Outbuilding on non RCD side of C/U?

[Nicebloke]

If running power to an outbuilding (usually a garage at the bottom of someones garden), I normally run it from the non RCD side of the C/U in the house and then put a 2 way RCD protected C/U in the garage. I was however, told that I can no longer do this as it must be connected to an RCD in the house C/U. I was also told that any outbuilding more than 20 metres away from a house must have it's own TT system?

 

[Midwest]

How come the upfront S type then? I'm interested now.

Ohh dear, we have the DP RCBO v SP RCBO TT supply debate all over again. Thought I understood it the last time, gonna get all confused again, I can see it

 

[Leesparkykent]

Because some single pole rcbo's will only disconnect the L leaving the N connected in a fault, so if you had a N - E fault the rcbo would trip, but the N - E fault would still be present and current will be flowing. the upfront s type would detect this and trip. Kind of like a backup really if that makes sense.

Yeah I get what you mean but don't see why you installed RCBO's. This is one time a dual RCD board would have been better. This way at least half the circuits would stay on in the event of a neutral earth fault where as in the event of a neutral earth fault with that setup your loose the lot. You may as well of installed a 30mA upfront RCD and used circuit breakers...Would of been cheaper and still would of had the same affect. If I was your assessor you would have got a bemused look from me as well

 

[Wilko]

Lee I see you too have joined the Gorilla Appreciation Society ?

 

[Leesparkykent]

Lee I see you too have joined the Gorilla Appreciation Society ?

LOL, @GMES keeps changing my one....I never notice it but he normally tells me after its been up for a few days for everyone to see haha.

 

[Midwest]

Yeah I get what you mean but don't see why you installed RCBO's. This is one time a dual RCD board would have been better. This way at least half the circuits would stay on in the event of a neutral earth fault where as in the event of a neutral earth fault with that setup your loose the lot. You may as well of installed a 30mA upfront RCD and used circuit breakers...Would of been cheaper and still would of had the same affect. If I was your assessor you would have got a bemused look from me as well

I said I would get all confused again.

Surely even a dual RCD A3 CU, will require a 100mA S Type RCD (as main switch or upfront) on an installation forming part of a TT supply, as the S Type will provide fault protection for the single insulated conductors supplying the 30mA RCD's?

 

[DPG]

It's too early for all this. My brain hurts already.

 

[Wilko]

I said I would get all confused again.

Surely even a dual RCD A3 CU, will require a 100mA S Type RCD (as main switch or upfront) on an installation forming part of a TT supply, as the S Type will provide fault protection for the single insulated conductors supplying the 30mA RCD's?

Agree - this is the design flaw of the metal CU, that there are single insulated cables and connections inside an earthed metal box that only have the DNO fuse for protection, and if it's TT that won't melt real fast ... unless it's one of MY rod installs

 

[Ib≤In≤Iz]

I said I would get all confused again.

Surely even a dual RCD A3 CU, will require a 100mA S Type RCD (as main switch or upfront) on an installation forming part of a TT supply, as the S Type will provide fault protection for the single insulated conductors supplying the 30mA RCD's?

That's why I use RCBO's on TT. Don't personally like the idea of unprotected single insulated conductors supplying the 30mA RCD's. I know we still have the busbar and tails to think about, but the busbar is solidly in place and the chances of that falling out is slim and if the tails are secured well so are they. But leesparkykent does have a good point about loosing the whole board in a N-E fault.

 

[Midwest]

That's why I use RCBO's on TT. Don't personally like the idea of unprotected single insulated conductors supplying the 30mA RCD's. I know we still have the busbar and tails to think about, but the busbar is solidly in place and the chances of that falling out is slim and if the tails are secured well so are they. But leesparkykent does have a good point about loosing the whole board in a N-E fault.

The OSG gives guidance on TT supplies and use of RCBO's & RCD. Suitable use of glands would mitigate the tails shorting against the metal enclosure on entry. The issue with using RCD in such an enclosure, is the single insulated cable supplying each RCD, which may be damaged or pinched, and shorting with the metal enclosure. Using an up front S Type would provide fault protection in this case.

The issue with using SP RCBO's and upfront S Type, is in the case of a neutral-earth fault, is the S Type would eventually trip, but would then isolate the entire installation. Using dual RCD's, the same fault would only isolatate one half of the installation. Some guidance I received from my scheme Elecsa;

Good morning and thank you for your enquiry.

In regards To a TT and isolation of circuits regulation 537.2.1.1 must be taken into consideration. This regulation requires every circuit to be capable of being isolated from each of the supply conductors. (except in a TN-S or TNC-S)

This can be done various ways and it is permissible to isolate more than one circuit from a single device (GN2 section 4.3).

In the situation you describe with RCBO’s a double pole device will meet this requirement.

For further guidance refer to Guidance Notes 2 Isolation and Switching section 4 and the OSG as advised.

The diagram in the OSG that you mention (3.6.3.(iii)) is just one acceptable arrangement of RCD’s in a TT system. Other suitable arrangements are shown in 3.6.3.(ii) – (v).

It may be worth reading sections 2.2.5 & 2.2.6 (pages 20 & 21) in the OSG and also sections 3.6.1 – 3.6.3 (pages 30 – 32) and also section 3.6.3.4 on page 35 of the OSG.

It may also be worth reading regulation 531.4.1 & also regulation 537.1.2.

Some sections of BS7671 state the use of DP RCD’s, such as installations that fall under sections 705 & 708.

 

[telectrix]

The OSG gives guidance on TT supplies and use of RCBO's & RCD. Suitable use of glands would mitigate the tails shorting against the metal enclosure on entry. The issue with using RCD in such an enclosure, is the single insulated cable supplying each RCD, which may be damaged or pinched, and shorting with the metal enclosure. Using an up front S Type would provide fault protection in this case.

The issue with using SP RCBO's and upfront S Type, is in the case of a neutral-earth fault, is the S Type would eventually trip, but would then isolate the entire installation. Using dual RCD's, the same fault would only isolatate one half of the installation. Some guidance I received from my scheme Elecsa;

Good morning and thank you for your enquiry.

In regards To a TT and isolation of circuits regulation 537.2.1.1 must be taken into consideration. This regulation requires every circuit to be capable of being isolated from each of the supply conductors. (except in a TN-S or TNC-S)

This can be done various ways and it is permissible to isolate more than one circuit from a single device (GN2 section 4.3).

In the situation you describe with RCBO’s a double pole device will meet this requirement.

For further guidance refer to Guidance Notes 2 Isolation and Switching section 4 and the OSG as advised.

The diagram in the OSG that you mention (3.6.3.(iii)) is just one acceptable arrangement of RCD’s in a TT system. Other suitable arrangements are shown in 3.6.3.(ii) – (v).

It may be worth reading sections 2.2.5 & 2.2.6 (pages 20 & 21) in the OSG and also sections 3.6.1 – 3.6.3 (pages 30 – 32) and also section 3.6.3.4 on page 35 of the OSG.

It may also be worth reading regulation 531.4.1 & also regulation 537.1.2.

Some sections of BS7671 state the use of DP RCD’s, such as installations that fall under sections 705 & 708.

BORING.BORING.

 

[telectrix]

CAN'T KEEP MY EYES ON THE CIRCLING SKIES.

 

[Midwest]

CAN'T KEEP MY EYES ON THE CIRCLING SKIES.

Sure its not a wake of vultures circling above your head, waiting for the right time to pounce

 

[Leesparkykent]

I said I would get all confused again.

Surely even a dual RCD A3 CU, will require a 100mA S Type RCD (as main switch or upfront) on an installation forming part of a TT supply, as the S Type will provide fault protection for the single insulated conductors supplying the 30mA RCD's?

Yeah but the point I was making was the RCBO and upfront combo in the event of a neutral earth fault they would ending up loosing the lot as the up front RCD would still trip as the Wylex RCBO's are solid neutral. So it would be no different having a 30mA up front as you they would still loose the lot. That's why with TT its really RCBO's with no up front RCD or a dual RCD board with upfront RCD.

 

[Midwest]

Yeah but the point I was making was the RCBO and upfront combo in the event of a neutral earth fault they would ending up loosing the lot as the up front RCD would still trip as the Wylex RCBO's are solid neutral. So it would be no different having a 30mA up front as you they would still loose the lot. That's why with TT its really RCBO's with no up front RCD or a dual RCD board with upfront RCD.

I did allude to that in my #54, perhaps it was too boring

 

[Ib≤In≤Iz]

I'm going to start using dual RCD boards with 100mA s type upfront or DP RCBO's from now on on TT. Thanks @Leesparkykent for that, we learn something every day