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.

 

[JJONESY4]

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.

Thought it was inrush at first but only a tv and couple of lamps plugged into the circuit at the moment. It held with nothing plugged in but couldn’t wait around all day as clients quite a pain!. It also holds with loads in and trips unannounced not when client turns something on etc which is usually the problem. Thanks for reply

 

[Rockingit]

Thought it was inrush at first but only a tv and couple of lamps plugged into the circuit at the moment. It held with nothing plugged in but couldn’t wait around all day as clients quite a pain!. It also holds with loads in and trips unannounced not when client turns something on etc which is usually the problem. Thanks for reply

Is this domestic or commercial? My first thoughts are wondering if there’s a timer involved somewhere which makes me think if this is domestic has a heating system been spurred off somewhere?

 

[pc1966]

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.

Indeed!

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!

Trouble is if this is a domestic RFC in 2.5mm then pushing it to 40A is not a good idea due to CCC.

 

[pc1966]

Is this domestic or commercial? My first thoughts are wondering if there’s a timer involved somewhere which makes me think if this is domestic has a heating system been spurred off somewhere?

I was wondering the same.

As an RCBO it can trip for one o(or more) of 3 reasons:

• Too much leakage
• Too high inrush
• Too high load

If you cant find any big loads to explain things, it is either a leak or an intermittent fault. If going 40A C "fixed" it then load/inrush is expected, unless the RCBO was stick and only after testing/cycling and eventual replacement is the leak being spotted!

 

[brianmoooore]

This sounds to me like the equivalent of thick aluminium foil wrapped around the fuse in a 13A plug! A domestic RFC, not serving a kitchen, should normally be happy on a B16. If a C40 is tripping on the MCB part, then something needs urgent investigation.
If it's tripping on the RCD part, then equally it needs urgent investigation.

 

[westward10]

This is the problem RCD side or MCB side. The evidence suggests MCB as the higher rated device does not trip. I know what I would do but couldn't possibly suggest such a thing.

 

[brianmoooore]

Unless this domestic property has a sideline hiring out their living room as an industrial welding workshop, then the MCB won't be tripping as a result of plugged in load.
If the MCB is tripping on load, then it must be a serious intermittent arc fault between live and neutral - the type that causes fires.
I wouldn't be leaving this circuit powered until I'd found the problem, especially with a C40 as OPCD.

 

[baldelectrician]

If you had spare ways you could split the ring in to 2 radials to narrow things down

What make is the consumer unit?

 

[JJONESY4]

Is this domestic or commercial? My first thoughts are wondering if there’s a timer involved somewhere which makes me think if this is domestic has a heating system been spurred off somewhere?

Hi, it’s a large domestic property all heating is on another circuit, thanks though.

 

[JJONESY4]

Unless this domestic property has a sideline hiring out their living room as an industrial welding workshop, then the MCB won't be tripping as a result of plugged in load.
If the MCB is tripping on load, then it must be a serious intermittent arc fault between live and neutral - the type that causes fires.
I wouldn't be leaving this circuit powered until I'd found the problem, especially with a C40 as OPCD.

I was wondering the same.

As an RCBO it can trip for one o(or more) of 3 reasons:

• Too much leakage
• Too high inrush
• Too high load

If you cant find any big loads to explain things, it is either a leak or an intermittent fault. If going 40A C "fixed" it then load/inrush is expected, unless the RCBO was stick and only after testing/cycling and eventual replacement is the leak being spotted!

No big loads on the circuit which would cause inrush or high loads, all tests satisfactory, nothing being plugged into the circuit is causing the fault either, just seems strange although I bought the b32 from eBay as they are now redundant so wondering if that was just faulty but all tests through the rcbo where fine. It stays on for a while then trips, every time I’m not there it seems…
Thanks for reply