How can I tell if an RCD in the consumer unit is faulty

[oldbutnotdead]

Annoying, can't find mine now but it had similar functionality to this one Test plug - doesn't do full testing by any means but better than nowt.

 

[ericmark]

A plug in tester can test many things.
1) Is the neutral same voltage as earth, this will show if line - neutral swapped.
2) Is the voltage between 207 and 253
3) Is the loop impedance low enough.
4) Will the RCD trip at 30 mA.
5) Will the RCD trip in 40 mS.

The RCD can be seen to work by pressing the built in test button. However with the voltage allowed to vary 207 to 253 with a simple resistor at 210 volt it needs 7000 ohm if I am doing my calculations right, but 6000 ohm at 240 volt. So it can only give a rough result. The one built into a 10 mA RCD socket (MK) would trip the main 100 mA RCD so clearly not really a very good test.

As to loop impedance most cheap plug in units start at 1.9 ohm but with a ring final the pass mark is below 1.4 ohm.

So the big question is how often does it need testing, if we think once every 10 years they answer is to hire for a day, if being used each time caravan or motor home plugged in then since TT not TN-C-S then 60 ohm is good enough.

And with two RCD's in series unlikely both will fail.

 

[JohnW2]

The RCD can be seen to work by pressing the built in test button. However with the voltage allowed to vary 207 to 253 with a simple resistor at 210 volt it needs 7000 ohm if I am doing my calculations right, but 6000 ohm at 240 volt. So it can only give a rough result.

We've discussed this before, and it seems that the situation is probably a bit 'worse' than you suggest - since those of us who have looked inside RCDs have found that the resistor value is usually such as to create an L-N imbalance roughly double the IΔn (i.e. about 50-60 mA for a "30 mA" RCD).

Kind Regards, John

 

[ericmark]

We've discussed this before, and it seems that the situation is probably a bit 'worse' than you suggest - since those of us who have looked inside RCDs have found that the resistor value is usually such as to create an L-N imbalance roughly double the IΔn (i.e. about 50-60 mA for a "30 mA" RCD).

Kind Regards, John

As an electrician I have never had a cheap plug in tester, looking at The Socket & See SOK 36 data sheet says it tests a 30 mA RCD but no details, it says it uses less than 18 mA and should not be used for more than 2 minutes, but nothing to say how much current it draws when the RCD test button is pressed.

 

[JohnW2]

As an electrician I have never had a cheap plug in tester, looking at The Socket & See SOK 36 data sheet says it tests a 30 mA RCD but no details, it says it uses less than 18 mA and should not be used for more than 2 minutes, but nothing to say how much current it draws when the RCD test button is pressed.

I haven't got a clue as to what residual current a plug-in tester uses to test an RCD - but, as I said, it seems that the test buttons on the RCDs themselves use a current appreciably greater than the IΔn of the device.

Kind Regards, John