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

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.