT
system should be removed from the Electricity
(Wiring) Regulations for installations where the
transformer is on the premises. The
requirement for a separate consumer's earth
electrode system should be discontinued.
Improved safety and reliability will come from
the provision of one substantial earth.
        Given the volume and suitability of the
steel in the foundations of Hong Kong's
building the earth electrode should be formed
using this as the earth connection in addition to
any supply company provisions. This could be
incorporated in the consumer's provision for
granting a supply, i.e. the consumer details an
earth connection using substantial parts of the
foundation metalwork and provides a terminal
in the transformer room to which the supply

                                                987

								
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