531.2.9 what does it mean?

brman · Mar 24, 2013

Ok, I am sad and have just been re-reading the regs on RCDs for additional protection.
This lead me to 531.2.9 which is talking about the need for descrimination on series RCDs. Ok so far.

but then it has a note; "in such cases the downstream RCD may need to disconnect all live conductors"

To me this is saying that the downstream RCD might need to switch neutral as well as line(s). ie. not be a single pole RCBO.

Is the the right interpretation and why would this be the case?

telectrix · Mar 24, 2013

not sure, b ut could it be that the downstream RCD is used as an isolation device for a sub-main and as such needs to disconnect all live conductors inc. the neutral.

brman · Mar 24, 2013

telectrix said:
not sure, b ut could it be that the downstream RCD is used as an isolation device for a sub-main and as such needs to disconnect all live conductors inc. the neutral.

Thanks, yes, use as isolation was all I could think of but then why single out the downstream RCD specifically. Surely that could equally apply to the upstream RCD?

guest119 · Mar 24, 2013

Is an rcbo not a dp switching device?

brman · Mar 24, 2013

Boydy said:
Is an rcbo not a dp switching device?

someone might correct me but I think most domestic RCBOs are single pole. I believe you can get DP ones but I don't think they are common.

I've just been reading a thread about SP RCBOs and the fact they do not break the neutral. The arguement being that this is not much good if it is a N->E fault. I suppose if the upstream RCD is DP and the downstream SP then a neutral to earth fault would trip both of them, meaning you don't have discrimination.

EDIT: thinking more I am still not sure. I am trying to think of a situation where a neutral to earth fault on a final circuit would not be fed by that specific circuits Line, meaning the downstream sp RCBO would effective clear the fault by isolating the line conductor. If the neutral for the whole system is significantly above earth then I would be happy that it needs the upstream RCD to clear it. Not sure if that makes sense.....

telectrix · Mar 24, 2013

brman said:
Thanks, yes, use as isolation was all I could think of but then why single out the downstream RCD specifically. Surely that could equally apply to the upstream RCD?

i think the answer to that is "local isolation"

brman · Mar 24, 2013

telectrix said:
i think the answer to that is "local isolation"

I think you might be right, but I still can't help wondering why it is specifically mentioned in the reg on RCD discrimination. Maybe I am over thinking things

Richard Burns · Mar 24, 2013

If there is a neutral earth fault after the downstream SP RCD that causes it to trip disconnecting the line conductor, there is still a neutral earth connection "visible" to the upstream RCD which will still trip because the downstream RCD did not clear the fault.
Although the current will be reduced by tripping the line there will still be transient current flowing possibly from another circuit that can use the earth path.

brman · Mar 24, 2013

Richard Burns said:
If there is a neutral earth fault after the downstream SP RCD that causes it to trip disconnecting the line conductor, there is still a neutral earth connection "visible" to the upstream RCD which will still trip because the downstream RCD did not clear the fault.
Although the current will be reduced by tripping the line there will still be transient current flowing possibly from another circuit that can use the earth path.

That was sort of what I was rambling on about in post 5 although I can't see a current flowing from another circuit as, if there was coupling of circuits the RCDs would not like it in the 1st place.

However transients from connected equipment is an interesting one, I can make sense of that, thanks.

So I guess this is saying that (for example) a TT with up front 100/300mA time delay, DP, RCD followed by 30mA RCBOs in the CU might not provide full discrimination. Maybe not a disaster in a general domestic situation but not allowed when discrimination is a safety issue, eg a dangerous situation could be created if another circuit was affected.

Des 56 · Mar 24, 2013

brman said:
someone might correct me but I think most domestic RCBOs are single pole. I believe you can get DP ones but I don't think they are common.

I've just been reading a thread about SP RCBOs and the fact they do not break the neutral. The arguement being that this is not much good if it is a N->E fault. I suppose if the upstream RCD is DP and the downstream SP then a neutral to earth fault would trip both of them, meaning you don't have discrimination.

EDIT: thinking more I am still not sure. I am trying to think of a situation where a neutral to earth fault on a final circuit would not be fed by that specific circuits Line, meaning the downstream sp RCBO would effective clear the fault by isolating the line conductor. If the neutral for the whole system is significantly above earth then I would be happy that it needs the upstream RCD to clear it. Not sure if that makes sense.....

I just noticed that assumption and maybe,its not the case
As far as I am aware single pole rcbos can also have a switched neutral,they take up 2 modules, single pole solid neutral are one module

brman · Mar 24, 2013

Des 56 said:
I just noticed that assumption and maybe,its not the case
As far as I am aware single pole rcbos can also have a switched neutral,they take up 2 modules, single pole solid neutral are one module

I was referring to single module domestic RCBOs which I believe are solid neutral. Am I am wrong?

Richard Burns · Mar 24, 2013

brman said:
I can't see a current flowing from another circuit as, if there was coupling of circuits the RCDs would not like it in the 1st place.

If you have a single pole RCBO trip out on a neutral earth fault then, for any other circuits in that board there is still a current path for neutral current (once it has passed normally through its own RCBO (and so not tripping it) to take an alternative route from the neutral bar up the tripped circuit neutral and so via the fault to earth.
If the upstream DP RCD is, well, upstream then the current now flowing to earth creates a L-N imbalance and trips.

Rather vague picture showing a N-E fault not cleared by the first RCBO tripping and the orange line shows a current path that can then bypass the upstream RCD.

[ElectriciansForums.net] 531.2.9 what does it mean?

brman · Mar 25, 2013

Richard Burns said:
If you have a single pole RCBO trip out on a neutral earth fault then, for any other circuits in that board there is still a current path for neutral current (once it has passed normally through its own RCBO (and so not tripping it) to take an alternative route from the neutral bar up the tripped circuit neutral and so via the fault to earth.
If the upstream DP RCD is, well, upstream then the current now flowing to earth creates a L-N imbalance and trips.

Rather vague picture showing a N-E fault not cleared by the first RCBO tripping and the orange line shows a current path that can then bypass the upstream RCD.
View attachment 18065

Thanks Richard, got it!

cobrauk · Mar 25, 2013

I think you are wrong and that all Rcd/rcbo's are double pole but please correct me if I am wrong

brman · Mar 25, 2013

cobrauk said:
I think you are wrong and that all Rcd/rcbo's are double pole but please correct me if I am wrong

Looking at the diagram on the side of a wylex RCBO and it shows it as single pole.
On this site: NHXSBS1B10 10A 30mA RCBO SP Type B

click on the bottom picture and you will see what I mean.

531.2.9 what does it mean?

brman

telectrix

-

brman

guest119

brman

telectrix

-

brman

Richard Burns

-

brman

Des 56

-

brman

Richard Burns

-

brman

cobrauk

brman

News and Offers from Sponsors

Similar threads

OFFICIAL SPONSORS

YOUR Unread Posts