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?

 

[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.

 

[Darkwood]

arty:

 

[Kev2632]

Thanks for the drawing there, so in that set up in will trip the rcbo1 yea? As it senses an imbalance yea ?

 

[Knobhead]

If you look I've put the currents in the various legs. So yes there out of ballance.
Either could trip, both are out of ballance.

 

[Kev2632]

Yea ive noticed that yea, so what I'm wondering is would both rcbo trip or would it just be one? As they are both out of balance??

 

[Knobhead]

You got in before I finished editing my last posy. It'e a ---- up which trips.

 

[Darkwood]

Just to re-iterate kev, when one trips it effectively isolates the other rcbo from the fault so usually on one trips, but occasionally it can trip both together but its rare as background leakage on the circuits will put one rcbo closer to the threshhold than the other as well as trip times and mA needed to trip will very slighty from rcbo to rcbo.
So in a nutshell, several factors are in play as to which one is gonna trip thus its rare for them to trip together, and after one has tripped it blocks the return path so the other rcbo holds.

 

[Kev2632]

Just to re-iterate kev, when one trips it effectively isolates the other rcbo from the fault so usually on one trips, but occasionally it can trip both together but its rare as background leakage on the circuits will put one rcbo closer to the threshhold than the other as well as trip times and mA needed to trip will very slighty from rcbo to rcbo.
So in a nutshell, several factors are in play as to which one is gonna trip thus its rare for them to trip together, and after one has tripped it blocks the return path so the other rcbo holds.

Thanks very much Darkwood,

So the reason the path is blocked is because the Neutral has been shut off with RCBO tripping yea, and isnt able to travel back along the neutral lead to the other rcbo? Is that correct?

 

[Darkwood]

Thanks very much Darkwood,

So the reason the path is blocked is because the Neutral has been shut off with RCBO tripping yea, and isnt able to travel back along the neutral lead to the other rcbo? Is that correct?

Correct!

 

[Kev2632]

Correct!

Thanks, i was just having a looking at that drawing there on page two tony drew, say for instance there was a neutral to earth fault on RCBO 1 , and it trips RCBO 1, then on RCBO 2,Current flows up the live and back down the neutral as normal, but wont any current flow across that neutral link and touch the earth fault? or how will that work???

 

[Knobhead]

In that instance both will trip.

View attachment 11925