SWA armoured to a shed/workshop

[Trefor]

Hi
Many years ago I was an electrician (well ok nearly 40 years ago actually)!

Now I have built a workshop in my garden, having calculated the power requirement I have decided to install 10mm SWA (I don't really need that size as 6mm would be ok, but to give some leeway.....). Now because of the current regulations I have asked a local electrician to connect and test the installation, however I intend to dig the trench and lay the cable.

When I was an electrician I'm sure that this was done with two core SWA, but I'm told that I need to install 3 core SWA nowadays with the the third core as an earth. I can't get hold of 'my' electrician to check with him as he is on his hols. Can anyone advise me on this matter?

 

[lxboy]

Yes, this is the advised method.

Regards,

Lxboy

 

[bernardgreen]

It may not be appropriate to export the earth from the house to the shed.

In some circumstances the "earth" in the house should not be exported. In these situations the outbuilding is supplied with ONLY live and neutral via an RCD at point of entry to the outbuilding. The earth ( CPC ) in the outbuilding being connected to an effective ground rod. The steel wire armour is connected to the "earth" from the house but must NOT be connected to the CPC in the outbuilding. Accidental connection to the SWA in the outbuilding should be prevented.

 

[Trefor]

Thanks to you both for your help.

Quick question, if you don't mind. I thought that I shouldn't earth the shed/workshop to the house only if there was another metal connection, i.e a water pipe, providing an earth. Is this what you mean? In the case of my installation the only outside connection will be the SWA so I was going to connect to the house earthing.

It may not be appropriate to export the earth from the house to the shed.

In some circumstances the "earth" in the house should not be exported. In these situations the outbuilding is supplied with ONLY live and neutral via an RCD at point of entry to the outbuilding. The earth ( CPC ) in the outbuilding being connected to an effective ground rod. The steel wire armour is connected to the "earth" from the house but must NOT be connected to the CPC in the outbuilding. Accidental connection to the SWA in the outbuilding should be prevented.

 

[lxboy]

yes, you are correct. If you have no other services i.e., water supplies you can export the installation earth.

Regards,

lxboy

 

[JohnW2]

When I was an electrician I'm sure that this was done with two core SWA, but I'm told that I need to install 3 core SWA nowadays with the the third core as an earth. I can't get hold of 'my' electrician to check with him as he is on his hols. Can anyone advise me on this matter?

Despite what you've been told here, it would seem that many, probably most, electricians still favour what you were doing 40 years ago (regarding use of a third core as an 'unnecessary waste'/expense). That 'old practice' is certainly still compliant with the regs. The fact that it is extremely difficult (if not impossible) to find 3-core SWA with blue/bown/G-Y cores suggests that the manufacturers don't think that many people will be using 3-core for single phase supplies. For most common sizes of SWA, the armour represents a copper equivalent CSA appreciably greater than would be provided by a third core.

Kind Regards, John

 

[ericmark]

I seem to remember there was a paper on this posted on IET website.

I have a chart which I think came from this listing the required CSA and the actual CSA of SWA cable.

Up to around 70 mm² the SWA is of a large enough CSA to be used but after that point depending on manufacturer and number of cores the CSA of SWA cable is often not enough to use it as the earth.

Clearly this will not affect what you are doing but it is because of this that it is often reported that you need a copper core as an earth. For 10mm you need a CSA of the SWA to be 22.6 mm² and it is around 41 mm² for two core.

There is another point often raised that should the outer cover be damaged then the SWA could be compromised due to rust. Where ELI readings are taken and kept at each EICR then any damage will likely to be obvious but where records are not kept or the prescribed tests are not carried out then there could be a problem.

However to not do something because some one in the future may do something non compliant would mean we can do very little so I can't see this argument holding much water. Where I have found damage to SWA then only option is either renew or go to TT earthing. Due to thick concrete I have been forced into latter option.

 

[Trefor]

Uhm so as I understand the posts, it is reccomended that a third core be used as an earth but it is not a regulation, at least not when using 10mm SWA. Most/many(?) electricians would still use the armouring as the earth connection.

Maybe my best bet is to wait until 'my' electrician is back from his hols and ask him what he prefers after all he will be testing the insatllation!

Thanks to everybody for your help.

 

[JohnW2]

I have a chart which I think came from this listing the required CSA and the actual CSA of SWA cable.... Up to around 70 mm² the SWA is of a large enough CSA to be used but after that point depending on manufacturer and number of cores the CSA of SWA cable is often not enough to use it as the earth.

Indeed. There are such tabulations in this forum's wiki ( click here ).

Kind Regards, John

 

[JohnW2]

Uhm so as I understand the posts, it is reccomended that a third core be used as an earth but it is not a regulation, at least not when using 10mm SWA. Most/many(?) electricians would still use the armouring as the earth connection.

It (using a third core) has been 'recommended' (described as 'the advised method') by one person who has replied to you, but, as you say, I think you'll find that many, quite probably most, electricians take the opposite view. As you say, it's the view of 'your' electrician which really matters - but I don't think he would be able to argue that use of the armour as CPC would be non-compliant with the regs (or even contrary to 'common practice'), even if he personally 'favours' use of a third core.

Kind Regards, John

 

[bernardgreen]

The SWA armour is steel and the fittings are brass ( or other non ferrous metal ) so special attention is needed when making connections to the armour if the safety "earth" of the system relies on those connections.

Steel rusts, SWA laid underground with a damaged outer sheath may over a few years loose continutiy along the armour.

With temperature changes the brass and steel have different expansion so a incorrectly made mechanical joint steel to brass will loosen faster than a brass to copper joint.

 

[Trefor]

Uhm its all very interesting.

One point looking at the tables that were posted I note that for 10mm SWA XPLE the CSA of the armouring is only 26mm^2 and should be 31.09 ^2 it is therefore not compliant. The supplier, that I'm thinking of using, for the SWA says that all of his cable is XLPE is this the same (a missprint maybe?). If this is the case I guess I would have to install 3 core.

 

[lxboy]

Yes.

Lxboy

 

[Risteard]

Uhm its all very interesting.

One point looking at the tables that were posted I note that for 10mm SWA XPLE the CSA of the armouring is only 26mm^2 and should be 31.09 ^2 it is therefore not compliant. The supplier, that I'm thinking of using, for the SWA says that all of his cable is XLPE is this the same (a missprint maybe?). If this is the case I guess I would have to install 3 core.

It's difficult to get anything other than XLPE nowadays. However, I'm sure that if you apply the adiabatic equation you will find that it is fine. I have no problem with using 2-core 10mm squared SWA. That's what 2-core SWA is for, after all!

 

[Trefor]

Thanks I'm now intrigued by the adiabatic equation . Back to the books , well ok probably the internet.

Thanks to you all for your help.