Wiring Matters 93 - Nov 22 - High Protective Conductor Currents

There is a bit about the sum of all the conductors in a multicore cable - between (iii) and (iv) of 543.7.1.203, but as I read it it only applies where you go for the duplicate  ordinary-sized c.p.c.s option (option (iii)) and have both of them in the same multicore cable (protective conductors (plural) are both incorporated in a multicore cable (singular)). Presumably on the basis that it's pretty pointless having duplicate c.p.c.s if the same mishap is likely to break both simultaneously - and that's less likely to happen  with larger more robust cables. I didn't read it as applying when the two c.p.c.s were in separate cables.

The single ≥10mm² conductor is a completely different option - (i).

I haven't read the article myself yet - but from what you say it does sound confusing.

   - Andy.

"total csa of all of the live and protective conductors will easily exceed 10 mm^2" 

^{Does not seem like any sensible interpretation to me}

depends if you are considering the tensile strength of the twin and earth compared to single 10mm2 core as the only limiting factor.

However, I think it is a dangerous simplification. In a ring safety comes from the two paths not following the same route and not likely to both be damaged in a way that breaks the CPC but not the live.

The cross section part is a read herring.

In a straight line  pull test I agree the strength is the summed strength of all the cores, but not being pulled around a corner, where one core will see most of the force and fail first, or being flexed repeatedly where the thinner core (CPC) may well not fail at the same time as the others.

I think the authors interpretation of cross sections is 'brave' ;-)

But he is right that a ring is a very good topology for socket circuits where total leakage may be high - just for the wrong reason - it saves the CPC return you would need on a radial.

The spur is a bit of a red herring as you would not put an un-fused commando socket on a spur from the ring - would you?
A 13A socket on any one  spur should not be adding more than 3,5mA of earth leakage.

(to save others the effort of looking the actual article is here

https://electrical.theiet.org/wiring-matters/years/2022/93-november-2022/high-protective-conductor-currents-in-electrical-installations/

)

M.

It appears to me that 543.7.1.202 and .203 contradict one another. .203 requires a 10mm conductor although installed whereas flexible cables to a plug may be 4mm2. Surely the 4mm cable is at much greater risk of failure, particularly loose connections than the fixed cables? Surely too the discussion of ring final circuits (that are normally used to supply sockets) is that they must be protected by 30mA RCDs, and are therefore not really suitable to supply more than 1 load with above 10mA of leakage? It is quite obvious to me that a 32A ring final circuit is non-compliant with any of this regulation group. I suppose it might be if:

1. It has no sockets, only fixed loads

2.It has no RCD of a sensitive rating, ie no additional protection (of course with these Earth precautions such would not be needed with a suitable method of cable installation.

The article needs a bit of a rethink, because it simply magnifies the confusion of the regulations themselves.

I think the author has just got it all confused. As I quoted previously, he says ring circuits with 2.5mm T+E satisfies 543.7.1.203 (presumably (iii)) but it should say it satisfies  543.7.2.201 by virtue of (i) "A ring final circuit with a ring protective conductor. Spurs, if provided, require high integrity protective conductor connections complying with the requirements of Regulation 543.7.1". Although this is implying you need to comply with 543.7.1.203 for the whole circuit

As 543.2.9 requires ring circuits with CPCs not formed from a metal enclosure containing all the ring conductors to be connected as a ring as well, and 543.7.1.204 has been deleted by AMD 2, meaning dual earth terminals in sockets are no longer required, the author is right that almost any ring final circuit wired in T+E will comply as a High-Integrity Earth circuit, just not quite for the reasons they gave

With regard to .202 and .203 contradicting each other, they kind of do, however the key difference is .202 is for a single item of equipment, .203 for multiple. 

The example being a radial circuit supplying two 16A Commando BS EN 60309-2 sockets, to serve to Data Racks. The supply to the sockets would need to comply with 543.7.1.203 while, after each socket, it only needs to comply with 543.7.1.202. 

.202 also says "shall be connected to the supply by". It is trying to impose conditions outside the fixed wiring after the BS EB 60309-2 socket, which are normally "outside the scope" of BS7671.

With regards to Ring final circuits, it is only where "the total protective conductor current is likely to exceed 10mA" for fixed equipment, or "known or reasonably to be expected that the total protective conductor current in normal service will exceed 10mA" (543.7.2.201) for socket circuits, so it could supply many loads where there are a number of possible devices that together can add up to more than 10mA

The typical one for this is a socket circuit in an office. Lots of switch mode power supplies for monitors, PCs, printers etc. These can go up to 3.5mA per device as per IEC-60950-1 limits. 

Well Matt, there is nothing to say that. You use the data centre description but a single 60309 may well supply a lot more than one item of equipment, often a rack full which may have a lot of leakage. However in a data center the Earthing is not just the mains leads for signal integrity reasons, so surely this is all nonsense anyway? Gridded Earthing is the norm, Earth impedance is very low indeed, and there are a myriad of Earth paths. This article is written by a probable non-expert author, the Editor ought to know better.

The example of the ring circuit is also somewhat spurious, and addresses another very unlikely scenario. How likely is it that a fixed wiring Earth conductor feeding a number of sockets fails, and then because it is a ring another point suffers the same fate? A single item may loose an Earth from its flex or plug, but this is not a high risk situation is it as the leakage is less than 3.5mA (which may not even be felt). If leakage approaches 30 mA the RCD should disconnect the circuit as additional protection.

For some reason the issue of high leakage seems to be tied up with plugs and sockets, it is not obvious to me why this should be. A set of stage lighting dimmers (500 of them) each with fairly large RFI filter capacitors, may "leak" many amperes, does this matter and is it unsafe because the Earth conductor may fail? For many years now this has not been a problem. It seems to me that this article is written to worry simple electricians with situations they are very unlikely to encounter. This happens all the time, because the training is failing the industry. The concept of risk has changed to "what if", and the what if is now always the worst possible outcome, with undefined circumstances. Surely this is ridiculous?

It appears to me that 543.7.1.202 and .203 contradict one another. .203 requires a 10mm conductor although installed whereas flexible cables to a plug may be 4mm2.

I don't read it like that - .203 only demands a 10mm² if it's exposed - permitting 4mm² if enclosed (it gives the example of flexible conduit, but a flex sheath where it is suitable for the environment will likely give similar protection). .202 aligns with that pretty well - other than it permits  a reduction to 2.5mm²  for 16A plugs - presumably just driven by the practical limitations of getting 4mm²  into a 16A terminal.

   - Andy.