It is trying, and perhaps not very clearly, to say that an RCD can be used to detect a fault current in the CPC, i.e. first fault is live to the correctly earthed case of some equipment, the RCD provides ADS. You only get  short duration shock, if you are unlucky enough to be touching it at the time the fault comes on.


It should NOT be used to justify omitting that CPC, where the RCD relies on some poor b***** getting a shock to trip it, as when the fault comes on it keeps working with a live case and nothing trips until then.

It may come as a surprise to UK readers who have not traveled outside the reach or IEC based rules,  but in some parts of the world such RCD and no earth installations are seen as OK - a sort of Class 0 + RCD.

but in some parts of the world such RCD and no earth installations are seen as OK - a sort of Class 0 + RCD.

I can confirm that in some EU self catering type accommodation that is as good as it gets.....hob keeps tripping RCD.... no problem the resort will " fix it"  by disconnecting the CPC

Thank you Gents.

The query was - 

This appears to be inconsistent. Is 531.3.6 meant to apply to only one type of neutral  earthing system?

Regards

I don't see how 531.3.6 applies to any one type of earthing system 531 is all about protection of electric shock by ADS, and 531.3 is about RCDs.


I don't see a discrepancy;  if you are going to use a RCD for fault protection, you still need to following the requirements for earthing and bonding as well as basic protection.

Isn't the point that


415.1.2 implies that OPDs must be used and 


531.3.6 states that that is not the case?

I think PG is asking if this only applies to TT instalations.

No the 2 sections are looking at different things.


RCDs can be used to provide additional protection on all earthing systems.


Also you may decide /need to rely on  an RCD as the interrupting part of an ADS system that has a CPC, even if it is TN-s or TNC-s - it may be that Zs is not low enough to blow a large fault fast enough for safety of life, a 40A C type MCB needs 500A to break for certain in the 'half a  heartbeat' required, and  on a 0.8 ohms TN-s (230V /0.8 ohms = 287A ) may come unstuck for example. The solution is an earth fault relay or an RCD at the origin of the circuit to be protected.


What you may not do however is to decide to use  the RCD as the sole means of ADS and omit the CPC, you still need the earthing and bonding.

Thanks very much,

geoffsd was correct in guessing the path of my uncertainty.

I now follow the point that reasoning in BS7671.



Regards

geoffsd:

Isn't the point that


415.1.2 implies that OPDs must be used and 


531.3.6 states that that is not the case?

I think PG is asking if this only applies to TT instalations.

• ADS (earthing, bonding and basic protection) (reg 411) or

• Double or reinforced protection (reg 412) or

• Electrical separation (reg 413) or

• SELV or PELV (reg 414)

I don't follow.


How do you get ADS without an OPD if the RCD is only allowed as additional protection?  Additional to what?

You still need a OPD for overcurrent (overload and fault current(short circuit))


the RCD can be used for fault protection see reg 411.4.5