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.

 

[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.