LCD TV tripping the RCD - any ideas

[TeeKay]

What else is on that ring? Outside lights, pond pump, computers, etc etc???

There really is nothing connected - Honest.

I'm now coming to the conclusion that the TV is injecting something into the ring which results in the RCD detecting an inbalance. I don't know what it could be but I think having a higher impedance ring could aggravate it, and as much as anything else the alternatives now make less sense.

I don't have an IR tester but it doesn't make sense that the insulation would suddenly breakdown on both halves of the ring.

My money is now on the TV being the culprit, though trying to persuade Toshiba to change the PSU is going to be interesting.

 

[TeeKay]

Are all the rings protected by that RCD?

If your Fluke has an IR tester, use it (having isolated everything on that circuit) to test IR. Start with L/E & N/E so as not to fry anything you may have forgotten.

Yep - all four rings protected by the same RCD. Unfortunately I don't have an IR tester but I measured the DC resistance between L/N and and it is open circuit. The Earth and Neutral are as good as a short circuit.

But remember if I split the ring in half in the CB and at a socket and then try the TV it still trips the RCD whichever side of the ring I connect. The probability of it being insulation breakdown has to be low.

 

[flameport]

The Earth and Neutral are as good as a short circuit.

Was that measured between E/N with the wires disconnected from the consumer unit?

 

[TeeKay]

The Earth and Neutral are as good as a short circuit.

Was that measured between E/N with the wires disconnected from the consumer unit?

nope - sorry that is connected. disconnected it is O/C

 

[securespark]

You say you are convinced the TV is the culprit, but then you said earlier that it does not trip on all the rings, only one. If this is the case, a faulty TV is unlikely.

 

[sparkyspike]

I then just reconnected one side of the ring in the consumer box and chose a socket about midway round the circuit and similarly split the circuit.

Did you disconnect live, neutral and earth on one side of the ring, or just live?

 

[sparkyspike]

The Earth and Neutral are as good as a short circuit.

Was that measured between E/N with the wires disconnected from the consumer unit?

nope - sorry that is connected. disconnected it is O/C

They shouldn't be short circuit, unless you're measuring with the main isolator still on!

 

[TeeKay]

Thanks to all of you especiallySparkyspike, Securespark and Flameport for their last few posts which had me look in more detail at the Earth/Neutral resistance.

It never occured to me that the fault could be leakage between earth and neutral but that was what it was.

I split the ring again but this time not just the live but also the neutral. This time one half of the ring was OK when I connected it and plugged in the TV, but the other half tripped.

After a bit more investigation I realised that on the tripping half of the ring there was about 2.5Meg of (DC) leakage between earth and neutral which I'd previously ignored

Well I traced that to the shed cable which comes off of a fused spur off this ring then thru' armoured cable to an RCD socket in the shed. I'd pulled out the fuse on the spur to rule it out, but it never occured to me that a neutral/earth leakage could cause this problem.

Anyway, have now disconnected the neutral on spur and all is working just fine.

If anyone can explain this I'd be interested. Thanks again to all for their posts.

 

[TeeKay]

The Earth and Neutral are as good as a short circuit.

Was that measured between E/N with the wires disconnected from the consumer unit?

nope - sorry that is connected. disconnected it is O/C

They shouldn't be short circuit, unless you're measuring with the main isolator still on!

Yes I was. But they are nominally at the same potential right? Why not measure this with the main isolator still on?

 

[flameport]

L and E are at the same potential as they are connected together either in the supplier cutout, or further back at the substation transformer. Measuring between them with the main switch on is therefore useless. Further, measuring resistances with the main switch on and wires connected does give the possibility of unwanted (even dangerous) voltages appearing on the circuit you are testing.

The L/E fault will cause tripping when a load is applied, as some of the current will return via the earth wire instead of the neutral. This will cause an imbalance between L and N, which the RCD will detect.
The actual percentage of current flowing in the normal neutral and the fault will depend on the resistance of the various cables involved and the fault itself.