Supply to outbuilding - Protection of SWA and RCD discrimination

[SW1970]

Hi folks,

Rural domestic property with 2 outbuildings. TN-C-S with additional earth rod (believe was upgraded to underground TN-C-S feed from old overhead TT feed and rod was left in place). Main house has the only RCD (30mA) in the whole property as the main switch and there is much nuisance tripping. After testing and a lot of trial and error there is believed to be several causes of nuisance RCD tripping. Currently the outbuildings are each fed from an MCB in the main board over SWA to a sub board with several MCBs. I've proposed a gold standard upgrade to the client which he is keen on but not just yet (c. £1,200) and wants a more cost effective approach tried first. One of the outbuildings is believed to be a major contributor to the nuisance tripping. What I could do is redirect the feed to the suspect outbuilding to the external meter cupboard via a Henley block and take a feed directly from the meter to the outbuilding (this is easily done). The SWA to the outbuilding appears to have been correctly installed, but is under a tiled garden patio the whole way, with about 12 inches visible up an external wall to a junction box, and about 3 feet up a wall inside the outbuilding to a sub board. I do not know if the SWA has been installed to a suitable depth, irrespective of the fact that it's now covered by patio.

What I want to do is upgrade the sub board in the outbuilding to several RCBOs (it's a home office with a lot of IT equipment so hence the division of circuits to tackle relatively large residual current - this is one of the suspected reasons for tripping), but the question is what am I required to do by way of protection of the SWA cable. I've got a few options but I'm stuck on whether I need to be mindful of the rural nature of the property (hence why option a is included):

(a) a DP 100mA time delayed RCD in the meter cupboard, or
(b) a DP main switch with an MCB for overcurrent protection of the SWA, or
(c) just a DP isolator in view of the well covered nature of the SWA with only a little on show. I can easily protect the exposed SWA using external capping.

Thanks loads! SW

ps. Please no input about reasons for nuisance RCD tripping or earthing for the outbuilding; well across that thanks.

 

[Chr!s]

(a) wheres the over current protection?

(b) Mcb feeding mcb, you will struggle with discrimination especially short circuit.

(c) Again whats providing over current protection

Id suggest,RCD 100 mA time delayed, and a suitable rated fuse for protection of the swa. You could omit the RCD.

This set up will provide the protection for the distribution circuit plus afford better discrimination possibly total.

Regards Chris

 

[malcolmsanford]

Bottom line is what is the Zs for the sub main. Chances are it will be to high to disconnect under a fuse/mcb protection device and therefore need RCD protection. so really a 100mA time delayed RCD is most likely the best way to go

 

[SW1970]

(a) wheres the over current protection?

(b) Mcb feeding mcb, you will struggle with discrimination especially short circuit.

(c) Again whats providing over current protection

Id suggest,RCD 100 mA time delayed, and a suitable rated fuse for protection of the swa. You could omit the RCD.

This set up will provide the protection for the distribution circuit plus afford better discrimination possibly total.

Regards Chris

Thanks all. I was thinking along Malcolms lines, haven't measured yet. Looks like best solution is 100mA time delayed RCD plus MCB if Zs low enough.

 

[Chr!s]

Thanks all. I was thinking along Malcolms lines, haven't measured yet. Looks like best solution is 100mA time delayed RCD plus MCB if Zs low enough.

You will seriously struggle with short circuit discrimination usung a mcb, id go for a fuse.

Also the distribution circuit is upto 5 secs so this will give you a little more leeway and allow a non rcd circuit possibly.

Regards Chris

 

[SW1970]

Thanks chris, same solution where the feed to the outbuilding is taken directly from the meter? Thanks.

 

[Chr!s]

Thanks chris, same solution where the feed to the outbuilding is taken directly from the meter? Thanks.

Well i may of misunderstood. If you take straight from the meter its doubtful and debatable as to whether you could rely on the DNO'S fuse, so a suitable fuse will be required to protect the distribution circuit. Use the fuse manufacturers info for max Zs this to may offer more leeway than those in the BRB.

Regards Chris

 

[SW1970]

Thanks Chris, here's a pic. I may have confused by my reference to discrimination in the title of the thread. So now with the pic, is a 100mA TD RCD required because of the uncertain installation method of the SWA and considering that this is a rural location ie. to provide supplementary protection to the earthing arrangement, or is that overkill? Overcurrent protection is provided by the fuse or MCB (depending on Zs).



Thanks, SW

 

[Chr!s]

Thanks Chris, here's a pic. I may have confused by my reference to discrimination in the title of the thread. So now with the pic, is a 100mA TD RCD required because of the uncertain installation method of the SWA and considering that this is a rural location ie. to provide supplementary protection to the earthing arrangement, or is that overkill? Overcurrent protection is provided by the fuse or MCB (depending on Zs).

View attachment 8017

Thanks, SW

That seems fine, SWA meets most requirements so in that respect no RCD would be required, for earth fault protection you could use a RCD or if design allows a fuse/mcb.

I wouldn't call it overkill, but if discrimination between the circuits is required you will need a fuse upfront.

Regards Chris

 

[SW1970]

Hi Chris, thanks loads. Surveyed today, will use DP isolation switch and MCB. (Zs is low). If I have this DP isolator and MCB adjacent to point of origin as in pic above, is the outbuilding board still considered a sub board? Many thanks, SW

 

[Chr!s]

Hi Chris, thanks loads. Surveyed today, will use DP isolation switch and MCB. (Zs is low). If I have this DP isolator and MCB adjacent to point of origin as in pic above, is the outbuilding board still considered a sub board? Many thanks, SW

That all seems fine to me, is discrimination an issue, MCB to MCB, a short will probably take both MCB'S out. If discrimination not an issue MCB to MCB is fine.

Regards Chris

 

[SW1970]

Thanks - from a certification point of view do you see that the DP isolator and the MCB mean that the outbuilding board is considered a sub board?