RCD's Misbehaving

[Sparkz123]

Hi,

I have a problem with 17t edition consumer unit (MK Sentry) on a TT system that has completely left me baffled. I'd be very grateful for an advice.

On the left hand side of the board I have a 100 amp main switch. Immediately next to that a 30ma RCD controlling 3 MCB's and 2 spare ways, followed by a second MCB controlling 4 MCB's and one spare.

If I test the3 left hand circuits the L/H RCD fails to operate, it also fails to operate when tested from the board. If I test any of the circuits controlled by the R/H RCD both RCD's trip. If I swap the RCDs around nothing changes, the L/H one fails to operate unless the R/H one is tested.

All my neutrals are connected to the correct connector blocks and there are no IR readings between the two blocks.

To leave the entire installation RCD protected overnight I swapped the main switch and L/H RCD's around, so the main switch was an RCD, but this failed to operate, even though it was fine when controlling the L/H circuits.

I'd appreciate any input.

Thanks,

Pete.

 

[Pete999]

Shared neutral on the landing light 2 way?

 

[Farmelectrics]

think pete could be right quick fix try putting all lights on one rcd and see if it stops tripping

 

[Pete999]

think pete could be right quick fix try putting all lights on one rcd and see if it stops tripping

Let me know will you please

 

[Octopus]

So is this an existing set up or one you have changed?

Need a bit more info me thinks!

 

[VoltzElectrical]

Surely if there was a shared neutral, the RCD WOULD trip, rather than FAIL to trip?

 

[R G]

Disconnect all circuits from the board and carry out RCD tests with prods , see what happens , if all works correctly for both RCDs then its an issue with your wiring , or faulty RCDs ,

 

[VoltzElectrical]

It may or may not be relevant here, but I recently installed an MK Sentry dual RCD board, where one RCD failed to trip on test. I had to leave the installation covered by the one good one as it was getting dark.

 

[telectrix]

It may or may not be relevant here, but I recently installed an MK Sentry dual RCD board, where one RCD failed to trip on test. I had to leave the installation covered by the one good one as it was getting dark.

probably decided to trip at last and take the lights out.

 

[tony mc]

Any chance of a picture of the CU close up I take it you have checked and double checked that its connected up correctly !!!!

Just out of interest what is your RA at the board ?

 

[Sparkz123]

It may or may not be relevant here, but I recently installed an MK Sentry dual RCD board, where one RCD failed to trip on test. I had to leave the installation covered by the one good one as it was getting dark.

That seems a bit of a coincidence, what was the outcome? Was it a faulty RCD or something else?

I'm confident that there are no shared neutrals, I have tested between the 2 neutral blocks and the readings were clear. It's not a new installation, I've just replaced an old board covered by a voltage operated trip. I'll disconnect all the circuits tomorrow and see what happens but I can't think of a wiring fault that could cause this. If it fails to work properly once the circuits are disconnected it's got to be a fault on the boards internal wiring, but God knows what.

Thanks everyone for the input.

 

[Sparkz123]

I'll get a picture tomorrow Tony. I don't recall the exact rA but it was nothing out of the ordinay, I'll make a note of it when I certify it

 

[VoltzElectrical]

That seems a bit of a coincidence, what was the outcome? Was it a faulty RCD or something else?

I'm confident that there are no shared neutrals, I have tested between the 2 neutral blocks and the readings were clear. It's not a new installation, I've just replaced an old board covered by a voltage operated trip. I'll disconnect all the circuits tomorrow and see what happens but I can't think of a wiring fault that could cause this. If it fails to work properly once the circuits are disconnected it's got to be a fault on the boards internal wiring, but God knows what.

Thanks everyone for the input.

Same thing, replaced board in brother in law's house, up front VO trip and re-wireable board, two new rings to give adequate sockets outlets in bedrooms and lounge, also to overcome lose of earth in lounge sockets. Come to test and one RCD won't trip. Faulty RCD. Also the mcbs wouldn't sit straight, and the factory wired neutral links came loose from the crimped lugs.

Horrible board, won't ever use another through choice.

 

[telectrix]

Same thing, replaced board in brother in law's house, up front VO trip and re-wireable board, two new rings to give adequate sockets outlets in bedrooms and lounge, also to overcome lose of earth in lounge sockets. Come to test and one RCD won't trip. Faulty RCD. Also the mcbs wouldn't sit straight, and the factory wired neutral links came loose from the crimped lugs.

Horrible board, won't ever use another through choice.

was that a MK board by any chance?

 

[7029 dave]

The last mk sentry board I fitted was 1980's.

 

[VoltzElectrical]

was that a MK board by any chance?

Yes, MK Sentry.

Sorry, I thought I made that clear. I've actually posted about the shortcomings of this once noble brand a few times now.

 

[Lucien Nunes]

An RCD that fails to trip by its own test button or an MFT while installed but works OK on the bench or with all circuits disconnected, usually indicates an N/E fault on the installation downstream. This causes the test button current or MFT test current to 'miss' the N side of the RCD by running to earth. In your case it suggests a fault on the L/H group of circuits, which I recommend re-testing.

I'm guessing the L/H RCD trips when using the MFT to test the R/H one but not with the test button? If so, this corroborates it as an N/E fault, as the R/H test button doesn't cause current to flow in the CPC and hence doesn't influence the L/H one. But the MFT dumps the test current to the CPC, some of which ends up returning via the L/H RCD neutral and tripping it.

 

[R G]

The old LN sentry range from the 80s and 90s were good , but this new stuff is not , the sockets get me , has anyone noticed how tight the terminal screw threads are , a customer had some for me to fit for him ...

 

[rich.250]

High integrity board by any chance?? I've seen them come wired wrong. Outgoing neutrals in wrong bars etc.

Have you got any dead shorts neutral to earth on the connected circuits? Could prevent the rcd from operating when testing.