Touch Voltage - NFPA 70 comparison to BS7671

[davva]

Is there a similar requirement in NFPA for disconnection times, maximum earth loop impedance and touch voltage as there is in BS 7671 (British standard 16th edition wiring regs)?

Specifically the BS gives guidance on fixed apparatus (5 second disconnection requirement), or shorter disconnection times for portable or handheld apparatus. Based on this a maximum earth loop impedance shall not be exceeded.

If there is an correlation for a similar requirement I would be grateful of the relevant code/section.

 

[alehman]

The NEC requires a maximum earth resistance of 25 ohms for services. There are no specific timing requirements in the NEC for phase overcurrent protection.

Ground fault protection for equipment must operate within 1 second for ground current >3000A. Class A GFCI (personnel protection) must trip when a ground fault current exceeds 5 milliamps.

 

[Kiribanda]

davva,

In my opinion you are trying to compare apples to orange.

There are several differences in NEC Electrical installation methods and BS/ IEE/ IEC installation methods.This is almost similar to Zone vs Div system in Hazardous area classification.

Per NEC & CEC, the domestic installation is more or less equivalent to TNC+TNCS of IEC 364/ BS 7671.So if somebody strictly follows the NEC tables and size the wires/MCCBs etc., then the ground fault protection is provided by the branch overcurrent MCCB. Therefore, NEC does't talk about maximum earth fault loop impedance and maximum disconnection times for fixed equipments.

NEC does not allow to pass the ground fault current through the soil under any circumstances. But BS 7671 allows earh fault current to flow through the ground to trip ELCB in a TT system.(This is mainly that we cannot rely on soil specially during NA winter conditions etc)

NEC allows upto 25 Ohms maximum for the safety ground electrode where as in BS 7671/ IEC 364 you can allow upto 1666.6 Ohms (50 Volts/30 mAmp)for the ground electrode in a TT system!!

NEC 2008 is going mandotary the installation of AFCI/ GFCI for all 120 V, 15 A & 20 A circuits in houses for human protection whereas 30 mA ELCBs are very common in electrical installations in any circuits done per BS 7671/ IEC 364.

Therefore, if we work in North America we have to follow the NEC/ CEC code whereas if we do any work in Europe/ Far Esat then we have to follow BS 7671 installation methods. That's it.

But it is always interesting to dig into the issues seperately to see how human protection is provided in each case.

Hope this helps
Kiri

 

[davva]

I was coming from the point of cable selection criteria. Where with BS7671 remaining within maximum earth loop impedance could be the difference between using a greater CSA cable (in the absence of a ground fault element).

I could not find a similar clause in the NEC but wasn't to be sure. Thanks for confirming that Alehman and Kiri.