cabling swa to cu from outhouse

[jimshall]

1. I am replacing the cable from my house CU to an outhouse which involves a short (3m) external cable run along walls from the house loft. I will use 10mm SWA for this but wonder whether to connect it to 10mm T/E via a junction box in the loft for the several metres run along joists and drop into the house CU, or alternatively use SWA for the complete run. I haven't worked with this size SWA before and am worried that it is very difficult to work, T/E of this size is bad enough.
2. The outhouse is currently supplied from an MCB in the house CU. I am intending to put a new CU in the outhouse with its own RCD and MCB's. Is this ok or should I see if I can take a new tail to supply the outhouse direct from the house supply?
Thanks for any advice, Jim.

 

[Taylortwocities]

I would run 10mm SWA all the way and finish it off with a SWA gland into a metal adaptable box butted up to the CU. Strip back a good long length of the SWA cores and connect directly into the breaker in the CU. Keep the joints down to zero if you can.

You MUST NOT do what you describe in 2. The cable must be protected by a suitably sized fuse/contact breaker. Otherwise the only thing between your new bit of 10mm SWA and the substation is the DNO's service fuse.
A 100A BS88 fuse makes one hell of a bang (do not try this at home children).

 

[ban-all-sheds]

You can do (2) if after the split you take the tails into a switchfuse, and the SWA from there.

In many ways it's the best solution as it completely removes the outbuilding supply from the house CU.

What are you going to do about testing?

 

[ColinJacobson]

My cousin did a similar exercise and these three options came up.

1. Have the outhouse SWA cable off a non-RCD-protected mcb at the house CU and have a small CU with an RCD in the outhouse. Similar to what Taylortwocities said, but with a CU and RCD in the outhouse. There is no influence from other circuits. The only negative point is that the SWA cable is not RCD protected.

2. For full protection, for the outdoor cable and outhouse, fit an RCD at the CU only connected to the SWA cable and a simple small CU with no RCD in the outhouse. The whole lot, outhouse and outdoor cable, is then protected with no influence from other circuits. If a problem in the outhouse then the house RCD at the house CU will need reseting, so not independent and confined to the outhouse.

3. For full independent and self-contained outhouse protection - the outdoor cable and outhouse - fit a lower acting RCD at the CU connected to the SWA cable and a simple small CU and a faster acting RCD. The whole lot, outhouse and outdoor cable, is then protected with no influence from other circuits. If a problem in the outhouse, the outhouse RCD trips before the house RCD.

I would be inclined to do No. 2, if an RCD can easily be fitted at the house CU, if not then No. 1.

 

[1john]

My cousin did a similar exercise and these three options came up.

1. Have the outhouse SWA cable off a non-RCD-protected mcb at the house CU and have a small CU with an RCD in the outhouse. Similar to what Taylortwocities said, but with a CU and RCD in the outhouse. There is no influence from other circuits. The only negative point is that the SWA cable is not RCD protected.

2. For full protection, for the outdoor cable and outhouse, fit an RCD at the CU only connected to the SWA cable and a simple small CU with no RCD in the outhouse. The whole lot, outhouse and outdoor cable, is then protected with no influence from other circuits. If a problem in the outhouse then the house RCD at the house CU will need reseting, so not independent and confined to the outhouse.

3. For full independent and self-contained outhouse protection - the outdoor cable and outhouse - fit a lower acting RCD at the CU connected to the SWA cable and a simple small CU and a faster acting RCD. The whole lot, outhouse and outdoor cable, is then protected with no influence from other circuits. If a problem in the outhouse, the outhouse RCD trips before the house RCD.

I would be inclined to do No. 2, if an RCD can easily be fitted at the house CU, if not then No. 1.

You would be inclined to do an awful inconvenient install, judging by you other posts, it is quite safe to say that what you would do should be ignored.

scenario 1 - why is it a negative to have an SWA cable without an RCD protecting it??

scenario 2 - why is it safer to have an RCD to reset in the house when you are in an outbuiling when it operates?

scenario 3 - what RCD would you fit at the consumer unit end and what RCD would you fit at the out building end?

 

[ColinJacobson]

scenario 1 - why is it a negative to have an SWA cable without an RCD protecting it??

The reason for having an RCD is obvious.

scenario 2 - why is it safer to have an RCD to reset in the house when you are in an outbuiling when it operates?

It is not safer. It is just that the outhouse is not self-contained. The RCD is in another building.

scenario 3 - what RCD would you fit at the consumer unit end and what RCD would you fit at the out building end?

A slower acting RCD at the CU and faster reacting RCD at the outbuilding. That is a 100mA trip time delayed RCD at the house CU and a faster 30mA trip RCD at the outhouse CU.

 

[Chri5]

But then the rcd on the house board is unlikely to be exclusive to the outbuilding, and having a board with MCB's covering the house via a 100mA rcd could be fatal.

The cleanest way is to make the swa a seperate service. So DNo head supply, henley block, a set of tails to the existing board, a set of tails to a metal clad sub board with 100 mA rcd and suitable rated MCB.

The swa then gets terminated via the sub board (and being metal allows for banjo fixing to earth the armour) and feeds out to the outbuilding CU with 30mA rcd + mcb's or to a board with x number of 30mA rcbo's

 

[ban-all-sheds]

If a problem in the outhouse, the outhouse RCD trips before the house RCD.

And then a few ms later, if it's a neutral fault and you've used SP breakers, the house RCD trips as well.

[EDIT] - for the benefit of any future readers, this isn't true, but clearly CJ didn't realise it. [/EDIT]

I would be inclined to do No. 2, if an RCD can easily be fitted at the house CU

It's pretty unlikely that the house CU is going to be structured in a way that it could support an extra independent RCD and have the necessary spare 3-module space in the right place.

The reason for having an RCD is obvious.

Please explain what the obvious reason for having an RCD protecting armoured cable is, because I'm sure that it isn't obvious to many people here.

< sensible opinions are fine but only the regs matter >

And please tell us which regulation(s) require such an RCD.

It is not safer. It is just that the outhouse is not self-contained. The RCD is in another building.

Given that you said that would be your preferred option will you please explain how preferring an RCD in a separate building is compliant with 314.1 (i).

A slower acting RCD at the CU and faster reacting RCD at the outbuilding. That is a 100mA trip time delayed RCD at the house CU and a faster 30mA trip RCD at the outhouse CU.

Will you please explain what types of fault a 100mA delayed RCD at the house will clear, and who and/or what will be protected against what risks by doing that.

 

[ColinJacobson]

But then the rcd on the house board is unlikely to be exclusive to the outbuilding, and having a board with MCB's covering the house via a 100mA rcd could be fatal.

The point was a dedicated RCD for the outhouse.

The cleanest way is to make the swa a seperate service. So DNo head supply, henley block, a set of tails to the existing board, a set of tails to a metal clad sub board with 100 mA rcd and suitable rated MCB.

The swa then gets terminated via the sub board (and being metal allows for banjo fixing to earth the armour) and feeds out to the outbuilding CU with 30mA rcd + mcb's or to a board with x number of 30mA rcbo's

Yep, I get that. If there is a newish CU there may be space on the DIN rail for dedicated RCD for the outhouse. If an old CU then as you say, fit a separate smaller box for a RCD and mcb.

It is probably better to have DP RCBOs on the outhouse CU.

Why metal clad? Can't banjo fittings be fitted into plastic?

It is nice to get a sensible response.

 

[ColinJacobson]

And then a few ms later,

All your concerns have been addressed by other posts.

 

[ban-all-sheds]

Actually they haven't.

You have decided that you should be giving advice on electrical installation design, so it is reasonable to ask that you show why it is good advice by addressing concerns raised and by explaining how your design ideas are safe and comply with the regulations.

If you are unable to do those things then you should not be giving advice because you are not competent to do so.

And then a few ms later, if it's a neutral fault and you've used SP breakers, the house RCD trips as well.

Please show us where you have addressed that concern. Even if that RCD does not cover any house circuits it is still in the house, so you'll still have a remote RCD tripping.

The time delay will not stop it tripping if the fault in the outbuilding is a neutral one and the breakers there are SP.

[EDIT] - for the benefit of any future readers, this isn't true, but clearly CJ didn't realise it. [/EDIT]

It's pretty unlikely that the house CU is going to be structured in a way that it could support an extra independent RCD and have the necessary spare 3-module space in the right place.

Whether you continue to ignore that or not it will still be true.

Please explain what the obvious reason for having an RCD protecting armoured cable is, because I'm sure that it isn't obvious to many people here.

And please tell us which regulation(s) require such an RCD.

Those questions have not been answered by you in previous posts, so will you please answer them now.

Given that you said that would be your preferred option will you please explain how preferring an RCD in a separate building is compliant with 314.1 (i).

Since no-one else has mentioned that particular regulation it is clearly utterly impossible for you to have addressed it in other posts.

Will you please now explain how your preferred design would comply with the regulations.

Will you please explain what types of fault a 100mA delayed RCD at the house will clear, and who and/or what will be protected against what risks by doing that.

You have not provided that information in previous posts.

Will you do so now, please.

 

[ColinJacobson]

You have decided that you should be giving advice on electrical installation design,

I gave the options given by a superb elec engineer to th e same scenario. He was not a spark, but the designer of big stuff.

The rest of your concerns were addressed in the thread by many knowledgable people.

 

[mfarrow]

The time delay will not stop it tripping if the fault in the outbuilding is a neutral one and the breakers there are SP.

That's only a problem if he installs SP RCBOs as protection. Colin/OP is describing an RCD in the outhouse, so if if that said RCD is a DP RCD acting as the main switch then there will be none of the problems you describe.

 

[ban-all-sheds]

That's only a problem if he installs SP RCBOs as protection. Colin/OP is describing an RCD in the outhouse, so if if that said RCD is a DP RCD acting as the main switch then there will be none of the problems you describe.

Quite. Good to see that some people are confident of their knowledge

 

[ban-all-sheds]

I gave the options given by a superb elec engineer to th e same scenario. He was not a spark, but the designer of big stuff.

But you don't appear to actually understand it, or to be able to answer questions relating to it. Which since some of it is highly questionable is a bit of a drawback.

The rest of your concerns were addressed in the thread by many knowledgable people.

How many more times are you going to make that same patently false claim? (Assuming, FGS, you're talking about this thread, not another one of which we have no visibility...).

Please explain what the obvious reason for having an RCD protecting armoured cable is, because I'm sure that it isn't obvious to many people here.

And please tell us which regulation(s) require such an RCD.

Where have you, or the superb electrical engineer, or many knowledgeable people answered those?

Given that you said that would be your preferred option will you please explain how preferring an RCD in a separate building is compliant with 314.1 (i).

Where have you, or the superb electrical engineer, or many knowledgeable people explained that? And, OOI, who else asked the question?

Will you please explain what types of fault a 100mA delayed RCD at the house will clear, and who and/or what will be protected against what risks by doing that.

Where have you, or the superb electrical engineer, or many knowledgeable people answered those?