RCBO's and borrowed neutrals

[Golden_Boy]

If a board is fitted with all RCBO's would a borrowed neutral show up and cause any problems? I reckon not but a mate of mine reckons it would cause trippng ??

 

[nickblake]

yes it would if a neutral shares between 2 circuits protected by 2 RCBO's then you find that both of the RCBO's will trip when a load is applied ,if this is the case most commonly in lighting circuits you need to identify which circuits have a shared or borrowed neutral by a process of elimination and testing

 

[Golden_Boy]

yes it would if a neutral shares between 2 circuits protected by 2 RCBO's then you find that both of the RCBO's will trip when a load is applied ,if this is the case most commonly in lighting circuits you need to identify which circuits have a shared or borrowed neutral by a process of elimination and testing

Thats what my mate said as well. But suely all the neutrals are connected together in the CU ? and as the rcbo's are single pole devices the neutrals will still be connected together even if one of them trips????

 

[WDMDL]

It's not about where the neutral ends up, it's the path that it takes to get there!
For a 30mA RCD or RCBO, the current going out down the line and returning down the neutral must be equal - if the imbalance exceeds 30mA it will trip.

So:
If your line current goes back to another neutral, you have an imbalance in both circuits - and if both are RCD'd, both RCDs will likely trip (though with something like a clock radio only drawing a few mA, maybe only one will trip)

Even though RCBOs are single pole devices in terms of overcurrent, switching and isolation, in order to work the neutral current for that circuit MUST flow through the toroid inside, so that the (hopefully) equal and opposite currents result in no current being applied to the trip solenoid.

simon.

 

[nickblake]

Ah yes they are connected in the CU but after ists gone through the electronics within the RCBO as wdmdl has said its the paths the neutrals take and the line and neutral must be equal, the RCBO/ RCD is perfectly balanced just like a set of scales so anything that changes the balance slightly ,incorrect neutral path earth leakage etc exceeding the 30 ma then the RCBO will trip

 

[Golden_Boy]

It's not about where the neutral ends up, it's the path that it takes to get there!
For a 30mA RCD or RCBO, the current going out down the line and returning down the neutral must be equal - if the imbalance exceeds 30mA it will trip.

So:
If your line current goes back to another neutral, you have an imbalance in both circuits - and if both are RCD'd, both RCDs will likely trip (though with something like a clock radio only drawing a few mA, maybe only one will trip)

Even though RCBOs are single pole devices in terms of overcurrent, switching and isolation, in order to work the neutral current for that circuit MUST flow through the toroid inside, so that the (hopefully) equal and opposite currents result in no current being applied to the trip solenoid.

simon.

I'm away to get some humble pie.........

 

[johnboy6083]

RCBO's are double pole devices, hence the neutral flyleads from them. They might only isolate one pole though.

 

[nickblake]

this may help