[Question] Earth Loop Impedance on COC test.

[Jasond MIE]

I just need some clarification on the earth loop test that's required on a COC, where can I find the acceptable values for this test In the SANS book?

And can this test be done with the standard loop tester if it's over 100A at the main switch as I know we need to calculate for PSCC If over 100A?

I have recently been doing factory COC and most if not all the Main Db's and Sub Db's main swiches are over 100a.

Any help would be appreciated.

[ACEsterhuizen]

Page 276:

8.7.5 Earth fault loop impedance at the main switch:

8.7.5.1 At the main switch, the impedance shall be such that an earth
fault current double the rated current (or higher) of the main protective
device automatically disconnects the supply to the installation.

Table 8.2
indicates the earth fault loop circuits for different distribution systems. Page 277

https://www.tlc-direct.co.uk/Book/8.6.2.htm

http://www.ee.co.za/article/earth-loop-impedance-and-prospective-short-circuit-psc-testing-methods.html

http://www.elek.com.au/Files/Understanding%20Earth%20Fault%20Loop%20Impedance.pdf

http://mandrelectrical.com.au/karatings.php

So the Loop Test Result (eg 0.24 Ohms) in Ohms expressed in amps: A=V/z = 230/0.24 = 958 Amps
(Use the actual voltage and impedance readings.) The tester give you the psc amps anyway.

So your Main breaker is eg 60A. Your Earth Fault Impedance (0.24 Ohms) is such that a fault will result in 958 Amps flowing which is more than the "double or higher of your main protective device". (60A x 2( main breaker rating x double)=120A) as required by the reg. - so the earth fault impedance satisfy the regulation 8.7.5 because if you double your main breaker amps it is 120A and the earth fault that will flow is more than that and/or higher and will trip the breaker.

Lets say you loop impedance is 3 Ohms. So now 230/3=76AMps. So now the earth fault that will flow IS NOT more amps than the "double or higher than 120A" (which is double your main breaker 60A as required by regulation).

Note that the 76A fault might trip the breaker, but it does not satisfy the criteria required by regulation.

Your max amps that will flow on a fault is 958 amps, divide that by 2 (double the breaker rated amps) so the MAX amp breaker you can use on that loop impedance would be 479 amp breaker (theoretically)

So you record the 0.24 Ohms on the coc because it satisfies the regs and is correct.

[Jasond MIE]

Thanks that does help, and with regards to measuring over a 100a for the loop on a tester is it permissible?

[ACEsterhuizen]

a 100A earth fault on a 50A main breaker will work because the 100A fault is equal to double the main breaker rating.

a 100A earth fault on a 60A main breaker will not work because a 100A is NOT more or higher than double the main breaker rating which is in this case is 120A.

In fact a 100A earth fault will only satisfy a breaker rated 50A or less acc to reg 8.7.5 (excluding all other relevant regulations of course!)

Also see:

6.11.3 If, for practical reasons, the supply to the installation cannot be
automatically disconnected by an earth fault current double the rated current
(or higher) of the main protective device, as an alternative, an earth fault
detection and disconnecting device may be installed at the supply of the
installation. The earth fault detection and disconnecting device shall be so
installed that it operates at a current related to the earth loop impedance which
will limit prospective touch voltages under short-circuit fault conditions to 25 V
for a period not exceeding 5 s. This alternative does not relieve the supplier
from the responsibility of providing a supplier’s earth terminal
(see SANS 10292).

and:

7.12.3.2.1 The UPS shall be fitted with protection devices designed to
disconnect the output of the UPS in the case of earth faults that occur in the
part of the installation supplied by the UPS. If normal protection devices are
fitted, the earth loop impedance, including the internal impedance of the UPS,
shall comply with the requirements of 8.7.5.

[Justloadit]

Interesting about UPS's and earth.

UPS's that I have seen, usually have an isolating transformer on either the boost side, or the load side, isolating the battery from the load during its conversion from DC to AC.
There is no earth connection, or even any circuitry connecting any part of the load side to an earth connection. Effectively then it means that there can not be an earth fault.

So how to solve this one?
Does one connect one of the sides of the load side of the UPS to the earth terminal, and call this Neutral/Earth, and the other side of the inverter is now called Live?
With these UPS's as well, they have fast reacting internal current limit, so no high currents flow under fault conditions, which means a ELB must be installed before the load to keep in with the 7.12.3.2.1 clause.

Any comments

[Jasond MIE]

Good read, the 100a I was referring to was that of the main breaker size, if it exceeds 100a cab I do the loop test with a standard loop tester as the PSCC you cannot do if it's over 100a you then need to do calculation or get from supplier.

[ACEsterhuizen]

a 100A breaker would allow for a 200A or more earthfault.

If your meter cannot measure the earth fault current then measure the loop impedance, take your phase to earth voltage, do the sum and calculate your earth fault current.

Your not testing the breaker so breaker size should be irrelevant in the test itself, you are measuring the impedance of the circuit right back to the transformer windings where it comes from.

And then make an informed decision whether the breaker, circuit, earthing loads etc. is suitable for the application and complies according to the regulations.

http://www.kew-ltd.co.jp/en/support/mame_08.html

[bergie]

the common cheaper type loop testers can test up to 100 amp main breaker.guessing about 3KA that it can handle. i have a 2nd loop tester that i use for fault currents up to 20KA but it doesnt display the voltage before the induced load and after.

[ACEsterhuizen]

Interesting about UPS's and earth.

UPS's that I have seen, usually have an isolating transformer on either the boost side, or the load side, isolating the battery from the load during its conversion from DC to AC.
There is no earth connection, or even any circuitry connecting any part of the load side to an earth connection. Effectively then it means that there can not be an earth fault.

So how to solve this one?
Does one connect one of the sides of the load side of the UPS to the earth terminal, and call this Neutral/Earth, and the other side of the inverter is now called Live?
With these UPS's as well, they have fast reacting internal current limit, so no high currents flow under fault conditions, which means a ELB must be installed before the load to keep in with the 7.12.3.2.1 clause.

Any comments

Here might be a few ways, and it confuses the hell out of me.

http://www2.schneider-electric.com/documents/technical-publications/en/shared/electrical-engineering/dependability-availability-safety/low-voltage-minus-1kv/ect129.pdf