Neutral lead on RCBO'S

[Kev2632]

What exactly are the neutral leads for? and if a neutral to earth happened on a board with just RCBO'S in it, why doesnt the fault travel along that neutral and affect the other RCBO's?

 

[Kev2632]

So it is possible for two rcbos to trip if somebody borrowed a neutral from two different circuits and there was neutral to earth fault on one circuit yea?

 

[Darkwood]

A - if there was a borrowed neutral then this would trip the rcbo anyway as there is an imbalance of load out and return as the current would be higher on the return path when borrowed neutral load was switched on, but hypothetically if you could maintain the circuit and and then touch down neutral to earth it will only trip the rcbo that is leaking the current and has seen an imbalance, this will depend on many things like where the short is, cable resistance , volts drop etc etc all playing a part in where the neutral current will leak to and how much, its possibly alot more complicated than it should seem.

Also in answer to your question is that when the rcbo trips it will stop the current flowing in the borrowed neutral so the second rcbo dosen't see it, which rcbo trips will be down to the variables mentioned above.

 

[Knobhead]

I give up!

 

[Darkwood]

So it is possible for two rcbos to trip if somebody borrowed a neutral from two different circuits and there was neutral to earth fault on one circuit yea?

I would say it is possible but more than likely just one may trip, im just theorising here but your best bet is to build a little circuit and do as you said with the neutral then link one to earth, repeat several times and then see what happens, but i only suggest this if you are competent to do this, as you will be experimenting live. Probably the quicker way of finding out as your going around in circles here we've explained the best we can.

 

[Kev2632]

see attached rubbish drawing, but it gives you the idea im trying to explain what would happen if somebody borrowed a neutral one circuit and took into the other circuit which are bother RCBO protected, and there was a neutral to earth fault on the 1 circuit would it affect the other RCBO, and ive been told your not "borrowing the neutral because there is split board its a common neutral but why doesnt the RCBO detect two different neutrals from different circuits if they are joined together at some point ??

 

[Darkwood]

Your mixing things up here are you talking about 2 rcbo's which monitor only the balance of the circuit attached or as you have just suggested 2 x rcd's in a split board which have independant neutral bars for their respective rcd and these bars may monitor several circuits each, someone at work or college has obviously explained this but are you sure you explained what you meant to them as the answer you got may be to a different senerio.

Dont get confused about what a borrowed neutral is; in a split 2 x rcd board you have a dedicated neutral bar to each rcd, if the borrowed neutral you talk about is on a circuit on the same rcd then it wont be detected by the rcd, but if the borrowed neutral is pinched from the circuits on an opposing rcd then it will trip the rcd that detects the imbalance, i would for the moment forget about neutral/earth faults as the rcd's will trip regardless of this.

 

[Kev2632]

I'm talking about 2 individual rcbo detecting there own circuits what happen if the neutral from rcbo 1 happened to join up with rcbo 2? As in they have borrowed a neutral from circuit 1 and it used at somepoint in circuit 2 would that trip both rcd as the neutrals are mixed up or does it not matter as its one common neutral bar for the installation??? Sorry im just getting confused and cant get my head round this

 

[sedgy34]

And why do they have curly neutrals so you can't do a neat job!

they are recycled from the old telephones lol

 

[Darkwood]

I'm talking about 2 individual rcbo detecting there own circuits what happen if the neutral from rcbo 1 happened to join up with rcbo 2? As in they have borrowed a neutral from circuit 1 and it used at somepoint in circuit 2 would that trip both rcd as the neutrals are mixed up or does it not matter as its one common neutral bar for the installation??? Sorry im just getting confused and cant get my head round this

In this set up one rcbo will trip a majority of the time, once it trips it isolates the link to the other rcbo through its double pole switch, although its possible that once in a blue moon they could trip together as you have to remember that if once rcbo gains extra neutral current then the other rcbo will loose it and both are out of balance, but also to take into account is natural leakage existing so the rcbo that has a higher background leakage will operate the first and as mentioned before it then isolates the link between the 2 neutral leaving the other rcbo on.

 

[Kev2632]

Yea I'm understanding what you mean so for instance I've seen inside a contractor box where there which looks like more than lighting circuit passing through it, every circuit within this building is fed from it's each individual rcbo, but all the neutrals are joined inside the box in a connector block, wouldnt that affect each rcbo?!

 

[Darkwood]

if its the supply neutral to the rcbo's that are linked this is ok nd wont affect the operation of any rcbo, if you have neutrals on the out going side of an rcbo linked in a connector then they are neutrals of the same circuit and probably just been brought back to box for convenience. I feel you are mis-interpreting what you see.

 

[Kev2632]

Yea thats probably what it is, neutrals of the same circuit yea? But for example If the neutrals of different circuits were all joined together in 1connector block that will not work ? Because they are fed from two different rcbos is that correct yea? And the rcbo will see an imbalance is that right? Thanks so much for your help so far

 

[Darkwood]

Yea thats probably what it is, neutrals of the same circuit yea? But for example If the neutrals of different circuits were all joined together in 1connector block that will not work ? Because they are fed from two different rcbos is that correct yea? And the rcbo will see an imbalance is that right? Thanks so much for your help so far

By Jove i think you got it! correct.... !!!!!!

 

[Kev2632]

Hahaha!! Sorry lad thanks so much for your help, so basically you can't borrow neutrals from other circuits and use them in other circuits when they are rcbo protected as that will trip the rcbo as it senses as imbalance from
Another circuit yea? Thanks

 

[Knobhead]

View attachment 11915

This shows two RCBO’s with a shared neutral.
The circuit supplied by RCBO 1 is trying to draw 3A, RCBO 2’s circuit 1A. Because of the imbalance in the two currents 1A will flow through the shared neutral. This give the imbalance I the RCBO’s.