C40 rcbo on ring main changes to b32 causing nuisance tripping

[JJONESY4]

Ring main was on a c40 rcbo so I tested the circuit everything was fine and dropped down onto a b32, It’s quite a long circuit but only serves about 10 sockets.The rcbo stays on but the client says trips frequently, I’ve been back and the rcbo stayed on the whole time, I then left and over the weekend it has tripped again but went back and all stayed on fine the whole time I was there again. Type c is for higher inrush but there’s nothing on circuit which would cause this… any ideas would be appreciated.

 

[westward10]

Was the 40A tripping?

 

[brianmoooore]

Was the 40A tripping?

Could be more of a case of "was the 40A working?"

 

[JJONESY4]

Was the 40A tripping?

c40 stays on just wondered if the breaker I ordered of off eBay was faulty as you can’t get them new anymore. Just seems strange how it stays on?

 

[JJONESY4]

Could be more of a case of "was the 40A working?"

Hi, the 40a breaker doesn’t trip but passes trip test on mft. No E-n fault or anything, just seems strange as now a small job is an annoying one!
Thanks for the reply

 

[Rockingit]

This has all the hallmarks of an original design of a 32A OCPD being swapped out to a 40A one to 'solve' the problem that the OP now has.

If you have sufficient sized cable and a low enough Zs to satisfy the 40A (without relying on the 1667Ω of the RCD!) then I'd swap back - something about if it aint bust don't fix it!

 

[JJONESY4]

This has all the hallmarks of an original design of a 32A OCPD being swapped out to a 40A one to 'solve' the problem that the OP now has.

If you have sufficient sized cable and a low enough Zs to satisfy the 40A (without relying on the 1667Ω of the RCD!) then I'd swap back - something about if it aint bust don't fix it!

Hi thanks for reply, I need to make alterations to the circuit also so just wondered what could possibly be causing the breaker to trip maybe from others experiences, seems another sparks just whacked it on a c40 to stop it from tripping.

 

[Rockingit]

Hi thanks for reply, I need to make alterations to the circuit also so just wondered what could possibly be causing the breaker to trip maybe from others experiences, seems another sparks just whacked it on a c40 to stop it from tripping.

Two reasons for the trip - either overload (/inrush) or earth leakage. A clamp meter will help with the first and a full IR test the other. You say it’s ‘quite long’ - that always makes me suspicious of a joint or other connection somewhere that you haven’t found yet. An R1R2 measurement will help you know exactly how long (look up the resistance values in the OSG and do the maths). Also, does the circuit hold with zero appliances connected? Again, that splits it down to either the installed circuit or a faulty device.

 

[timhoward]

You might be able to get some useful data out of the client. It has been known!
Does it trip when he turns something on, or at certain times of day, or completely randomly when the circuit isn't in use much?

You could try a ramp test with the loads plugged in and see how close it is to tripping all the time.
You say it tests out ok, so one would err towards an issue with a load not the fixed wiring.

 

[Rockingit]

You could try a ramp test with the loads plugged in and see how close it is to tripping all the time.

Another useful step at that stage is to use a mA sensitive clamp over the cpc at the origin of the circuit and see what leakage you have initially, I'm always slightly sceptical of the ramp test method if you don't know enough about the circuit to start with and there may be alternative paths to earth.