High protective conductor currents - Deletion of 543.7.1.204

Yes it's gone, a sad day for me as I sat on the IEE panel that produced those regulations and that one was my suggestion.  It is the only thing I could claim to have actually got into the regulations.  I contributed to other regulations but so did everybody else on the panel. So my lifes work is now in the trash can.

Regards

Geoff Blackwell

GeoffBlackwell said:

So my lifes work is now in the trash can.

Well, not quite yet Geoff. We still have accessories with not one but TWO terminals for CPCs, which i believe we wouldn't have if it wasn't for your suggestion. Thank you.

We calls it our Blackwell Inheritance 

Agreed, Three Cheers for Geoff

OlympusMons said:

Well, not quite yet Geoff. We still have accessories with not one but TWO terminals for CPCs, which i believe we wouldn't have if it wasn't for your suggestion. Thank you.

If we are talking about socket-outlets, Regulation 411.3.3's development through 17th and 18th Editions has effectively removed the necessity for this. Socket-outlets up to 32 A are now required to have 30 mA RCD protection. 63 A socket-outlets are the next up in the range, meaning we would almost certainly have a cpc that meets the minimum 4 sq mm requirement of Regulation 543.7.1.203.

I'm struggling to understand how you could meet the requirements of 411.3.3 and have a circuit with high protective conductor currents for socket-outlets of 32 A or less (i.e. requiring "dual cpc"), unless it's somewhere like a data centre where you have additional earthing in any case via another path to comply with BS EN 50310.

If we are talking about socket-outlets, Regulation 411.3.3's development through 17th and 18th Editions has effectively removed the necessity for this. Socket-outlets up to 32 A are now required to have 30 mA RCD protection

But 30mA RCD protection is only needed for sockets, not necessarily the entire socket circuit - and now we have BS 7288 RCD sockets back - it's quite easy to imagine >>10mA protective conductor currents on a 32A circuit.  I can think of plenty of office environments originally wired with a ring covering a significant number of desks with wiring in dado trunking (so no RCD requirement for cables concealed in walls) where RCD sockets would be a very obvious choice.

I dare say some commercial lighting circuits will likely fall into this category too as we switch to electronic ballasts with their filtering capacitors and subsequent earth leakage.

Even if 543.7 doesn't apply as often, that doesn't seem to justify watering down its provisions for the remaining situations where it does still apply.

Well, not quite yet Geoff. We still have accessories with not one but TWO terminals for CPCs, which i believe we wouldn't have if it wasn't for your suggestion. Thank you.

+1 for that. In fact the twin terminals have proved very useful on several occasions where I've ended up with 3off or 4off 4mm² c.p.c.s to terminate - for some reason BS 1363's terminal capacity thinking seems to overlook that you often want a fly lead to the back box in addition to the two ring (or radial in and out) and one spur conductors.

   - Andy.

gkenyon said:

unless it's somewhere like a data centre

Or non-domestic where a risk assessment has been done. But i agree, RCDs put a practical limit on the level of protective conductor currents that can occur. 

Another example perhaps of our reliance on RCDs to hopefully do their job, in the absence of "belt and braces" methods.

I am probably getting too old for this, so I don't want to get too involved in the discussion.

However, my comments are -
The idea behind 'duplicate earthing' as opposed to using more robust earthing - is to ensure that two simultaneous failures would have to happen before a shock risk could occur.

Duplicate earthing consists of two earth paths and duplicate connections. A single failure could occur in an earth path or in a single connection - the result being that the other path or other connection would maintain protection.

1) This is similar to class 1 protection you need:
i) a fault to earth and;
ii) a failure of the automatic disconnection or other measures to mitgate the shock risk.
If i) and ii) occur a shock risk could exist.

2) Class II - the equivalent of two layers of insulation must fail before the risk occurs.

3) Duplicate earthing - two conducting paths for the cpc with duplicate connections at each termination - so two failures in the path of the circuit protective conductor must occur.

4) The suggestion appears to be that we can remove part of the duplicate earthing measure (i.e. the duplicate connections) and rely on RCD protection to save the day. In this case in the event of one fault occurring in a single point of connection of the circuit protective conductor, an appliance or similar could rise to a dangerous potential with respect to earth. If nothing else happens this will be a stable situation so the the RCD will not respond.

This potential has caused a shock risk to exist after just one failure - if a person touches the appliance current will divert though them causing an imbalance between line and neutral currents. This (we hope!) will trigger the RCD.

So we are relying on the RCD to prevent injury after just one failure has occurred.

If we take 1) above you could argue that the automatic disconnection could be achieved using an RCD so what is the difference between 1) and 4). Simply this - for 1) above the shock risk is only present after two failures and for 4), if it does not have duplicate connections, a failure in one connection could remove both paths of the duplicate cpc. So the shock risk could be present after just one failure.

I am not entirely happy with that.

Regards

Geoff Blackwell

What are we looking to address with 2 terminals?

If it's a case of the conductor itself breaking, 2 no. conductors in the same terminal would require 2 failures?

If a terminal is subject to failure, surely that's the case regardless of the size of cpc connecting to it? So what's special regarding 4 sq mm over 2.5 sq mm or 1.5 sq mm connecting to what is potentially the same terminal? But BS 7671 never covered off that requirement - it only required 2 independent terminals if 2 conductors were used?

OlympusMons said:

Or non-domestic where a risk assessment has been done.

But the risk assessment route has been tightened up a little more in Reg 411.3.3 in Amendment 2:2022