Rcbo's on a TT system

[paul]

Fitting rcbo's to a TT system using a Schneider distribution board I read the instructions to the rcbo's and it clearly states that they are not suitable for a TT system. I have not seen this on instructions to crabtree rcbo's and can only assume the issue is that when tripped the neutral is still connected as the rcbo's are only single pole switching but looking for clarification as confused?

 

[Deleted member 9648]

This has always confused me as well, and I've never seen a satisfactory explanation. The usual thing seems to go along the lines that a N-E fault will not be disconnected by the device even though it may cause it to operate, however the same applies to an RCBO on a TN system....so why pick on a TT? In fact if the Ra was low enough it would be permissible to use a SP mcb or fuse just to add to the mystery.
Manufacturers instructions and all that though.....

 

[paul]

Asked my manager but he didn't have a clue and just said others have fitted them but why would one manufacturer point this as a problem but others not?

 

[Des 56]

Most Rcbos are single pole + switched neutral
I understand the double pole main switch covers the regulation requiring disconnection of both poles, yet it seems to me that double pole rcbos would have been expected

 

[Deleted member 9648]

I once enquired on an NICEIC annual assessment visit and got a blank look.

 

[paul]

Still confused been on Schneider web site and going to ask the question but clearly states not suitable for TT or IT systems?

 

[Deleted member 9648]

Most Rcbos are single pole + switched neutral
I understand the double pole main switch covers the regulation requiring disconnection of both poles, yet it seems to me that double pole rcbos would have been expected

Agreed, the reg merely states for every circuit both poles must be capable of being isolated (except for TNS/TNCS) so applies to TT and IT,that is achieved by the main switch, it does not state that each protective device has to disconnect both poles.

 

[Richard Burns]

The lack of disconnection of the neutral would only be of concern if the board were part of a further system that has RCD protection as there would then be a lack of cascading/discrimination, though of course applicable to all earthing systems.
I agree with #7 so long as you have a DP main switch this covers the TT DP requirement, though a DP disconnection on each protective device would be ideal effectively the UK supply of RCBOs does not support this.
I did think that the majority of domestic SP RCBOs were not switched neutral but solid neutral.

 

[paul]

The distribution board is intended to be amendment 3 with main switch and all circuits with rcbo's protection on a TT system. I have suggested it would be prudent to fit a time delay rcd upfront in case of a neutral fault to earth in the DB but could prove problematic as a neutral fault on a circuit could trip an rcbo and the upfront time delay rcd.
I also think the majority of domestic so rcbo's are unswitched neutral

 

[paul]

Had a phone call from Schneider bod and issue is that the neutral is not disconnected when tripped when I suggested other manufacturers do not see this as issue he replied that they recommend using the double pole rcbo units which would take space of 2 MCB ways, so now have issue of against manufacturers instructions

 

[Deleted member 9648]

Tricky one, I'd say go with an S type RCD main switch, or if you are brave enough note it as a departure on the cert with the justification that the use of SP RCBO's is not precluded in Bs7671, and other manufacturers do not suggest their RCBO's are not for TT.
I wouldn't be brave enough and would use the S type main switch.

 

[paul]

That's what I'm suggesting to my manager, he can have the final say and be held responsible as his price and design

 

[DPG]

This is another case where the regs need to be clearer and more definitive.