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?

 

[GLENNSPARK]

because the neutral from the circuit being fed from the RCBO has to go through the RCBO before it gets to the neutral bar vie the RCBOs neutral lead.....so any imballances due to fault between neutral-earth are seperated from the other RCBOs/RCDs within the install....unless in series of course

 

[Kev2632]

because the neutral from the circuit being fed from the RCBO has to go through the RCBO before it gets to the neutral bar vie the RCBOs neutral lead.....so any imballances due to fault between neutral-earth are seperated from the other RCBOs/RCDs within the install....unless in series of course

So the fault is detected within the RCBO before it will travel back along the Neutral lead is at right yea?

 

[Guest55]

Q1 to complete the circuit

Q2 because only one rcbo will detect the imbalance and trip.

 

[Top Cat]

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

 

[Kev2632]

Q1 to complete the circuit

Q2 because only one rcbo will detect the imbalance and trip.

So the fault is detected within the RCBO before it will travel back along the Neutral lead is at right yea?​

 

[GLENNSPARK]

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

because their pigs TC...

 

[GLENNSPARK]

RCBOs have flying neutrals that terminate into a common neutral bar...yes?....so as the final circuit is connected THROUGH each individual RCBO....both line AND neutral.....so the RCBO will detect imballance without upsetting the ballance of any other RCBOs within that C/U....

 

[Darkwood]

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

Whether post in humour or not the reason is its fine wire and to terminate it it has to be crimped, the factory crimp it for you but if it was just one long lead ppl would shorten it more readily and probably ignore the regs and not crimp, this has a risk of poor termination which can cause overheating, nuisance tripping etc etc, the manufacturers would get more returns thinking rcbo faulty because of poor terminated neutrals due to sparkies cutting long leads to length.
I do recall they used to be long leads on some brands once upon a time.

 

[Guest55]

RCBOs have flying neutrals that terminate into a common neutral bar...yes?....so as the final circuit is connected THROUGH each individual RCBO....both line AND neutral.....so the RCBO will detect imballance without upsetting the ballance of any other RCBOs within that C/U....

i said that , but just with less words :-D

 

[Kev2632]

If somebody was to take a neutral from 1 rcbo circuit and it joined up with another rcbo circuit further down the line, and there was a neutral to earth fault on the first rcbo won't the fault travel along that neutral link and cause the other rcbo to trip as well? Or not??

 

[Darkwood]

your looking at this wrong rcbo's will only detect faults on the load side any neutral/earth faults on the supply side of the rcbo wont be seen, it looks at the current going to the load matches the current coming back, if their is a difference then it is leaking elswhere and if large enough will trip the rcbo. This will not effect another rcbo on the same neutral bar as that is monitoring only its own load.

 

[Kev2632]

Yea i understand what you mean, what I'm meaning is if there was a borrowed neutral on the load side of the rcbo " a link between two rcbo e.g on two lighting circuits" and there was a earth to neutral fault on one rcbo why won't that affect the rcbo number two ??

 

[Darkwood]

Yea i understand what you mean, what I'm meaning is if there was a borrowed neutral on the load side of the rcbo " a link between two rcbo e.g on two lighting circuits" and there was a earth to neutral fault on one rcbo why won't that affect the rcbo number two ??

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.

 

[Knobhead]

13 posts to find out it has absolutely nothing to do with the flying leads.

The RDBO’s will act as any two RCD’s would with a shared neutral. They will trip on current imbalance, it doesn’t even need an earth fault to cause the tripping.

 

[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.

 

[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

 

[derek preece]

As far as I understand it, if there is an N/E fault on RCBO1 and RCBO2 shares a neutral on the load side then RCBO2 will also see an Earth fault and so it will trip.

 

[Kev2632]

In that instance both will trip.

Yea thats what i thought yea, and that would happen no matter regardless of what you do there, until that borrowed neutral would be removed and then the N-E fault would only lie on RCBO 1 yea??

 

[Knobhead]

Hope you can see the fault direction arrows on the last drawing

 

[Kev2632]

Hope you can see the fault direction arrows on the last drawing

Thanks Tony you've been a massive help cheers

 

[Geoffsd]

As far as I understand it, if there is an N/E fault on RCBO1 and RCBO2 shares a neutral on the load side then RCBO2 will also see an Earth fault and so it will trip.

Correct.

A borrowed neutral N-E fault will trip both because both neutrals (or shared one) will be connected to earth.


Just to point out that single-module RCBOs are only single-pole switches.

 

[Kev2632]

Correct.

A borrowed neutral N-E fault will trip both because both neutrals (or shared one) will be connected to earth.


Just to point out that single-module RCBOs are only single-pole switches.

But arent most RCBO'S double pole switches?

 

[Geoffsd]

No. Not the ones normally in CUs.

 

[Kev2632]

No. Not the ones normally in CUs.

So how is the neutral isolated then when the RCBO trips?

 

[Geoffsd]

It's not.

That's why it trips when you touch N and E.

Edit - no that only applies to RCCBs with an MCB off.
RCBO already off

 

[Kev2632]

It's not.

That's why it trips when you touch N and E.

Yea but on page 3 there, Tony and Darkwood was saying that the neutral would be isolated ?? what is right then?