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For whatever reason, test buttons seem to have more than their fair share of intermittent contacts even factoring in how infrequently they are used. The test button circuit is independent of the RCD proper, so an RCD with an inoperative test button that tests fine on the MFT is not necessarily less reliable than one with a working test button, unless some physical factor like corrosion or dirt ingress is the cause (in which case the rest of the RCD could very well fail prematurely). But as pointed out above, not only does the test button test the RCD, if used as prescribed it also provides some exercise, which can avoid sticky tripping mechanisms. Therefore, by preventing the exercise taking place, the faulty test button could (in theory, if not in practice) lead to impaired safety, not just lack of proof of that safety. That said, I see the logic in the OP and would also like to be able to code it C2.5.
We've had a discussion about wiring faults that disable the test button in the past. There are multiple ways of implementing a test button circuit on a normal 2-pole RCD. If the schematics on the front of bog-standard 30mA are to be believed, a resistor between the line on one side and the neutral on the other, creating an imbalance without actually leaking current to earth, is the most common. Obviously there are two ways round this could be done; a) Supply line to load neutral or b) Load line to supply neutral. Considering the case of an N-E fault downstream of the RCD with no circulating current, configuration 'a' would be de-sensitised by the fault, since some of the test current would go from supply L to E, never passing through the toroid. Arrangement b) does not suffer from this problem since hard L-E shorts don't persist on a live circuit in the same way as hard N-E shorts. Therefore it would seem to be the better of the two, but IIRC we found both in practice, which would negate any requirement for L & N to be a particular way round unless the contacts themselves were sequenced.
Another way of implementing the test button is with a separate winding on the toroid, energised from L & N of the same side. The same MMF is created in the toroid as with the intended imbalance in the main conductors, although as the test coil can have more turns, the actual current can be lower. This makes it a more suitable method for RCDs with high IΔn values and renders the test function completely indifferent to polarity.
But, and it's a big but, the point PC1966 makes about the relative phasing of any current circulating via an N-E fault current more or less overrides any other considerations. Since this can pass through the N conductor in the toroid in either direction, increasing or decreasing the net imbalance when the device is tested, the test button threshold will always be subject to an error up to IΔn when an N-E fault is present, regardless of how the test circuit is implemented.
But until someone makes an RCD with a built-in finger that pops out like a cuckoo from a clock to press the test button exactly on cue, I don't suppose any of this matters very much.
We've had a discussion about wiring faults that disable the test button in the past. There are multiple ways of implementing a test button circuit on a normal 2-pole RCD. If the schematics on the front of bog-standard 30mA are to be believed, a resistor between the line on one side and the neutral on the other, creating an imbalance without actually leaking current to earth, is the most common. Obviously there are two ways round this could be done; a) Supply line to load neutral or b) Load line to supply neutral. Considering the case of an N-E fault downstream of the RCD with no circulating current, configuration 'a' would be de-sensitised by the fault, since some of the test current would go from supply L to E, never passing through the toroid. Arrangement b) does not suffer from this problem since hard L-E shorts don't persist on a live circuit in the same way as hard N-E shorts. Therefore it would seem to be the better of the two, but IIRC we found both in practice, which would negate any requirement for L & N to be a particular way round unless the contacts themselves were sequenced.
Another way of implementing the test button is with a separate winding on the toroid, energised from L & N of the same side. The same MMF is created in the toroid as with the intended imbalance in the main conductors, although as the test coil can have more turns, the actual current can be lower. This makes it a more suitable method for RCDs with high IΔn values and renders the test function completely indifferent to polarity.
But, and it's a big but, the point PC1966 makes about the relative phasing of any current circulating via an N-E fault current more or less overrides any other considerations. Since this can pass through the N conductor in the toroid in either direction, increasing or decreasing the net imbalance when the device is tested, the test button threshold will always be subject to an error up to IΔn when an N-E fault is present, regardless of how the test circuit is implemented.
But until someone makes an RCD with a built-in finger that pops out like a cuckoo from a clock to press the test button exactly on cue, I don't suppose any of this matters very much.
? as it's clearly the same rcd both visually and by your paperwork I'd just grab one off that site
