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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?
 
Thanks all for responding to my message.

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?

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.
 

Reply to the thread, titled "RCBO Testing" which is posted in UK Electrical Forum on Electricians Forums.

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Title
RCBO Testing
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UK 
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UK Electrical Forum
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7
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JXYM89,
Last reply from
davesparks,
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