It's more of the BS 7671 perspective that worries me - if BS 7671 demands something and we can't tell if we've met that requirement or not, can can we in all honesty sign it off?

It's not unknown for BS 7671 to ask for things product related that are over and above what the product standard says - e.g. BS EN 61439-3 doesn't demand ferrous/non-combustible enclosures, but BS 7671 421.1.201 does (for domestic), so there seems to be a precedent for going beyond just relying on simple compliance with an appropriate product standard.

(But on the other hand, some products clearly fall below normal BS 7671 general requirements (e.g. BS and ES lampholders - live part not even meeting IP2X etc) - so the principle doesn't feel entirely clear)

    - Andy.

Andy,

I'm still not 100 % sure that "levels of availability" discussed in 443.6.1, and "degrees of availability" in 443.6.2 are a direct read-across to the voltage ratings of cables and connectors, in the way they would be for switches and isolators.

That appears to be what the OP was told by a manufacturer?

Thanks everyone for sharing your thoughts on this; I think Graham is correct in suggesting that it probably isn’t a problem worth worrying about.

It has, however, prompted me to step back and consider just what section 443 is trying to achieve. 440.1 States that the requirements are intended to provide for the safety of electrical installations in the event of voltage disturbances. However, what aspect of safety is being considered given that protection against electric shock or thermal effects are addressed elsewhere? 443.1.1 Deals with protection of electrical installations against transient overvoltages of atmospheric origin or generated by the equipment within the installation, and 443.6.1 states that “Overvoltage categories are defined within electrical installations for the purpose of insulation coordination…” and provides Rated impulse voltages for equipment selected according to the nominal voltage to distinguish different levels of availability of equipment with regard to continuity of service and an acceptable risk of failure.

So, aside from the direct effects of a lightning strike or the consequences for any safety critical systems reliant upon the installation, is 443 really about reliability / availability rather than safety? Should we be interpreting it as guidance for the designer when choosing components or the need for surge protection devices, rather than a clear requirement that all equipment in the fixed installation must be rated for category III? If the latter was the intent, then we need to study a lot of product standards. The examples of Overvoltage category III equipment given in Table 443.2 include distribution boards, switches and socket-outlets, and the 17^th edition of the regulations included wiring systems in this list; manufacturer’s data for distribution equipment does commonly state the overvoltage category but I struggle to find that information for a socket outlet or a cable. I’m sure that the relevant product standards would reveal all; unfortunately, since retiring, I no longer have ready access to those. Wago do publish relevant information on their web site but I found it somewhat unclear, hence the original post.

Anyway, I’ve waffled on enough about something that probably isn’t a problem at all; I do still wonder about 443 though, requirement or guidance?

Geoff

Geoff Lyons said:

I struggle to find that information for a socket outlet or a cable

Cable you won't find.

Socket-outlets to BS 1363-2 used for fixed wiring (but not rewirable "free" socket-outlets etc.) have an impulse withstand test as part of the standard, and the standard itself states Overvoltage Category III.

For industrial outlets, BS EN IEC 60309-1:2022 states 'overvoltage Category II' (Clause 21.1.6)

That's useful to know, thanks Graham.

Slightly an aside but it is worth observing that the same category (I, II, III IV) for the same voltage (230V usually for us) requires different impulse test voltages, although the only thing that changes in going down to a safer a category is the fuse it has come though.

~Now this may seem a  bit odd, as clearly a fuse does not reduce the voltage of a transient - but it does limit the available current/time if something unexpected induces an arc event, and cuts the power off safely.

So the higher test  voltage has the effect of  ensuring a larger gap between bits of metal, meaning in turn that even though more current is available, the striking of a self sustaining arc remains unlikely.

This sort of thought is especially important when skating about with spindly meter probes where it is rare but not unknown to accidentally short adjacent terminals.  So removing the covers from the meter probes in effect lowers you a grade or two - i.e. a given length of  bare metal is more dangerous on the main bus bar than it is on the supply to a bell transformer.

As I said earlier, the actual spacing between a few free flapping wago's is a bit of an unknown. It is however very possible, and so required, to test the flashover between the internals of a 3 pin socket or a lamp holder where the metal and what holds them apart is fixed.

quoting definitions from https://www.gambica.org.uk/static/d96bb734-348d-478e-b0cba3b50fc35940/Guide-to-Overvoltage-Categories-and-Measurement-Categories.pdf

Note that we are now onto a later revision than this refers, but the general push is unaltered.

CAT I

In addition to the label “CAT”, the maximum voltage must be marked. This voltage is the maximum voltage between live and ground of the circuit or the same overvoltage range.

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mapj1 said:

quoting definitions from https://www.gambica.org.uk/static/d96bb734-348d-478e-b0cba3b50fc35940/Guide-to-Overvoltage-Categories-and-Measurement-Categories.pdf

But these are only according to BS EN IEC 61010-1, and not true of all product standards (and IEC 60664-1 along with BS 7671)

True, but 60664-1 uses the same impulse test  limits for the same voltage case and these in turn re-appear in in tables F1 and F5 and set the required creepage and air gap clearances.

So the choice of 3mm contact gaps in mains switches for showers etc comes immediately from the 4kV impulse test. If we really only had to hold off 230V RMS and not open against a high current, then a contact gap of about half a mm would probably do.

It is not too surprising that the various documents advise similar things.

M

mapj1  Agreed test voltage levels are the same.

However, the thing that's not the same is the descriptions of what the overvoltage Categories apply to vs BS 7671 and IEC 60664-1. They are "roughly the same" but the lines are very blurred I think.