Plug-in RCD question

[Vicario]

I have one of those three-pin plugs that serve as an RCD. It plugs into the socket and then you plug in at the back. I use it for the lawnmower/hedge-trimmer outside. When I press the test button before use, it trips on one of the sockets I use, but not the other. The earth wire in the socket is connected OK, so what else can I try short of replacing it with a dedicated RCD socket? SOD's law, of course; the dodgy socket is the only one conveniently near the garden door. Are these devices just as good as the ones built into some sockets?

 

[JohnD]

RCD test buttons usually work by creating a fault current, and will work even if there is no earth present. They sometimes have an additional electronic circuit. It sounds like your RCD is faulty. In any case it is not reliable. Buy a new one. If the new one doesn't work either your socket must be faulty but I don't know what the fault would be. It would be worth verifying the the Line and Neutral are not mis-connected. Check other sockets too as the circuit might be faulty.

RCD adaptors seem to me to be less reliable than RCD sockets, which seem less reliable than RCDs in Consumer units. I do not have any substantial statistics to prove it though.

All RCDs are capable of failing, and they fail to trip most often if they are not regularly tested. We had some test survey results published here a while ago.

 

[ericmark]

Normally even with no earth the test button will operate the device the test but is only to test the physical operation in most cases.
Having said that it is possible some circuitry is included to test the earth connection although I have not as yet seen one with it in and one wonders if it could be socket not RCD at fault.
There is in theory, no difference between built in and plug in types other than the current they can handle. Sockets may be 26 amp if double.
Try plugging into extension lead then plugging lead into the two sockets and testing if it works the same in both sockets then the socket it does not work in must be forcing the pins so stopping it working which is only thing I can think of which could affect how it works in different outlets I will watch with interest for the outcome.
Eric

 

[sm1thson]

If it repeatedly works on other sockets but not the one in question i would say there is more likely a socket /wiring fault. if you just buy another RCD adaptor it may work in a different way to the one you have an mask a faulty socket (if it is faulty).

edit:

it is possible some circuitry is included to test the earth connection although I have not as yet seen one with it in and one wonders if it could be socket not RCD at fault.

my parents had an old power breaker one that tested for earth and thus alerted them to the lack of earthing in the garage, not saying this is the same, just that designs do vary, such as active, passive and i believe some types with the functional earth do send the test current down it (there was a post ages back where someone had a problem as he used one of these on a portable distribution board).

 

[Vicario]

Thanks for all that. I'll buy a new one and see if that works OK in the socket in question. Can I buy one that tests the earth connection? I always thought that an RCD test detected a leakage to earth, so I've learned something new today.

 

[Vicario]

Sorry for the double post, but I've just had a thought (on a Friday too!!). Presumably the RCD on my electric shower circuit in the CU works by detecting a leak to earth, so what's the point of the test button there? Would it fail to work if the earth connection was dodgy? I had no idea that the buttons were merely mechanical testing switches.

 

[JohnD]

they aren't

an rcd measures the current flowing through the P and N conductors, and compares them.

the test button puts a test current through a resistor. due to internal wiring, it only flows through one conductor through the detector. thus creating an imbalance. the RCD detects the difference in current through the two conductors, and causes a trip

you will find that the test button does not cause a trip if the RCD is not connected to the electrical supply. with no supply there can be no current, so no imbalance, so no trip.

 

[ban-all-sheds]

The earth wire in the socket is connected OK

That does not mean that the socket itself isn't faulty, nor does it mean that the "earth wire" is actually earthed...

You should get the socket checked out.

And try this - put a rewirable plug on an extension lead (if you don't have one with a non-moulded plug already), and temporarily disconnect the earth in the plug (and don't leave it until "another time" to reconnect it ), then see if the RCD works in the extension socket - if it doesn't work with the lead plugged in anywhere it's a fair bet that the test button generates a real earth fault.

 

[Vicario]

Thanks for all that. I'll go out and get a new one tomorrow (at work at the moment). Any recommendations for a plug-in RCD? Wouldn't mind something that tested the earthing as well if possible.
How often should I push the test button on my shower's RCD? Will it trip the MCB too? Don't like fiddling if everything's working OK though.

 

[JohnD]

once a month but quarterly at least, no.

 

[Adam_151]

Don't like fiddling if everything's working OK though.

Its not as much fiddleing, as making it do what it was designed to do, actually in reality the advice to test quarterly is not just to test it, but also to make sure the mechanism doesn't lock up due to inactivity

EDIT: Do please ensure that the shower is not in use at the time though