UK Class II enclosure + gland recommendation for divorcing SWA armour at TT outbuilding/shed?

[rossinator]

As the title says, I am looking for a recommendation for an enclosure and gland arrangement to electrically isolate or divorce the earth/armouring of a two core SWA supplying an outbuilding/shed. The supply to the property is overhead TT, so DNO provide phase and neutral only, a sub 20 ohm rod provides the main earth for the house DB. The DB in the wooden outbuilding/shed (which contains no extraneous metal or services) is supplied from the main house DB by a 2 core SWA. The shed DB which is two way with RCD main switch is currently using the main house rod as it's earth, utilising the armour of the SWA as it's cpc. The plan is to give the shed it's own low ohm rod and divorce the earth from the house at the outbuilding end, whilst leaving the armour for the SWA connected to earth at the house end. The following webpage and image sums up the situation well. The recommendation I am looking for is the part of the image below that is labelled "All insulated Class II enclosure". I've read that some people just bung the SWA in a stuffing gland in a wiska. I asked Wiska to recommend something and the said they couldn't offer a suitable Class II product and suggested Pratley. So here I am asking for recomendations for a strong reliable product / solution? The shed DB is metal. Many thanks.

Voltimum - armoured cable supplies to outbuildings

 

[suffolkspark]

Why do you want to divorce the existing earthing arrangement out of interest?

 

[pc1966]

As above, if the main house is already a TT arrangement why do you feel it needs a separate earth at the out-building?

Adding another earth rod at the out building is not a bad idea, but it can happily be connected to the house's own rod via the SWA armour.

 

[pc1966]

Of course the feed to the out-building needs to meet ADS so realistically it has to be RCD protected. That could be off the main house incomer if its something like 100mA delay and MCB for over current protection, then the out building can have weak earth fault selectivity via 30mA RCD/RCBO. It is never going to have high current selectivity though if fed from usual MCB, but probably good enough for most purposes.

Or it could come off a 30mA RCD/RCBO but no selectivity at the outbuilding. I would strongly advise not taking it off a dual RCD or similar arrangement as then an outbuilding fault will take out half the house's supplies!

 

[littlespark]

To answer the question in the OP without asking why it’s necessary…. Any old plastic adaptable box between the db and the cable gland will suffice. Just don’t connect the brass gland to anything.

 

[brianmoooore]

Don't see the point of this if the supply is TT, but going from a TNC-S supply to a remote building that is going to have TT earthing, then I wouldn't use a brass gland at all. If it's connected to the TNC-S earth via the SWA armour, it has the potential to become an exposed live terminal in the TT environment.

 

[pc1966]

If I were doing this (irrespective of the arguments as to why) I would take the SWA in through a plastic IP68 style gland and inside tape up the end to cover the armour wires with self-amalgamating tape or similar. Of course those glands don't grip as strongly as SWA ones so you really do need to make sure the cable is properly cleated in place.

 

[brianmoooore]

If I were doing this (irrespective of the arguments as to why) I would take the SWA in through a plastic IP68 style gland and inside tape up the end to cover the armour wires with self-amalgamating tape or similar. Of course those glands don't grip as strongly as SWA ones so you really do need to make sure the cable is properly cleated in place.

That's exactly how I've done it, although it can be a challenge to find a gland big enough sometimes.

 

[pc1966]

I found CEF has some that will do larger SWA sizes. This link ought to show the 40/50/63mm hole fitting examples:

Buy Pvc Cable Glands | Cable Accessories | CEF

Manufactured to British standards and designed to offer the same level of performance over a wide temperature range, our range of PVC cable glands are available in a variety of finishes and are competitively priced.

www.cef.co.uk

They cover cables from 22mm to 44mm OD so a fair range of SWA.

 

[rossinator]

Thank you all for the thoughts, amazing. To clarify things, the house is TT (not TN-C-S) and outbuilding/shed also TT. House is currently on a re-wireable fuse board with a standalone upsteam 30mA RCD protecting the whole installation, so no selectivity. Outbuilding is old skool metal Wylex with power and lighting on MCB's. New DB's are planned for both house and outbuilding. The house is currently destined for a full rcbo board with SPD. It will have double pole RCBO's and a built-in Type 1+2 SPD as required to cover the overhead TT supply. For a belt and braces approach the main switch will also be a time delayed 100mA RCD to protect any non-RCD wiring such as the wiring/MCB in the db for the SPD.

The outbuilding currently only has power and lighting circuits, but there is potential to expand in the future. To enable selectivity, the main switch on the new shed DB was going to be a 30mA RCD with individual MCB's for the shed's circuits. The 2-core SWA supply to the shed DB was going to be put on it's own MCB on the house DB, rather than an RCBO. House and shed are only about ten meters apart if it makes a difference.

The diameter of the SWA is only 20mm, so a stuffing gland in an enclosure with armour wrapped/heat-shrunk would not be too taxing, but as mentioned, cable needs to be properly secured.

The reason for divorcing the earth was to avoid a potential difference between the outbuilding and the house. Maybe this isn't required... Maybe the shed doesn't need it's own rod and as suggested the outbuilding just use the house rod through the SWA armour. I guess I'm keen to aim to make the house and shed installs as safe as possible. Maybe the optimum is a rod at the shed in addition to the house and leave both DB's connected to both rods through the SWA armour. Your thoughts are appreciated, aiming for safe as possible especially given pets in vicinity...?

 

[LewisM]

I've done it like this in the past.

 

[pc1966]

Thank you all for the thoughts, amazing. To clarify things, the house is TT (not TN-C-S) and outbuilding/shed also TT. House is currently on a re-wireable fuse board with a standalone upsteam 30mA RCD protecting the whole installation, so no selectivity. Outbuilding is old skool metal Wylex with power and lighting on MCB's. New DB's are planned for both

OK. But if you have a separate 30mA RCD/RCBO feed for the outbuilding that Wylex board could stay.

house and outbuilding. The house is currently destined for a full rcbo board with SPD. It will have double pole RCBO's and a built-in Type 1+2 SPD as required to cover the overhead TT supply. For a belt and braces approach the main switch will also be a time delayed 100mA RCD to protect any non-RCD wiring such as the wiring/MCB in the db for the SPD.

That is a sound approach. Personally I would use an up-front RCD as you suggest since it means there is no single point of failure in terms of electronics for earth-fault disconnection. OK you might lose the 30mA 'additional' protection if an RCBO fails but you don't risk the whole CPC system becoming live since the incomer will trip.

The outbuilding currently only has power and lighting circuits, but there is potential to expand in the future. To enable selectivity, the main switch on the new shed DB was going to be a 30mA RCD with individual MCB's for the shed's circuits. The 2-core SWA supply to the shed DB was going to be put on it's own MCB on the house DB, rather than an RCBO. House and shed are only about ten meters apart if it makes a difference.

If the out building needs are really simply just feed the sub-main from a 20A RCBO for the sockets and take the lights off with a 3A FCU. No need for a 2nd CU then!

If you plan on using it as a workshop or similar where loss of lights is a safety issue then consider putting in one or more light that has built-in emergency power (often available for baton lights and the similar anti-corrosive lights).

If you do seek selectivity then your proposal of taking it off the house CU's 100mA protected supply is fine, but then use RCBO in the outbuilding CU otherwise you might as well go with the RCBO+FCU suggestion as trip due to sockets also takes out lights!

Getting over-current selectivity is hard when fed from a MCB, you can get some of the way there by using a MCB with as high a rating as the SWA supports and going for a D-curve to push up the "instant" trip point, since your earth fault disconnection is on the delay RCD anyway.

Personally unless you need anything special I would just go with the RCBO feed and FCU approach. You can use an adaptable box for the SWA gland and then some plastic glands for T&E out of there, a few companies make ones with oval slots for that job, or if using metalclad sockets then gland directly to one of them, etc.

The diameter of the SWA is only 20mm, so a stuffing gland in an enclosure with armour wrapped/heat-shrunk would not be too taxing, but as mentioned, cable needs to be properly secured.

OK.

The reason for divorcing the earth was to avoid a potential difference between the outbuilding and the house. Maybe this isn't required... Maybe the shed doesn't need it's own rod and as suggested the outbuilding just use the house rod through the SWA armour. I guess I'm keen to aim to make the house and shed installs as safe as possible. Maybe the optimum is a rod at the shed in addition to the house and leave both DB's connected to both rods through the SWA armour. Your thoughts are appreciated, aiming for safe as possible especially given pets in vicinity...?

Having more than one rod for TT is a good plan, more reliable and lower Ra so no risk of failing to disconnect if fault during drought spell when Ra rises. I would simply link both via the armour but include a 2nd rod. In fact linking the earths is the best way to reduce any difference!

Probably the biggest thing you can do towards greater safety beyond the delay RCD incomer is regular trip-testing of the RCD & RCBO!

 

[davesparks]

The reason for divorcing the earth was to avoid a potential difference between the outbuilding and the house.

But by doing that you would be increasing the chances of a potential difference!

 

[rossinator]

Having considered everyone's input, it's looking like the house and the outbuilding are no longer getting a divorce and are going to live happily ever after.

There will be a rods for both the house and the outbuilding with both connected together via the SWA armour.

I will be considering the various merits/costs of these three options:
(a) removing the second DB from the outbuilding and putting lighting on 3A FCU
(b) a standard inexpensive garage board (30mA RCD main switch with MBC's like Fusebox F100GRA @ £33 ) vs
(c) a mini rcbo board (such as Fusebox F2004M @ £31 + a couple of type A double pole RCBOS's @ £13 + blanks which for £57 total would leave a couple of ways for future expansion).

I've yet to identify what csa the SWA is, the diameter of the 2 core cable including the outer sheath is 20mm which is pointing to 16mm2 but until the outbuilding needs more than sockets and lighting, thinking the supply will be run from a 20A MCB/RCBO.

 

[pc1966]

2C 10mm and 16mm SWA are not that different from the outside, but once you get the ends visible (and safe) then a check with callipers might answer it, or you might find the rating embossed along the site of the cable.

Both are quite capable on the current carrying capacity front, but maybe needed for voltage drop? What is the approximate cable route distance from house to outbuilding?

You are not going to overheat them at 20A supply (I think even 1.5mm is just about OK for that) but if you have future plans that might involve a bigger load then knowing what margins you have is useful.