RCD for whole install tripping

[Welsh_sparky_21]

Hi, thought I’d ask in here and hopefully someone with more fruit finding experience than me can help. Customer saying Rcd tripping and they believe it was the sockets, as the socket mcb tripped once when it happened. Now I had unplugged everything that I could see, couldn’t see anything else at all plugged in Or any FCUs on this circuit but was getting a reading of 0.00 between live and neutral, when I joined the live and neutral together and tested between this and earth I was getting >999. Nothing tripped the whole time I was there. It was in a care home so it’s hard to establish if there is something else being fed from this circuit, the reading I was getting obviously indicates this

 

[timhoward]

I'd actually start with everything plugged in, and the L+N > E test. You might have unplugged the problem?

 

[pc1966]

Seeing as low as 0.00 on a Megger is not so meaningful, as anything below 10k ohm would read that so it could be a fridge plugged in, doorbell transformer on FCU, defrost heater somewhere, or similar and you are getting a few hundred ohms of motor coil.

While it is possible a L-N fault also managed to trip the RCD if it originally involved all 3 conductors, it is more likely the fault was in some appliance as @timhoward suggested.

But...if everything really is out, and there are no dodgy USB sockets or wifi repeater sockets in use, then it would suggest something deeper is wrong.

 

[brianmoooore]

First rule of fault finding is to disturb as little as absolutely possible before making initial tests, especially IR tests from L&N to E.

 

[Welsh_sparky_21]

I'd actually start with everything plugged in, and the L+N > E test. You might have unplugged the problem?

Didn’t even think of it from that point of view. Find it hard trying to find an intermittent fault, least in a house you have a good chance of finding everything being fed off that circuit

 

[timhoward]

I'm also thinking that a care-home having an RCD covering the whole install isn't very ideal, was it TT earthing?

 

[pc1966]

Agreed, a single RCD is quite an old installation. Either for shock protection, or an incomer for TT (where it ought not to be the only RCD if 100mA delay...)

 

[Welsh_sparky_21]

I'm also thinking that a care-home having an RCD covering the whole install isn't very ideal, was it TT

Agreed, a single RCD is quite an old installation. Either for shock protection, or an incomer for TT (where it ought not to be the only RCD if 100mA delay...)

Its not for the whole install, it’s split into a few dbs, that one DB just feeds a small section of the carehome

 

[littlespark]

thought I’d ask in here and hopefully someone with more fruit finding experience

Fruit finding?

Oh look, a strawberry!


Could it be cumulative earth leakage if it’s an rcd mainswitch? LED fittings and sensors in the lights perhaps? Even emergency lights?
Something on the sockets is just pushing it over the edge.

Have you done a ramp test on RCD?

 

[timhoward]

A ramp test or high resolution clamp meter is a certainly a good place to start.
If nothing else it tells you how likely the fault is to occur ‘right now’.
If you then disconnect the outgoing N, turn off all breakers, and test RCD normally and then ramp test again then you can prove the RCD itself is fine too.
After that IR test each circuit L+N to E.
If nothing shows up after all that make a cup of tea and think for 5 minutes!

 

[Rockingit]

Boil it down, it's one of three options:

1) A faulty appliance / device;
2) Faulty wiring ('faulty' in this context could also include poor original circuit design);
3) A faulty RCD.

The fact that you believe an over-current device also activated at the same time would suggest that it's unlikely to be (3). You can break apart the circuit in question at the outlets, a chunk at a time, and do continuity and IR tests to search to see if you still have a (1) in place and at the same time it will deal with (2).

(ps..... I'll start a sweepstake that says it's a kettle....)

 

[timhoward]

(ps..... I'll start a sweepstake that says it's a kettle....)

Agreed - and that's actually why I suggested making a cuppa if nothing jumped out
My old boss used to say if you can't find an intermittent fault in an occupied building, it's always worth making a cuppa and leaving the fridge door open when you get the milk out. Thus activating the fridge cooling and the kettle, two likely contenders! And if it wasn't them at least you are more likely to think straight for having stopped for 5 minutes.

 

[James]

Another likely contender for the known facts is a mouse.

an IR test would confirm

 

[brianmoooore]

(ps..... I'll start a sweepstake that says it's a kettle....)

Most kettles are cordless now, but back when they were corded, I traced many tripping faults to the triangular type plug (not the older round type) at the kettle end.
There would be carbon tracking along the moulding marks on the front face of the plug, between the live socket and either the earth or neutral. Could be seen on the white plugs, but not so obvious on the black.
While on the subject of kettles, how on earth are the standard cordless kettle bases considered safe? It's upward facing live connectors in a damp environment.

 

[pc1966]

While on the subject of kettles, how on earth are the standard cordless kettle bases considered safe? It's upward facing live connectors in a damp environment.

They never seem to last long either. A friend of mine has the obnoxious habit of lifting the kettle off/on before it has finished boiling and thus drawing current! Don't think that helps it much...

 

[brianmoooore]

They never seem to last long either. A friend of mine has the obnoxious habit of lifting the kettle off/on before it has finished boiling and thus drawing current! Don't think that helps it much...

Absolutely guilty of that, but the switch moves to off as you lift it. Don't know whether the switch breaks before the socket for sure, but I've never seen evidence of an arc.
Never had a problem with one of these bases either, in spite of my reservations about their design. Of course, now that I'm 'retired', I don't come into contact with other peoples' problems any more, just the five kettles under my jurisdiction.

 

[pc1966]

I get, el chepo Tesco white one, as lasts as long as fancy Italian named one (also made in China) at 4 times the price. They don't cut out on lifting, unless you deliberately poke the switch, and they resume boiling when put back.

Nasty design