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TN-C-S (PEN) RCBOs Discussion

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Burnmenot

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I'm currently planning for a CU upgrade on the domestic premises, and following previous advice on this forum, I was told that I should consider upgrading to a Type A RCBO CU.
I have been doing a bit of reading and came across some confusing information, stating that an RCD would not work on a PEN system, RCCD was not permitted on a PEN system and consequently, RCBO would not work and it will have to have separate earth.

I believe I have a PEN system as the earth it's taken at the DSO service fuse, and I also seem to have a small green hatch on the ground for rod servicing...
I can add some photos for clarification if needed.

This is confusing to me as I do seem to have split distribution with an RCD protecting the socket rings, immersion, and boiler, and on the other side, all the light rings an RCBO for the shower.

I would appreciate any comments
 
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Burnmenot said:
I'm currently planning for a CU upgrade on the domestic premises, and following previous advice on this forum, I was told that I should consider upgrading to a Type A RCBO CU.
Click to expand...
The "problem" there is that whatever your motive is for sitting down to think about whether Type AC is still suitable or not, and the process you go through which leads you to decide it isn't, if all that is done properly you might well find that Type A isn't suitable, and you need Type F, at least for some circuits.

Burnmenot said:
I have been doing a bit of reading and came across some confusing information, stating that an RCD would not work on a PEN system, RCCD was not permitted on a PEN system and consequently, RCBO would not work and it will have to have separate earth.
Click to expand...
Got a link to that? It must surely be talking about the situation where N & E remain combined throughout the installation....
 
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I think there is confusion and you might be thinking of comments that an RCD or RCBO wouldn’t protect you in the event of a broken supply Neutral in a TNCS system?
 
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Soi disant said:
The "problem" there is that whatever your motive is for sitting down to think about whether Type AC is still suitable or not, and the process you go through which leads you to decide it isn't, if all that is done properly you might well find that Type A isn't suitable, and you need Type F, at least for some circuits.


Got a link to that? It must surely be talking about the situation where N & E remain combined throughout the installation....
Click to expand...
the last page of this Hager Tech datasheet mentions that RCCB is not permitted in PEN.
Aso this thread on the IET forum states that an RCD will not work on a PEN system: https://----------------------/discussions/viewtopic/1037/25035

regarding the comment you made about the type, being F perhaps more suitable for some circuits, I'd say that given all the modern equipment we have, (water pump, modern washing machines, dishwashers, and all the IT equipment) maybe even a type B may be more appropriated?
 

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timhoward said:
I think there is confusion and you might be thinking of comments that an RCD or RCBO wouldn’t protect you in the event of a broken supply Neutral in a TNCS system?
Click to expand...
thanks for your comment, I didn't really refer to that, I understand that is a very remote possibility.... to lose N and not lose L in the concentric cable from the DSO would be a very unlikely event.
 
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Burnmenot said:
thanks for your comment, I didn't really refer to that, I understand that is a very remote possibility.... to lose N and not lose L in the concentric cable from the DSO would be a very unlikely event.
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Sorry! I’ll read the stuff you posted later as it sounds interesting.
 
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I am very confused with what you are saying, and the links you have provided don't work.


You talk about having a TN-C-S - which obviously will have a PEN upstream, but then talk about the manual not allowing the use of any RCBO/RCCB in the PEN - why on earth would you want to put it in the PEN, this would clearly be prohibited?


You say you are looking to replace the CU, well that is the right place to put the RCCB/RCB/RCBOs, under no circumstances should you consider placing any of these in the PEN.

You need to get someone in to do this, just get the electrician to change the CU, you can use one that's fully populated with RCBOs, or get a split board with two or more RCD sections.

I would recommend type A RCD/RCBO, usually B curve RCBO/MCB - but your electrician will size them accordingly.

Edit:

Reading your post again, do you have a TN-C-S or TT - you also talk about having a Rod?

The protection requirements are different between TNCS and TT - although in both cases RCD/RCBO can be used in a TT you will need to use a RCD for fault protection (not additional), and usually a type S ahead of the CU
 
Last edited:
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The Hager document last page is saying you can’t have a PEN conductor downstream of the device, not you cant install a device on a TNCS system. They are basically saying that if any fault current returned via the same conductor as the N the device wouldn’t know it is fault current. You would never do this anyway.
 
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timhoward said:
The Hager document last page is saying you can’t have a PEN conductor downstream of the device, not you cant install a device on a TNCS system. They are basically saying that if any fault current returned via the same conductor as the N the device wouldn’t know it is fault current. You would never do this anyway.
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gotcha, thanks for the clarification, much appreciated
 
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[ElectriciansForums.net] TN-C-S (PEN) RCBOs Discussion
[ElectriciansForums.net] TN-C-S (PEN) RCBOs Discussion

Julie. said:
I am very confused with what you are saying, and the links you have provided don't work.


You talk about having a TN-C-S - which obviously will have a PEN upstream, but then talk about the manual not allowing the use of any RCBO/RCCB in the PEN - why on earth would you want to put it in the PEN, this would clearly be prohibited?


You say you are looking to replace the CU, well that is the right place to put the RCCB/RCB/RCBOs, under no circumstances should you consider placing any of these in the PEN.

You need to get someone in to do this, just get the electrician to change the CU, you can use one that's fully populated with RCBOs, or get a split board with two or more RCD sections.

I would recommend type A RCD/RCBO, usually B curve RCBO/MCB - but your electrician will size them accordingly.

Edit:

Reading your post again, do you have a TN-C-S or TT - you also talk about having a Rod?

The protection requirements are different between TNCS and TT - although in both cases RCD/RCBO can be used in a TT you will need to use a RCD for fault protection (not additional), and usually a type S ahead of the CU
Click to expand...
thanks for the comments, I appreciate I will have to get a RECI to do it, I'm just trying to understand what would suit better my needs ahead of the call, educating oneself about electricity and what goes on one's home never hurt anyone I believe.
I understand that I may have a TN-C-S system as the earth seems to come from the DSO service fuse, and I seem to have an earth rod too, I attach the pics for clarification.

[ElectriciansForums.net] TN-C-S (PEN) RCBOs Discussion
 
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You've a TN-C-S (aka PME) system and need to replace that 16mm T&E before worrying about anything else!
 
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Burnmenot said:
oh ok, is there a problem with that wire?
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You'll need to consult with a reputable electrician to you, but in the U.K. that would not meet our regulations.
 
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Burnmenot said:
oh ok, is there a problem with that wire?
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Years ago it would be OK, now it either needs to be replaced by armoured, or surface/contained, depending on how far it goes it may need local protection (such as fuses etc), it also looks a little small - depends on your fuse size but it will be a maximum of 80A
 
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