Cooker RCBO trips when any other RCBO tested?

HappyHippyDad · Jan 17, 2023

I will start with apologies, as I have not thought this through very much (at all) yet and have just sat down with a beer.... meaning I thought I'd ask the collective.

CU change today. I did my pre checks and I knew I had a major IR fault on the cooker circuit (N-E 0.3ohms, yes that figure is correct). The existing 30mA RCD was not tripping (have seen this before on a TT, @davesparks explained it to me and it makes sense). The RCD was fine, tripping within all parameters when all loads disconnected.

I realise I should have done the fault finding first, but I didn't. That will be tomorrow. Cooked circuit left disconnected for the night.

When I tested the RCBO (RCD test (x1 and x5 and ramp) ) on the cooker circuit by plugging in the tester at the socket on the cooker switch, the RCBO did not trip. When I RCD tested any other RCBO, the tested RCBO tripped and so did the cooker RCBO.

I'll track down the IR fault tomorrow, but can't get my head around why the above is happening?

Cheers in advance chaps (and SC and Julie)

Dave OCD · Jan 17, 2023

A difficult one to explain but I'm pretty certain it'll stop once you locate and fix the cooker circuit IR fault, then testing that RCBO should have no effect on other circuits or vice-versa. Good luck !

Lucien Nunes · Jan 17, 2023

As I understand your description, the cooker RCBO was connected to a circuit with an N-E fault, and it tripped simultaneously when any other RCBO was tested.

Traditional RCD test buttons (where there is no functional earth) test by transferring current from load L to supply N or vice versa, so they create an imbalance within themselves but not upstream, as no current actually goes to earth. Therefore, they won't interact with any other residual current sensing devices and would not cause this behaviour.

However, I think the test button in the RCBO you are testing dumps the test current to earth via its functional earth lead. As this is a TT system with significant Ra, instead of going to earth some of the test current diverts from the MET up the the cooker circuit CPC, through the fault, back down the cooker circuit N and through its RCBO to the supply neutral, causing an imbalance and tripping it. On a TN-C-S system this would not happen, as there would be a much lower resistance to earth from the MET, so the current would not divert via the cooker circuit.

It's kind-of the reverse situation to an RCD / RCBO trip on an N-E fault, when other circuits are carrying load. That causes the voltage on the neutral terminal to rise relative to the MET, due to the voltage drop along the main neutral, diverting current through the N-E fault. In your case the voltage on the MET is rising relative to the main neutral and the diverted current is flowing through the fault in the opposite direction.