Type A R.C.D. 6mA tolerant.

All this talk about RCDs is bamboozling me. ?

I think that I may have been misled by my VFD which tripped a type AC RCD, but not a type A one because it failed subsequently and the warranty replacement is compatible with my SRCD.

As I understand it, there is a hierarchy:

Type AC: trips on ordinary AC, which is all you have in a traditional filament light, one-speed hoover, electric heater, etc.

Type A: trips on ordinary AC and rectified AC = pulsating DC, so if there is a diode anywhere, a fault downstream of it would be detected whereas a type AC would (probably) not trip.

Type F: trips on both of the above and high frequency leakage such as may be found in the synthetic waveform at the output of a VFD. So now we are getting further away from the supply and it seems to me that any fault is unlikely to be of “negligible impedance”.

Type B: trips on all of the above plus genuine smooth DC. This requires a DC circuit to be in proximity to the mains one so that a fault may develop between them such as in an EV lead.

“Blinding" is more like getting the sun in one's eyes - the FCD can still ”see", but has to make a little more effort.

So taking, for example, my chum's new boiler, type A is recommended because of the possibility that a fault in the electronics might make the case live, but with insufficient impedance to trip an MCB. 134.1.1 of BS 7671 simply requires us to “take account of manufacturers' instructions” and the only requirement of RCDs in 415.1.1 is that they have a “rated residual operating current not exceeding 30 mA”. So a judgement might be made that substituting a type A RCD for a type AC one is disproportionate and the installation would comply.

If I have got the wrong end of the stick, please say so!

All this talk about RCDs is bamboozling me. ?

I think that I may have been misled by my VFD which tripped a type AC RCD, but not a type A one because it failed subsequently and the warranty replacement is compatible with my SRCD.

As I understand it, there is a hierarchy:

Type AC: trips on ordinary AC, which is all you have in a traditional filament light, one-speed hoover, electric heater, etc.

Type A: trips on ordinary AC and rectified AC = pulsating DC, so if there is a diode anywhere, a fault downstream of it would be detected whereas a type AC would (probably) not trip.

Type F: trips on both of the above and high frequency leakage such as may be found in the synthetic waveform at the output of a VFD. So now we are getting further away from the supply and it seems to me that any fault is unlikely to be of “negligible impedance”.

Type B: trips on all of the above plus genuine smooth DC. This requires a DC circuit to be in proximity to the mains one so that a fault may develop between them such as in an EV lead.

“Blinding" is more like getting the sun in one's eyes - the FCD can still ”see", but has to make a little more effort.

So taking, for example, my chum's new boiler, type A is recommended because of the possibility that a fault in the electronics might make the case live, but with insufficient impedance to trip an MCB. 134.1.1 of BS 7671 simply requires us to “take account of manufacturers' instructions” and the only requirement of RCDs in 415.1.1 is that they have a “rated residual operating current not exceeding 30 mA”. So a judgement might be made that substituting a type A RCD for a type AC one is disproportionate and the installation would comply.

If I have got the wrong end of the stick, please say so!