UK RCBO Testing

JXYM89 · Apr 22, 2024

Just wondering if someone could help provide an explanation as to why this might be the case when testing an RCBO. Testing the RCBO at the DB recorded a value of >300ms (test probes connected N&L load side, and E bar), yet when running the same test at a socket-outlet of the same RCBO tripped at <40ms?

westward10 · Apr 22, 2024

So it didn't trip at the board but it does at the socket?

mhar · Apr 22, 2024

Make of rcbo? Some (MEM?) were known to not trip when testing at their output terminals but attach even a modest length of conductor and test from that and they behaved.

littlespark · Apr 22, 2024

Grime on the terminal screws not making a good connection?

Fault with the probe lead, but not the lead with plug fitted?

westward10 · Apr 22, 2024

mhar said:
Make of rcbo? Some (MEM?) were known to not trip when testing at their output terminals but attach even a modest length of conductor and test from that and they behaved.

Yes Memshield 2.

JXYM89 · Apr 22, 2024

Thanks all for responding to my message.

mhar said:
Make of rcbo? Some (MEM?) were known to not trip when testing at their output terminals but attach even a modest length of conductor and test from that and they behaved.

I can confirm that it was an Eaton RCBO/MEM TP-DB. Out of interest, are there other brands of RCBO that this occurrence is common? - or is Eaton/MEM particularly renowned for not tripping when under test at the breaker?

littlespark said:
Grime on the terminal screws not making a good connection?

Fault with the proketbe lead, but not the lead with plug fitted?

Connections looked good to me. Test probes could be a consideration, I have subsequently ordered a new set to err on the side of caution. There were a number of RCBOs within the same DB that were giving me the same >300ms recording when under test at the DB, and a result of <40ms when using the MFT socket adapter on the respective circuit.

James · Apr 22, 2024

memshield 2, the live terminal screw is before the current coil so you will not get it to trip.
the design is that the cable you install passes through a tube that holds the current coil.

davesparks · Apr 22, 2024

JXYM89 said:
or is Eaton/MEM particularly renowned for not tripping when under test at the breaker?

It is not that they don't trip, it is that putting a probe in the front of the RCBO to contact the terminal bypasses the sense coil so the RCBO does not see the test current.
If you insert the probe where the conductor would normally be it will trip as expected.