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?

 

[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?

 

[Kev2632]

So when the rcbos are single pole devices then, what would in the scenarios we have described with the borrowed neutral case, and the borrowed neutral with the N-E fault on Rcbo 1? What happen in both of these scenarios?? :/

 

[Geoffsd]

No. It's not isolated.

So, if borrowed or connected to another neutral it will divert current from the second RCBO.

If there is an N - E fault it will be on both RCBOs.

 

[Kev2632]

No. It's not isolated.

So, if borrowed or connected to another neutral it will divert current from the second RCBO.

So the current will travel back along RCBO1 Neutral cable through the neutral lead and trip also the RCBO 2 is that correct??

 

[Geoffsd]

Yes...

 

[Kev2632]

Yes...

So just to clear this up for the single pole RCBO, in relation to the drawing on page 3- bearing in mind they are single pole RCBO's instead, when we have just a borrowed neutral on RCBO 1 & 2, It will trip both RCBO, when RCBO 1 trips the current will go down through RCBO 1 neutral cable, through the RCBO1 neutral lead and trip RCBO 2 as well yea?? and when we have the borrowed Neutral and neutral to earth fault on RCBO1 it will trip RCBO1 and will it no affect all the other RCBO's as well now??? as the neutral is not isolated from the rest of them??

 

[Kev2632]

??? anybody keen to answer that ^^ please

 

[Geoffsd]

So just to clear this up for the single pole RCBO, in relation to the drawing on page 3- bearing in mind they are single pole RCBO's instead, when we have just a borrowed neutral on RCBO 1 & 2, It will trip both RCBO,

Yes, because the current is going out through one RCBO and (some of it) back through another - so imbalance.

when RCBO 1 trips the current will go down through RCBO 1 neutral cable, through the RCBO1 neutral lead and trip RCBO 2 as well yea??

Yes, probably at the same time if lamps on both circuits turned on.
It's easier to see if you draw the circuit and follow the current path(s).

and when we have the borrowed Neutral and neutral to earth fault on RCBO1 it will trip RCBO1 and will it no affect all the other RCBO's as well now??? as the neutral is not isolated from the rest of them??

Not sure if you meant now or not.

Don't forget that an N-E fault will only trip the RCBO whose neutral has faulted to earth and the one whose neutral is connected to the neutral of the first - not any others because the fault is on the load side and load side neutrals are not connected (except in these fault conditions).

 

[Kev2632]

Yes, probably at the same time if lamps on both circuits turned on.
It's easier to see if you draw the circuit and follow the current path(s).

So the current wont travel back up the supply side of an rcbo then?

Not sure if you meant now or not.

Don't forget that an N-E fault will only trip the RCBO whose neutral has faulted to earth and the one whose neutral is connected to the neutral of the first - not any others because the fault is on the load side and load side neutrals are not connected (except in these fault conditions).

Yea i meant now sorry, ahh right so once the RCBO 1 has tripped the fault is isolated from the rest of the circuits as the current wont get past the supply side of RCBO 1 yea??

 

[derek preece]

Sorry just read your last post so edited mine

 

[Geoffsd]

Yes, as no other circuits are connected to it (unless by these faults) none of the other RCBOs will be affected.

 

[Kev2632]

Yes, as no other circuits are connected to it (unless by these faults) none of the other RCBOs will be affected.

And by saying "these faults" i assume you mean the borrowed neutral affecting the other RCBO2? yea???

 

[Geoffsd]

Yes.

The term 'Borrowed Neutral' is a bit misleading. Think of it like this -

The first RCBO has a borrowed Neutral from the second RCBO, and...

...the second RCBO has a borrowed Live from the first RCBO so both will have an imbalance.

As I said, if you draw it, it becomes a lot easier to see.

 

[Kev2632]

Yes.

The term 'Borrowed Neutral' is a bit misleading. Think of it like this -

The first RCBO has a borrowed Neutral from the second RCBO, and...

...the second RCBO has a borrowed Live from the first RCBO so both will have an imbalance.

As I said, if you draw it, it becomes a lot easier to see.

Yea i understand what you mean there, so when we have No "borrowed Neutral" just the neutral to earth fault on RCBO 1, and it trips RCBO 1, leaving that circuit isolated, with the fault i know on the load side of RCBO1, what stops the other RCBO's from seeing that fault?? once the current has returned through the neutral on RCBO 2 and goes back to the neutral bar thats its job done, it wont go along and find the fault because it cant get past the supply side of RCBO 1, is that correct?? this is the FINAL question Geoffsd!!

 

[Darkwood]

ive PM you and should have explained i was refering to DP rcbo's only you will appreciate throwing a single pole in the mix would have made your confusion worse, but should have gone that route first, it just habit on my part as its what i fit in my control panels but yes domestic is usually single pole.

 

[Geoffsd]

Because each RCBO only 'senses' the current going through it and returning along its neutral, there's no way it can get to the place of the fault.

Anything that happens on the supply side of the RCBOs (where all the Ns and Es are connected will not affect the RCBOs.

 

[Kev2632]

ive PM you and should have explained i was refering to DP rcbo's only you will appreciate throwing a single pole in the mix would have made your confusion worse, but should have gone that route first, it just habit on my part as its what i fit in my control panels but yes domestic is usually single pole.

I replied to your last PM darkwood, I dont have anything back in return? you sure you sent it ? :/

 

[Kev2632]

ive PM you and should have explained i was refering to DP rcbo's only you will appreciate throwing a single pole in the mix would have made your confusion worse, but should have gone that route first, it just habit on my part as its what i fit in my control panels but yes domestic is usually single pole.

Got it replied to it

 

[Darkwood]

I replied to your last PM darkwood, I dont have anything back in return? you sure you sent it ? :/

Just now, remember and will add to this thread too for other to remember; in a tns and tncs the neutral and earth bar will be linked together if the main switch is on due to the center tap in transformer (tns) and the combined N/E in the cutout on tncs.
So it shows that an E/N fault feeding back to the E and N bar through 1 or more rcbo's will not effect other rcbo's that are clear of this fault as its not on their load side.

 

[Kev2632]

Just now, remember and will add to this thread too for other to remember; in a tns and tncs the neutral and earth bar will be linked together if the main switch is on due to the center tap in transformer (tns) and the combined N/E in the cutout on tncs.
So it shows that an E/N fault feeding back to the E and N bar through 1 or more rcbo's will not effect other rcbo's that are clear of this fault as its not on their load side.

Perfect thanks, i sent you a pm lad

 

[steenie738]

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

Is this the case with SP RCBO's as although they supervise the load side N and L they only operate the L phase. In the ideal sittuation each circuit would have its own dedicated earth however if the neutrals are linked at the supply side then where is the break in the circuit in the case of an N-E fault. I would have though an imbalance in current flowing in the faulty neutral wire would also be detected on other RCD's or RCBO's. Clarification on this would be appreciated. The regs seem to explain that a main switch isolator can be used with a seperate RCBO for each circuit but I am unsure if they mean SP or DP RCBO's. DP being mega expensive so quite reluctant to buy these even though I only have 4 ways going out on my current fuse box. Any clarification on this would be much appreciated. Hello to everyone on the forum btw.

Regards
Steve

 

[Des 56]

Is this the case with SP RCBO's as although they supervise the load side N and L they only operate the L phase. In the ideal sittuation each circuit would have its own dedicated earth however if the neutrals are linked at the supply side then where is the break in the circuit in the case of an N-E fault. I would have though an imbalance in current flowing in the faulty neutral wire would also be detected on other RCD's or RCBO's. Clarification on this would be appreciated. The regs seem to explain that a main switch isolator can be used with a seperate RCBO for each circuit but I am unsure if they mean SP or DP RCBO's. DP being mega expensive so quite reluctant to buy these even though I only have 4 ways going out on my current fuse box. Any clarification on this would be much appreciated. Hello to everyone on the forum btw.

Regards
Steve

This is the way I see it after having a read

I think the rcbo, whether single pole -n or double pole,both will disconnect both poles
However, in the single pole rcbos,the live is the rated contacts
The neutrals are not for breaking the load,they operate after the live so do not switch the loads
Hence the neutral tail used, so that it isn't confused with the live terminal

The double pole rcbo has both poles monitored and rated, so no need for the neutral tail, both poles disconnect

 

[steenie738]

This is the way I see it after having a read

I think the rcbo, whether single pole -n or double pole,both will disconnect both poles
However, in the single pole rcbos,the live is the rated contacts
The neutrals are not for breaking the load,they operate after the live so do not switch the loads
Hence the neutral tail used, so that it isn't confused with the live terminal

The double pole rcbo has both poles monitored and rated, so no need for the neutral tail, both poles disconnect

If you look at the circuit diagram on the side of a single pole RCBO you can clearly see that there are no contacts between the N in (tail) and the N out therefore it would seem only the L is disconnected. DP RCBO's diconnect both poles however they are far more expensive and I am unwilling to install these even though I only have 4 ways outgoing possibly 5.
Regards
Steve

 

[yellowvanman]

Is this the case with SP RCBO's as although they supervise the load side N and L they only operate the L phase. In the ideal sittuation each circuit would have its own dedicated earth however if the neutrals are linked at the supply side then where is the break in the circuit in the case of an N-E fault. I would have though an imbalance in current flowing in the faulty neutral wire would also be detected on other RCD's or RCBO's. Clarification on this would be appreciated. The regs seem to explain that a main switch isolator can be used with a seperate RCBO for each circuit but I am unsure if they mean SP or DP RCBO's. DP being mega expensive so quite reluctant to buy these even though I only have 4 ways going out on my current fuse box. Any clarification on this would be much appreciated. Hello to everyone on the forum btw.

In the case of a neutral/earth fault current could flow from the neutral bar through the RCBO to earth through the fault - in which case the RCBO will trip. None of the other RCBOs should trip because there isn't a path to earth through the neutrals of the other circuits. Remember the neutral busbar is 'down wind' of the other RCBOs.

Single Pole (SP) RCBOS are just that, they disconnect the Line only. If SP RCBOs are used in conjunction with a down stream DP RCD then the RCBOs will not clear a neutral/earth fault and the down stream RCD will trip and won't be able to be reset until the fault is cleared.