Shed Supply on TNCS/PME

[Lewis Ashton]

Hi,

I know this is a very commonly debated topic, but I have never found a solution agreed by a majority.

I am in the process of building a wooden shed, approximately 20metres from the house (up to 30 metre cable run to the CU and meter cut out). I have a PME/TNCS supply to the property and no extraneous metallic services/structures in the shed. I would like to install a handful of IP rated sockets both inside and to the exterior of the shed, along with an interior LED light and a couple of LED floodlights/bulkhead lights outside (it will be next to my patio/seating area).

My proposal is to install a 40A RCBO, supplying a glanded SWA cable (earth connected to house CU earth). The SWA will be ducted (to allow replacement-I'm thinking of the long term as I don't intend on moving again, and to allow provision of other services possibly). This SWA would then enter the shed and terminate in to a garage type CU with RCD mainswitch, one 20a MCB for sockets and a 6a MCB for my lighting requirements. I propose not to connect the earth of the SWA to the shed CU so as not to export the earth, but instead use a sufficiently deep enough earth rod (depth dependent on testing of course) and connect this to the shed CU via a 10mm earth cable to provide a local TT arrangement.

My question is, does this seem the safest way of installing such a shed supply? I realise that most people wouldn't duct the SWA, etc. but as I am doing the garden up, it's my own house and I do not intend on moving, I wish it to be future proof (able to replace SWA if ever necessary, etc.) Likewise, I would prefer to have a separate CU in the shed to allow future increased loads, although initially there will be no, "heavy duty" workshop tools/high current loads.

Would I be better using a time delay RCBO at the house to allow discrimination between that and the house RCD? Would I be best terminating the SWA in to a plastic enclosure prior to the shed CU to ensure the SW does not touch and
innadvertantly earth the shed CU?

I'm used to working on industrial, rolling stock based installations, rather than domestic installations, so please excuse any question that may seem obvious!

 

[Spoon]

the railways are quite strict on the old drugs and alcohol!!

Always thought it would be interesting work, working on the railways. Is it?

 

[DPG]

Using a good reliable existing earth sounds better than a hopeful earth rod banged in next to the shed.

 

[Lewis Ashton]

Always thought it would be interesting work, working on the railways. Is it?

I work on the Ballast Cleaning System; it's a massive train that excavates the track ballast, screens it the poor ballast and puts the ballast and track back, adding any additional ballast as required. It's a pretty impressive machine and one of the longest trains on the network (over half a mile long!) I love my job, but hate living out of a hotel and the often unsocial and long working hours.

Using a good reliable existing earth sounds better than a hopeful earth rod banged in next to the shed.

I always thought that DNO's took issue with exporting of the earth. I had planned on testing it of course. I would prefer to use the existing earth if possible, as you rightly say, it would be better to use a known good earth.

 

[davesparks]

Presumably this is going to be a minimum of 10mm SWA, so why not just install a 3 core and use the DNO provided earth?

10mm 3 core gives you a cpc large enough to provide bonding to anything that may need it now or in the future.

 

[davesparks]

I always thought that DNO's took issue with exporting of the earth. I had planned on testing it of course. I would prefer to use the existing earth if possible, as you rightly say, it would be better to use a known good earth.

You can extend the equipotential zone to wherever you like, as long as you do it properly there is no problem.

 

[Deleted member 9648]

It's staggering that the myth about sheds and PME/TNCS still persists.

 

[Lewis Ashton]

Thanks Davesparks, that had been my preferred method if at all possible. As luck would have it, I spoke with a friend who works for the local DNO and he has confirmed exactly what you have just mentioned. I also spoke with another friend who has some 4-core 16mm SWA and 110mm twinwall ducting as an off-cut (he works on large private network installations and 25m is an "off cut" for them. I know the ducting is a bit over kill, but it's free, so why not!?

My intention is to use a none RCD protected MCB currently in the CU, utilising the 16mm SWA to supply an RCD-protected, small garage type consumer unit with a MCB for the sockets, and a MCB for the lighting. I will use the 3rd core as suggested. Hopefully I can take the SWA straight to the house CU, but I have a feeling that I will need to terminate the SWA and continue the supply in T+E back to the CU. Unfortunately, the house's CU is back in the centre of the house in an understairs cupboard.

It's staggering that the myth about sheds and PME/TNCS still persists.

Working on rolling stock, I have never really had much need to investigate this myth, I just remember hearing it mentioned on these sorts of forums in yesrs gone by.

 

[Dave OCD]

Well with a 4 core 16mm SWA Lewis in the case of your job you could have a 32mm2 CPC if you wanted !

 

[Lewis Ashton]

Well with a 4 core 16mm SWA Lewis in the case of your job you could have a 32mm2 CPC if you wanted !

Indeed Dave OCD! Might as well use it if it's available! Should certainly provide a much better solution than the earth rod.

Thank you everyone for your help and input!

 

[davesparks]

My intention is to use a none RCD protected MCB currently in the CU, utilising the 16mm SWA to supply an RCD-protected, small garage type consumer unit with a MCB for the sockets, and a MCB for the lighting. I will use the 3rd core as suggested. Hopefully I can take the SWA straight to the house CU, but I have a feeling that I will need to terminate the SWA and continue the supply in T+E back to the CU. Unfortunately, the house's CU is back in the centre of the house in an understairs cupboard.

[\QUOTE]

Converting to T&E may put you back at square one with the need for an RCD for the submain depending on how it is installed/routed.

The 'best' way to do it, in my opinion, would be to run the SWA back to the CU location and fit a switch fuse to protect it rather than an mcb.

 

[Lewis Ashton]

My thoughts exactly Dave. The more I look at the cable route through the house, the more I think it will be difficult. Unfortunately, the downstairs floor is solid concrete though. I should be able to SWA right up to the external wall and then terminate in to an IP box and run T+E under the upstairs floor and back down to the top of the cupboard where the CU is located. I am able to install it in trunking/conduit.

Alternatively there is a redundant piece of SWA which runs from the adjacent wall to the C.U., across the garage ceiling joists, down the side of the garage, under the decking to an outside IP box to provide a supply to the conservatory (I don't know why they did it that way either, so don't ask!!) That's now redundant now the conservatory is on the ring, so I could potentially use that, depending on it's size, integrity, etc. but it's routing is not ideal (looks like it's just chucked straight under the decking!)

I shall investigate further when I'm home, and let you all know the outcome/finished product.

 

[buzzlightyear]

I shall investigate further when I'm home, and let you all know the outcome/finished product.

post a video or pictures when finished from start to the end product .good luck .

 

[Top Cat]

Basic requirements for earthing and bonding
Where an installation serves more than one building, main protective bonding is required in each building served, except for a building where there are no extraneous-conductive-parts, such as metallic water, gas or oil installation pipes (Regulation 411.3.1.2 refers).

Option 1: Extending the earthing and bonding system

The image shows an example of the earthing and bonding arrangements where the installation in a house is extended to serve a detached outbuilding. A Building Earth Marshalling Terminal (BEMT) is usually provided in the outbuilding to marshal together the main bonding conductor(s), if any, and the circuit protective conductors (cpcs) in that building. The BEMT must be connected to the MET by:

• the cpc of the distribution circuit supplying the outbuilding, and

• a main bonding conductor, if there are any extraneous-conductive-parts at the outbuilding.

As shown, the functions of the cpc and main bonding conductor (if any) may be combined in a single protective conductor having a cross sectional area (csa) meeting the requirements for both functions.



Option 2: Making the installation in the detached outbuilding part of a TT system with its own installation earth electrode and MET

The image shows an example of the earthing and bonding arrangements where an installation in a house supplies an installation in a detached outbuilding, forming part of a TT system with its own installation earth electrode and MET. It is desirable for there to be no connection between the earthing and bonding arrangement of the installation in the outbuilding and that of the installation in the house (such as through a protective conductor or shared metallic pipeline), as shown.

Note: Although both options show an earthing facility provided by the distributor being used as the means of earthing (TN-S or TN-C-S system), the earthing conductor could be connected to an installation earth electrode instead (TT system).



Table 1 – Sizing requirements for cpc and main bonding conductor

Note. The requirements of Regulation 544.1.1, referred to in Table 1, can result in a main bonding conductor size greater than that of the live conductors or cpc of the distribution circuit supplying the outbuilding, particularly where PME conditions apply (TN-C-S system).

Table 1 summarises the requirements of BS 7671 for sizing the cpc and main bonding conductor to the outbuilding. If the functions of these two conductors are combined in a single protective conductor, this must have a csa not less than that required for either function. Where there is no such connection, the main requirements of BS 7671 applying to the earthing and bonding are as summarised in Table 2.



Table 2 – Requirements where the outbuilding installation forms part of TT system with no connection to the earthing and bonding arrangements of the house installation:

A further consideration applies where there is a connection between the earthing and bonding arrangement of the detached outbuilding and that of the installation in the house, if the latter installation forms part of a TN-C-S system where PME conditions apply.

Where this is the case, PME diverted neutral current may flow in the connection between the two earthing and bonding systems. To avoid harmful thermal effects caused by this current, the csa of this connection and of the main bonding conductors and the earthing conductor of the installation in the outbuilding needs to meet with the requirements for a main bonding conductor of the installation in the house (which are summarised in Table 1).

F me! did you swallow the reg's book!

 

[Spoon]

I should be able to SWA right up to the external wall and then terminate in to an IP box and run T+E under the upstairs floor and back down to the top of the cupboard where the CU is located.

Just be aware that the T+E may need RCD protection, if you do it this way.

 

[Lewis Ashton]

Just be aware that the T+E may need RCD protection, if you do it this way.

Am I correct in thinking that only RCD protection is required on the submain if it is in a wall and to a depth of 50mm or less? I can mechanically protect the run with galvanised trunking or similar (straight run across the floor and then a short 600mm run down the wall to the CU. Just not sure if I can achieve the bend radius to get the SWA up the side of the house, in beneath the ceiling/floor void and then down the wall. I'll have a proper look when I'm home and see which methods are possible with regard to cabling.