EN 61000-3-11 and EN 61000-3-12

No, I don't think so ... when we are talking about loads in excess of 40 A (per phase).

I think the wrong emphasis in the instructions, though, for loads <= 9 kW (otherwise this would appear on the instructions for a lot of domestic showers).

Apart from being a low reading 0.031776Ω is particularly precise. 

Which is rather frustrating, because we don't know whether that's based on 240 V or 230 V (or even C_max times either of those, i.e. 253 V or 264 V) ... which of course has a huge impact, i.e. is the prospective fault current we're looking at 8.3 kA or 7.2 kA ???

gkenyon said:

we don't know whether that's based on 240 V or 230 V (or even C_max times either of those, i.e. 253 V or 264 V) ... which of course has a huge impact, i.e. is the prospective fault current we're looking at 8.3 kA or 7.2 kA ???

I'm not sure it matters - whether it's 7kA or 9kA it's still well beyond what probably 99.9% of domestic supplies could offer. They seem to have come up with a standard that makes items that comply with it practically useless. Another BS 7288?

    - Andy.

I said to to the electrician who was also working at the house that it’s an EN, not a BS or a BS-EN, so we can ignore it now.

Is there now an equivalent BS?

I just answered my own question, yes there is.

standards.globalspec.com/.../BS EN 61000-3-12

The interesting feature of this standard is that it is completely unnecessary and foolish. LED lamps of the plain variety do not flicker with mains changes, they give a constant light output across the rated voltage range by design. Dimmable ones may flicker slightly with particular switching phases, but even these are designed to change brightness fairly slowly in cycle terms and one will probably not notice a slight change in brightness. Perhaps we are about to get get a new directive to change back to the horrible CFL lamps or even filament types?

As for these "magic" and rediculously small numbers, they were written by someone with no knowledge or experience of real world electrics, and rubber stamped by queues of others with no knowledge of anything! Such is the EU bureaucracy!

Presumably it starts with the IEC rather than an EN, the BS-EN is the next adoption and presumably is now a BS.

https://webstore.iec.ch/publication/69084

However it was devised, I cannot see it can be applied in a ground floor bathroom in a granny annexe on a house, the DNO network is not up to it.

So what is the answer, is it don’t fit electric instantaneous water heaters and showers in UK homes?

If it's a heater (so inrush not the issue), then probably it's thryristor controlled, meaning it can have rapid and repetitive changes of load, which gives a worry about supply quality. The  thyristor control of big heating loads tends not to chop the sinusoid shape (which would make strong harmonics, which are more regulated for big loads) but to take a selection of whole cycles or half cycles.  A load that could be tolerable when just on/off could be more disturbing when changing several times in a second. DNOs have long had criteria about impedance for avoiding annoying light changes from simple electric showers (on/off) switching. 
I've always perceived LED lamps as not changing so much as incandescents when the voltage changes, as David says.  They should be able to maintain light output, if designed for that, rather than for dimming. On the other hand I've read of there being some that respond more strongly to small variations. mapj1 mentioned cheap designs in the recent thread about flickering lamps and 220/240V.  Perhaps I've been lucky in getting good quality ones.

If the standard's concern is about effects on other customers in the network, rather than self-inflicted effects in the disturbing-equipment-owner's own installation, then logically the place for the low impedance to be checked is the point of common coupling to other customers, i.e. out on the street main (unless there's a looped service).  There, it's more plausible to fulfil the requirement if it's a biggish net. But you can't easily measure there. Not unless you get help from a neighbour on the right phase, and an extension lead from them onto a voltmeter, and a big load switching several times.