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

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