RCD and earth conductor sizing

[healyx]

Hi All,

I was once told during a course that if you had high earth loop impedance, instead of increasing cable sizes, you could use an RCD. I can't find the reference in AS/NZS 3000 (Australian Wiring Rules) to say this is OK? Does anyone know where to find it? Happy to get opinions on other IEC based wiring rules - they are usually similar.

If this is OK to do this, what size does the earth conductor now need to be? I guess you might not technically need it anymore? Would you just select the size as per the usual tables and/or size it to thermal constraints? - it obviously wouldn't be large enough to reduce fault loop impedance - hence the RCD?

Thanks.

 

[7anoter4]

If RCD means "Residual Current Device" this is a good protection device against electric shocks. But the grounding it is a general protection against electric shock and fire protection also by forcing the opening of a breaker protecting the circuit.
RCD will work also if the grounding connection will show a limited resistance to ground. If the minimum current to be sense by RCD could be 10mA a maximum resistance of VL_G/0.01 ohm has to be provided. A good security factor is 10 then R<=VL-G/0.1 will be better.
You have to check the RCD by pushing the disconnecting button at least once in a half of a year. The time to clear the fault is very critical mainly in the electric shock case as the human body has a limited time to survive under the certain potential rise.
Conclusion: the use of RCD is very indicating, but a good grounding it is compulsory.

 

[sibeen]

healyx, check out section B4 of As3000 - 2007 and have a read.

I guess you might not technically need it anymore?

You'd be guessing wrong. The earth wire in a TN-S system, the primary distribution method in Australia, is a safety earth. You don't go willy nilly removing equipment that is installed for safety reasons

Also, all conductors – line, neutral and protective – must meet the size requirements of the adiabatic equation. Clause 5.3.3.1.2 of AS 3000 in conjunction with Section 5.3 of AS 3008.1.

 

[healyx]

Thanks guys.

Sibeen, I am familiar with that section of the standard but it doesn't really answer my questions which are:

(1) If you can't economically achieve a low enough fault loop impedance with cables, is it OK to accept that and use an RCD?
(2) If you use an RCD in this case, what is the minimum size of the earth cable now? Is it just found via the adiabatic equation?

I would never remove the earth cable, I was just making the point that if you use an RCD, the earth cable isn't serving the purpose of providing a low enough impedance to trip the circuit breaker on an earth fault. Disconnection is now determined by the current sums of the active and neutral cables.