well the test button needs to introduce an imbalance and does so by passing a current from something on the load side that came through the sensing coils, back to the supply side without going back  through the sensing coils. (that is after all what happens in a fault too, it is just that the return path  is via the earthing cables or via a person and the general mass of terra-firma earth neither of which involve a return path through the sensing coils either.)

Ideally at least on the supply side neutral should be connected even if the load side is not needed, as it will confuse the unwary trying to test it. It is not dangerous as such, but it is a bit silly. Personally I would never leave it that way. There are special 3 pole RCDS made for the situation where there is no supply side neutral available, so if that is the case, then perhaps  one of those would have been a better choice.

You may be wondering why the test button does not return to earth, but then imagine what happens if there  is an RCD protected final circuit fed by RCD protected distribution - now the far end test button trips both, as a fault may do, but a test button really should only test its own RCD.
Mike

it looks like the test button is connected between N and E

That would be unusual I think (not to mention unreliable, as the N-PE voltage tends to be somewhat variable) - normally the test button (and appropriate resistor for 230V) is connected between one of the lines and N on the opposite side. If no N is present the N terminal can then be connected to either one of the other two lines via an additional external resistor (making the overall suitable for 400V).

I'd maybe check the RCD has actually been connected as the manufacturer intended...

   - Andy.

oops yes missed that, normally expect test to be L1-N or L1-L2 but crossed over the sensing coils. Apologies
M.

Just a thought, could you connect your own test button and a suitable resistor between two of the phases. Leaves what the manufacturer intended intact but allows you a test facility in that situation. Warning - only to be designed and assembled by someone with sufficient knowledge.

Thanks for the replies.

I found this from Schneider https://ckm-content.se.com/ckmContent/sfc/servlet.shepherd/document/download/0691H00000FJoVlQAL

States a 4W 2200ohm resistor between L2 and N on the outgoing side to make the test button work

Yes, basically what that`s showing is A/ if there is a N connected then use it or B/ If N is not connected then link another phase to it - not forgetting to disconnect that link if you later supply an actual N connection to it. 

thanks all, as always its helpful to discuss things so I understand whats happening properly!

It is a pretty poor show if the installer does not know how to do this though. The resistor is scarcely expensive though two 3watt 4k7s in parallel or 2 off 1k resistors in series may be easier to find in this time of random electronics component shortages.

Even so an example of something suitable here ; postage and £5 minimum order qty or whatever it is these days are probably the main costs.

Mike

mapj1 said:

It is a pretty poor show if the installer does not know how to do this though.

Wonder how it passed initial verification, as it's been a requirement of BS 7671 for some time to check the test button is working as part of initial verification?

I imagine he or she forgot and managed to tick the paperwork later from the comfort of the bar of the Dog and Duck, or some similar location.

The gap between the  "regs world" and what happens 'in the field' can be huge.

My ageing parents had their kitchen re-done a few years ago now,  and due to work, I was not about to go and supervise/ be a pain to the contractors.
I did manage to mention to my dad to ask for the test results and certs from the sparks, as the moving of a few sockets suddenly developed a need for a new CU to replace the one I had been merrily adding stuff to for years  (heard that before ?)  . Well it had all been switched on for a week or two when they came back with another chap to do a load of 'extra tests' specially for the 'extra certificate' he had requested.

My strong suspicions it that they had never been asked before, probably never bothered before, and were a bit thrown by it.

Quite a neat job though, and in the end the paperwork matched the installation and looked OK apart from  the dates.

Still waiting for the plumber who came with that team to register the replacement boiler he fitted though - and in the mean time I have redone about 90% of his wiring.  ...  :-)

meanwhile the company that oversaw and did the kitchen have folded.
Mike.