Very strange fault

[RF Lighting]

You seem to have ruled out all the usual suspectls like loose connections, borrowed neutrals etc, so it must be something more obscure, so I may go off on a bit of a tangent here

You say the supply is T-N-CS. Is the network definatly PME'd? We have seen a few posts on here recently about DIY conversions from TT.

Have you tried taking a voltage reading between the installation earth and true earth? I'm wondering if there might be a failing joint on the neutral somewhere locally on the network?

I had an intermittent RCD trip fault a while ago. It was sometimes at random, and sometimes co-incided with a light being switched.

I took an incredibly poor quality video on my phone showing what I tracked the fault down to

 

[ColJack]

TRANSIENT EARTH LEAKAGE CURRENTS
All Wylex residual current devices incorporate a high level of immunity
to tripping when subjected to transient earth leakage currents.
Such transients can occur when there is a significant level of
capacitance to earth as can result from cable capacitance (particularly
MICC) or RF filter networks. Wylex RCDs are therefore less
susceptible to nuisance tripping due to transient earth leakage
currents.

sounds like a faulty filter on the RCD...

 

[GaryMo]

You say the supply is T-N-CS. Is the network definatly PME'd? We have seen a few posts on here recently about DIY conversions from TT.

Have you tried taking a voltage reading between the installation earth and true earth? I'm wondering if there might be a failing joint on the neutral somewhere locally on the network?

The supply was installed by the DNO around 2 years ago.
Haven't taken a voltage reading though - will try that if I'm sent back.

 

[GaryMo]

TRANSIENT EARTH LEAKAGE CURRENTS
All Wylex residual current devices incorporate a high level of immunity
to tripping when subjected to transient earth leakage currents.
Such transients can occur when there is a significant level of
capacitance to earth as can result from cable capacitance (particularly
MICC) or RF filter networks. Wylex RCDs are therefore less
susceptible to nuisance tripping due to transient earth leakage
currents.

sounds like a faulty filter on the RCD...

Cheers Col - will also take another RCD if I return.

 

[securespark]

In your case Gary, I don't think this is the problem, but I had a fault on a TN-C-S install a couple of years ago. It was a split board change & the RCD tripped when an outside light was energised.

It was nothing to do with the outside light though....there was an intermittent NE fault on an RF circuit.

To make you feel even better, a colleague was chatting to me yesterday about how he had IR an installation top to bottom & everything was fine, yet upon energising the circuit, the RCD tripped almost instantaneously.

Turned out that when powered down, the circuit IR'd OK, but when energised, a fault appeared!!

 

[bernardgreen]

In front of me I have a circuit of a piece of industrial sensor equipment with a voltage free relay output to switch external circuits.

There is a 2.2 Megohm resistor between the relay contact common and the casing. I have seen similar to this before where the resistor is intended to leak static charge away from the external circuit. Perfect for installs where the relay is not switching mains and/or there no earth leakage detection involved.

Turned out that when powered down, the circuit IR'd OK, but when energised, a fault appeared!!

Did he find the fault ? I found some PIR's operate their relay for a very short time just after power up. About a tenth of a second or thereabouts and therefore not long enough for the lamp to visibly light up. Hence a cold low resistance lamp across the supply leading to a short duration inrush current. With several lamps doing that at the same time the inrush would be very high.

 

[securespark]

Yeah, it was some kind of outside lighting, either D2D or passives, I can't remember which.

 

[GaryMo]

I forgot all about this thread.

ColJack was correct, it was indeed a faulty RCD causing the problem even though it tripped within required times.