Explain why the rcd doesn't trip initially ?

[alvina]

There’s a low insulation resistance fault between the neutral and CPC, the fault current flows through toroid, through circuit neutral, through the CPC to cutout then back to supply neutral. There’s no induced magnetic field so the RCD won’t manually trip whilst the fault is still present on the circuit.

[rowles]

Would be great to see a video on this fault🎉

[pavierthermostat]

A N-E short exist on the ring. As N & E are (almost) at the same potential, only a small offset current passes through the N side of the toroid - but not enough to trip the RCD. When the test resistor is applied across the L side of the toroid a trip current flows but it is not enough to cause a trip due to the small N-E fault current flowing back in the other side of the transformer…. The sense circuit doesn’t see enough of a difference to initiate a trip.

[andrewcadby]

N-E short on one of the circuits.
When the test button is pressed, I believe a resistor shorts connects the outgoing L to incoming N of the RCD. The current that flows through this route should create an imbalance enough to trip a working RCD.
However, with the N-E short on one of the circuits, a portion of that current will flow back through the N of the RCD, to the fault, and then back to the transformer via the earthing system.
The result: the RCD 'sees' less of an imbalance, so doesn't trip! Most likely a TN system, as would need a fairly good earth path back to the tranny to make a difference.

[ForidKhan]

My thinking:

There’s a N-E fault in this ring after the RCD. Causing a current path which bypasses the coil in the RCD. A very minimal current to no current flows in the coil so no trip.

[MartinE]

Surely the question should be why the apparent delayed test trip on the 2nd RCD and then an apparent immediate test trip on the 2nd RCD after you disconnected the neutrals yet you didn’t appear to reset the RCD between the two trips?

My best guess / assumption is the 2nd RCD is knackered, one pole is not disconnecting

[jamiebourne]

It's a good one this! I've experienced this before, discovered a very low NE reading on socket circuit (nicked pattress pin), sorted, then rcd worked fine. I can't, however, explain the science behind it....

[paulmatthews]

My guess would be the circuits are off so N-E fault keeping it at the same potential either side and RCD not seeing the imbalance because of it if that makes sense? Other than that I couldn’t say.

[Timetochange]

Earth to neutral fault. Most likely a screw caught neutral when socket screwed back.

[CazBag]

L-E and N-E fault on the circuit. Test current doesn't cause enough of an imbalance in RCD to trip it as some goes up the line and back down the neutral via the CPC downstream of the RCD?

[carlstanton]

I'd like a clear answer on this one when you can mate.i barely do any testing so I'm interested to know (my mate owns the test kits so he generally does all our testing).i assume it's a n-e fault of some sort though?

[retrozmachine]

The test button does not pass current to earth like a normal earth fault. It passes from the outgoing line and the incoming neutral. The outgoing neutral must be lower impedance than the incoming with the ring fault in place causing the neutral side of the test circuit to operate of the outgoing side keeping the sense transformer current balanced or at least in a situation where there isn't enough imbalance to trip. N/E fault on the ring and perhaps a high impedance neutral issue on the incoming side.

This is all educated guesswork, not had a whole heap of experience with RCDs and it took me quite a bit of thinking and staring at the symbolic diagram on the side of an RCD.

Couple of edits to clean up what I was saying. I think this question or similar has come up before.

[edmondhodaj]

Seen happening on both rcds, rcds up position but no power, never understood why,

[RAS_StageWinner]

Would the fault still occur in the same way regardless of the earthing system of the installation, TT, TNS, TNCS?

[chrisjoe]

I’ve never come across this actually 🤷🏻‍♂️ I’m looking at getting into some EICR inspections. Not really to charge for it but basically just for extra knowledge. Do you or anyone out there know of any source of a possible ladder “ yes / no “ question system. Similar to what they have on fault finding in machinery and boilers. Or if there a training program or courses specifically for EICR .
Thank you for your time to read this . Thank you in advance. 👍🏻

[alip]

L-E fault, neutral doesnt have enough leakage current for the RCD to trip being electromagnetic. When neutral removed, by pressing the test button you simulate the fault for the RCD to function.

[ahhhh]

Always wondered..if a circuit is switched off and you touch neutral and earth together, why does it trip the rcd?

[damiandrozdek]

Because u have L in mcb witch is connected to second rcd ?

[JonElectron]

Test button current flowing through toroid, through circuit neutral, down CPC to cutout then back to supply neutral. No magnetic field induced so no trip

[SULWRS]

Neutral to earth fault on that ring circuit