Earth Fault Loop Impedance Testing For Traffic Signal Installations

Document Sample
Earth Fault Loop Impedance Testing For Traffic Signal Installations Powered By Docstoc
					                    Earth Fault Loop Impedance Testing
                       For Traffic Signal Installations

Author
Brendon Tong
Traffic Projects Engineer
Whangarei District Council


Introduction
This study came about as a result of a Contractor being shocked from an (electrically) live
signal pole. This pole was fortunately located in a splitter island away from pedestrian walk
lines. The controller showed no blown fuses or other controller faults.

Earth faults of this nature are not particularly common but can be very serious, and
investigations were carried out to determine why no fuses had blown or circuit breakers
operated. Discussion of what the AS/NZS 3000:2000 requirements are around fault condition
touch voltages/disconnection times and how they can be controlled by monitoring the earth
fault loop impedance is also presented.



Background

Protective earth systems are an essential part of all traffic signal installations. They prevent
exposed metal items such as signal poles becoming alive in the event of contact with an
electrically live part under fault conditions. This is achieved in two ways:

    •   By connecting all exposed metal items together(equipotential bonding), and

    •   Connection to the general mass of earth through a driven metal spike or other means
        (earth electrode) at the signal controller cabinet.

The Supply Authority also provides an earth electrode at its distribution transformers. The fault
current path is from the line conductor through the fault and returns by the combination of the
signals earth and the supply authority earth.

The resistance of the combined path from the fault to the Supply Authority earth and the fault
to the signals earth needs to be low enough to operate the protective devices.

AS/NZS 3000:2000, which is referenced in the National Traffic Signal Specification, sets out a
number of requirements around earthing systems and the manner in which protective devices
should operate in the event of a fault developing. These include:

    •   To disconnect the supply in 400mS (milliseconds)(1.7.4.3.4).

    •   In the event of a fault 50V ac touch voltage shall not be exceeded and also that the
        supply shall automatically be disconnected(1.7.4.3.2).

    •   The resistance of the main earthing conductor shall be not more than 0.5 Ω (5.4.3)
The Issue

A situation has arisen that even though checks of earth bonding were carried out on a regular
basis and shown to be satisfactory with earth bonding resistance to be less than 1Ω (ohm), and
most sites being 0.5Ω or less, an earth fault was not disconnected, leaving a signal pole alive.

This fault was not discovered until a contractor received a shock from the pole.
A programme of testing for earth fault loop impedance was undertaken as part of checks to
ascertain why disconnection had not occurred. This revealed a very high impedance in some
sections of the installation. Because of this we undertook tests of all the traffic signal
installations. The results of these tests will be discussed below.



Description of the controller fault protection

In the controller there is a network of protective gear which consists of a main circuit breaker,
sub-circuit breakers for the logic rack, signal displays and a general purpose outlet which
usually supplies a cabinet heater.

In the case of the signal displays circuit the circuit breaker (which is generally a 10A (ampère)
or 16A rated type B or C unit) is backed up by 20x5mm HRC fuses in the lamp circuit board,
one for each colour of each signal group.

The merit of this arrangement is that faults can be sectionalised and cleared by the group
fuses, leaving the rest of the lanterns operational.



The Earth Fault Loop Impedance Test

The earth fault loop impedance test measures the combination of the two fault current paths.
This can then be used with the requirement in AS/NZS 3000:2000(1.7.4.3.4) to disconnect the
supply in 400mS (milliseconds) and the combination of the fuse and circuit breaker
characteristic curves to ascertain the minimum required fault current, and hence the maximum
permissible earth fault loop impedance.

For the analysis the following assumptions were made to capture a “worst case” scenario:

   •   The signals circuit breaker has a type “C” characteristic curve, and

   •   The 20x5mm HRC fuses in the lamp circuit boards have an “aM” characteristic curve.

Relating this to the disconnection time identified above, a maximum earth fault loop impedance
of 1.85Ω has been derived. This translates to a minimum fault current for a 400mS
disconnection time in the region of 125A. See Appendix B for graph.

It is important to distinguish between insulation resistance testing and earth fault loop
impedance. Insulation resistance testing is intended to check for insulation breakdown to earth
or other earth faults. It is done with power disconnected and usually as part of the
commissioning tests unless earth faults show up or are suspected.

By contrast the earth fault loop impedance test is designed to be undertaken with power
applied, and checks the impedance( i.e. resistance and reactance combined) of the earth path.
The lower this impedance is the faster the protective device(fuse or circuit breaker) will operate
and the lower the voltage a potential victim will be exposed to. See Appendix E for additional
information.
Discussion of Testing

What follows from this is that if the earth fault loop impedance test returns a very high value, a
fault may not produce a sufficiently high current through the earth system to operate a fuse or
circuit breaker.

For example a value in the 129-139Ω range would only produce a fault current of about 2A,
well below the 125A necessary to disconnect in 400mS. The result of this would be that the
fault would remain unnoticed for an extended period, with consequent danger.

There may also be poor discrimination, i.e. the circuit breaker may operate, blacking out the
whole site where under more suitable conditions a signal group fuse would have operated to
disconnect the faulty section only, leaving the site mostly operational.

The results of the testing were classified for remedial works as follows:

   •   1.85-2.0Ω action programmed when work next being done on site(general maintenance)

   •   2.0-4.0Ω action programmed as soon as possible(medium priority)

   •   4.0Ω or greater – immediate action(high priority)

These correspond to fault currents of 115A(2Ω), 58A(4Ω) respectively. This 58A value returns a
disconnection time of around 20 seconds.

Note that the assumptions given above require further development to ensure that they are
universally applicable, given that the protective gear in use will vary between Road Controlling
Authorities. It may be that a more stringent immediate action limit is required to ensure that
AS/NZS 3000:2000 is complied with at all times. The 400mS time has been chosen based on the
likely damp/wet situation of the equipment.

Although AS/NZS 3000:2000 does permit a disconnection time of up to 5 seconds for
fixed wired equipment it is this author’s view that in a damp situation a person in poor health
touching a pole would have a greatly reduced chance of survival with such a long disconnection
time under fault conditions.



Pedestrian Call-boxes

In addition AS/NZS 3000:2000 requires(1.7.4.3.2) that in the event of a fault 50V ac touch
voltage shall not be exceeded and also that the supply shall automatically be disconnected. This
applies especially to pedestrian call-boxes.

The latest type ATTS call–boxes operate on extra-low voltage supply, so they are acceptable in
themselves, but there is a danger that their cases could be livened in the event of a fault
elsewhere on the pole if the earth fault loop impedance is not controlled.

For older type call–boxes such as the Harding PCB4 and PCB5 the need to control earth fault
loop impedance is even more important due to them operating at mains voltage. Any fault here
could liven a push button causing a dangerous situation for pedestrians.
Implementation

Road Controlling Authorities cannot control the Supply Authority’s distribution and earthing
system. However Road Controlling Authorities can reduce the impedance of their signals
earthing system and lower the impedance of the combined systems. This could be achieved by:

   •   Paralleling up extra cores in older type cables to increase the cross-section of the earth
       conductor. Note that for new installations a 4mm2 earth conductor is required, which is
       satisfied by the current generation of traffic signal cables. The older type of cable is
       around 1.5mm2 .

   •   Selective replacement/re-termination of signal cables – some old type terminals develop
       high resistance over time.

   •   Pole top replacement with modern units.

Older installations where cables have been direct buried and there are insufficient cores to
parallel present some difficulties in achieving compliance.

The possibility of adding an extra earth electrode at the pole in lieu of re-cabling is not
recommended as it would introduce a new earth path to the main earth electrode via the
resistance of the earth. This resistance could be quite high depending on the soil conditions and
will vary on a seasonal basis. It is also unlikely to comply with AS/NZS 3000:2000.


Recommendations

Road Controlling Authorities should consider the following:

   •   Adding earth fault loop impedance testing to their requirements as part of the
       commissioning process for new signals works.

   •   Undertaking a baseline survey of their existing network.

   •   Developing a remedial work programme based on the baseline survey.

   •   Undertaking an annual compliance survey of their network.

In relation to the National Traffic Signal Specification:

   •   The above recommendations be added to this document or related documents as
       appropriate.

   •   A summary of the AS/NZS3000:2000 earthing requirements be added, with emphasis on
       the requirements to follow the procedure set out in 6.3.2., and the details which follow
       in 6.3.3. The steps in this testing procedure are:

       •       Earth resistance test-continuity of main earth conductor
       •       Insulation resistance test of the installation
       •       Earth resistance test for other earthed and equipotential bonded parts
       •       Consumer’s main test – polarity
       •       Final subcircuit test – polarity and connections
       •       Earth fault loop impedance test
       •       Verification of residual current devices(if installed)

       This will generally incorporate into Clause 2.17 in the National Traffic Signal
       Specification.
   •   A review of the Specification be undertaken to ascertain consistency in requirements
       and language with AS/NZS3000:2000.



Acknowledgements
The author acknowledges with thanks the assistance of Mr. Harvey Limbert with analysing the
data and production of the earth system and fault path diagram.
Appendix A : Earth system diagram with fault paths
Appendix B : Disconnection Time Graph
“C”
Appendix C : Remedial Works Programme
                                       POLE
                                                                                fault
            1    2      3      4       5   6     7      8   9   10    11   12   count       comments/additional works
                                                                                                                                       >4 ohms - high priority
        1                    1.94    2.3                                                2   rusty terminations poles 5,8               2-4 ohms - medium priority
                                                                                                                                       <2 ohms - general
        2       1.9                 2.28   3   2.6    235                               5                                              maintenance
        3                                  2                                            1
        4                            2.6       1.9                                      2   some rusty terminations pole 12
        5                                  2                                            1
        6                                  2                                            1
        7                            2.3   2   2.3                                      3   rusty terminations poles 3,6,8
                                                                                            POLE NO. 8 - FIT NEW HOSE                  Note that data has yet to be
                                                                                            CONNECTORS POLE NO. 1 - MAST               supplied for Site 9(under
                                                                                            ARM TARGET BOARD BENT POLE                 construction), Site 18 and Site
                                                                                            NO. 5 - OLD CABLE, TERMINAL BOX            20
                                                                                            BEEN HIT. NEEDS REPLACING
                                                                                            POLE NO. 6 - POLE EXTENSION -
        8             2.3     2.8    2.4                                                3   OLD TERMINALS
        9
       10                     1.9    2.1                                                2   rusty terminations poles 2,6,7,8
       11                            2.2   2                                            2
SITE




                                                                                            POLES NUMBERED WRONG -
                                                                                            STARTS AT NO.2 POLE NO.6 IS
                                                                                            CONCRETE POWER POLE POLE
                                                                                            NO.1 NEW CABLE - NEW
                                                                                            CONNECTORS POL NO.2 & 3 OLD
                                                                                            CABLE - NEW CONNECTORS POLE
                                                                                            NO.3 LOW VOLTAGE - ONLY 220V -
                                                                                            NO TEST NO HEATER IN
       12                                                                               0   CONTROLLER CABINET
       13   4         129    139     2.5   2                                            5
       14   4   4.5    5.2    6.1    6.5                                                5
       15                       2    2.6                                                2
       16                           2.15   3   2.3    1.9   2                           5
       17                     1.9    2.1                                                2
       18
       19                                             1.9                               1
       20
                                                                                            rusty terminations pole 8 in Mk2 pole
       21                                                                               0   top
                                                                                            POLE NO.5 MAST ARM BOX
       22                                                                               0   DAMAGED


NOTES

The above result sheet is a compilation of the field record sheets.

Only non-compliant results are shown, and these have been manually assigned into the appropriate work priority as explained in the text. The value of this approach is
that sites which require immediate attention can be seen at a glance which aids work programming.
                                            TRAFFIC SIGNAL INSPECTION CHECKLIST



        POLES
                                 Volts         Earth Loop     Earth Bond      Cable Insulation      Terminations
           1                     222              1.4            0.39
           2                     221              1.9            0.25
           3                     222             1.51            0.16
           4                     222             1.68            0.15
           5                     222             2.28            0.18
           6                     221               3             0.5
           7                     222              2.6            0.15
           8                     222              235            1.03
           9                     224              0.8
          10
          11
          12
          13
          14
          15



  CONTROLLER                                                                                                       Sample result sheet showing typical data from an intersection. Note that even
                                                                                                                   though bonding results are generally in the acceptable range the earth fault loop
         Volts                   239                                                                               impedance results can vary markedly.

        Current                   2.9                                                                              As a part of the data capture process to produce accurate graphs the type numbers
       Earth Bond              ........                                                                            of the circuit breakers must also be noted. This is important to get the correct
          Peg                  ........                                                                            characteristic curve selected.
         Cabinet               ........
       Earth Loop
       Impedance                  0.2


   COMMENTS
MAIN           MG 63A 6KA
SOCKET          GE 16A 4.5KA
CONTROLS GE 6A 4.5KA
LAMBS           GE 16A 4.5KA


   Intersection                                                                      Serviceman:
                               Electrical
                                Testing                                                     Date:
                                                                                            Time:
Appendix D : Current NTTS specification items
2.6 Earthing (Bonding)
All metal components must be individually earthed in accordance with the AS/NZS 3000:2000 wiring
regulations, using a minimum size earthing cable of 4.0 mm2. Particular attention should be given to
poles(including mast arms), call boxes, finial caps, metal bodied signals, controller and cabinet, mast arm
termination box and audio tactile driver box.

2.11 Pole Terminal Assemblies
The top of each standard pole shall be fitted at the upper end with a terminal assembly unit and cover meeting the
requirements of AS 2339-1997 “Traffic Signal Poles and Attachments”. For mast arm poles a terminal assembly unit
and cover shall be mounted on the top section on the mast arm that supports the lanterns. The terminal cover or
finial cap shall fit snugly over the pole top to minimise the ingress of water, dirt and grime.

Metal finial caps must be separately earthed (bonded). The pole top terminal assembly must be designed to
provide for the mounting of the signal lanterns.

In addition, mast arm poles shall have a terminal assembly box fitted to the pole, mounted no lower than 3.5
metres above ground level, for termination of low level lanterns. The box shall be a standard waterproof type with
minimum dimensions of 305 mm x 160 mm x 160 mm and be made of aluminium or polycarbonate box
and have rating of IP65. The terminal assembly box shall be fixed to the pole with a minimum of two M6 bolts and
shall have a rubber seal or gland between the box and the pole metalwork to create a waterproof seal. All signal
leads shall enter through the underside of the box. (REV 2)

All cables shall be terminated in accordance with the details shown on the Cable Termination Chart.


2.18 Certificates of Compliance
(a) General
All new traffic controller cabinets being installed with new mains, switchboards and earthing systems
will require a certificate of compliance.

(b) Particular Requirements
The Engineer requires that all electrical work will be done in accordance with AS/NZS 3000:2000
The Contractor must comply with the NZ Electricity Act, in particular clauses 95, 108 and 114.
It will be the responsibility of the electrical contractor doing the signals installation work to ensure that
a certificate of compliance has been obtained and the required copies delivered to the Engineer prior to
commissioning of the signals.

3.5.5 Earthing (Bonding)
The earth pin and wiring connection shall be located in a protected inclosure not readily accessible to the public.
Appendix E : Technical notes on Earth Fault Loop Impedance
Further Notes on Earth Fault Loop Impedance

AS/NZS 3000:2000 and its companion handbook SNZ HB 30:2002 provide much useful information
regarding why earth fault loop impedance must be controlled and how it should be tested. It is vital for
safety that these requirements are adhered to.

Why is this?

   •   To prevent a dangerous situation where exposed earthed metal is made electrically alive in a fault
       and not immediately disconnected, which creates a danger for personnel.

   •   With the exposed metal being alive, a high earth fault loop impedance means that a high shock
       voltage is present, leading to a potentially lethal electric shock on contact.

   •   As the earth fault loop impedance increases, the time for the protective devices to operate and
       disconnect the fault also increases.

In the type of electricity supply used in New Zealand(multiple earthed neutral or MEN) electric shock
protection is afforded by suitable design which prevents contact with live parts(direct contact).

In the event of a phase/active to earth fault or phase-neutral conductor fault protective devices in the final
subcircuit level are operated to disconnect the fault and create a safe situation( SNZ HB 30:2002).

At the same time the “touch voltage” on the exposed metal under fault conditions measured to earth must
not exceed 50V ac before the fault is disconnected. This is necessary to prevent dangerous electric shock as
outlined above.
The above figure, simplified for clarity, details the elements which make up the earth system and what the
earth fault loop impedance test measures.

The earth fault loop comprises the path from the Supply Authority transformer through the service line and
protective devices and the fault (which is assumed to be of negligible impedance) and the exposed
metalwork to the installation earth electrode and back to the Supply Authority earth electrode at the
distribution transformer.

Note that in the traffic signal environment there will be multiple earth paths which need testing. A more
complete drawing is shown in Appendix A.

The “touch voltage” or the electric shock voltage which a victim would experience under a fault can be read
by the voltmeter V. It should be noted that if the earth fault loop impedance is near zero then this voltage
will also be small as the earth path through the victim is bypassed by the lower impedance of the earth
system.