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!

 

[Lewis Ashton]

With regard to the time delayed RCBO; my thinking was just incase the steel wire of the earth came in to contact with the localised TT system, though on hindsight it is probably overkill!!

 

[buzzlightyear]

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).

 

[Spoon]

My proposal is to install a 40A RCBO, supplying a glanded SWA cable (earth connected to house CU earth).

Could just use a 40A breaker as the SWA does not need RCD protection. This will also remove the discrimination needed with the RCD in the shed.
What size ducting are you thinking of?
If you are thinking of increasing the load in the shed in the future why not install a bigger cable now, saving money and time in the future?

 

[Lewis Ashton]

Thank you Buzzlightyear, it is very much appreciated and confirms my plans on a localised TT arrangement to the shed.

Would you have a preference over either method? I can see that a localised TT system to the shed could suffer from degradation of the earthing arrangements, i.e. earth rod corroding, ground conditions affecting earth impedance, etc.

 

[Lewis Ashton]

Thanks for that suggestion Spoon; I had presumed that RCD protection was required for any external circuit. I obviously stand corrected!!

I'm not sure on ducting size yet to be honest. I have no plans as such to increase the load, but just thinking of the future. I don't fancy spending £1000's doing the garden and Mrs Ashton asking for a hot tub next to the shed in 5 years time. Cue many cursed words and me ripping up the turf and patio!!

 

[buzzlightyear]

TT system to the shed could suffer from degradation of the earthing arrangement

i doubt that if installed correctly. In ducting ,then will not fail.rods copper any way.
when finished ,post up what you have done so we can have a laugh i mean look .

 

[Spoon]

How about price up the difference between 10mm and 16mm.
I bet the price of 16mm is a lot lower than 10mm and ducting.
You should not need more than 50A for a shed.

 

[Spoon]

I'm an industrial guy as well. This forum is cool as you can learn a hell of a lot from it. Some very knowledgeable people on here.

 

[buzzlightyear]

You should not need more than 50A for a shed.

course he needs it for selling bob hope.LOL.

 

[Spoon]

course he needs it for selling bob hope.LOL.

Isn't that better to do in the attic? He'd have to insulate the shed..

 

[buzzlightyear]

Some very knowledgeable people on here.
has Pete and Tele fallen sleep in there chairs .Lol.

 

[Spoon]

Some very knowledgeable people on here.
has Pete and Tele fallen sleep in there chairs .Lol.

Funnily enough, they are who I was thinking of, when typing that. Of course there are many more people who also fit the bill.


Not very good English, but it seems my brain can't be arsed switching on today...

 

[Lewis Ashton]

Thank you Buzzlightyear. Can I assume your preference would be a localised TT earthing arrangement (through an earth rod) for the shed then, rather than exporting? I shall let you all have a laugh (read: look). Glad to see you have confidence!

You're quite right about the excellent knowledge on here; every day is a learning day as they say Spoon. The idea of 16mm and no ducting probably is the better option long term.

Funny you should mention insulation; it actually is! No Bob Hope here though; the railways are quite strict on the old drugs and alcohol!!

 

[Spoon]

This is where the more knowledgeable people people can correct me but would it be easier to export the earth than use TT for this length of circuit?

 

[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.