Earthing arrangement for shed.

[daz3770]

hi there gents,

I want to run power to a new shed which will be 45 metres from main house/CU.

Can I use 3 core armourd cable and use one core for the earth? Or at that distance from the CU do I need to make it a TT system?

All suggestions welcomed, with in reason!!!

Kind regards

 

[Spark123]

What is your supply type - TN-CS, TN-S or TT?
Are there any extraneous metal parts such as a water supply in metal pipes in the shed?
It is possible to use a 3C SWA, you'd need to start by working out how much current you will be drawing as a maximum then from that it is possible to calculate the required cable size.

This work will be notifiable to your LABC under part p of the building regs.

 

[Taylortwocities]

This article from the IET has a lot of detail that will be of use. http://www.theiet.org/publishing/wiring-regulations/mag/2005/16-elect-inst-outdoors.cfm?type=pdf

Note that this was written before the 17th edition of the wiring regs came out.

 

[daz3770]

Thanks for your replies.

In the artical you refer to, it mentions using 10mm bonding.

If I use 4mm 3 core SWA, will the resistance of 1 of the cores, over 45m back to house, be low enough to provide an acceptable earthing arrangement?

Hope this isn't going a little OTT.

 

[RF Lighting]

Will there be any extraneous metal work in the shed?

 

[daz3770]

Hi there,

I think you mean water pipe, metal drain pipe; then no.

The power is just for portable power tools and lighting. There will be work bench and shelves, mainly constructed in wood. May have some metal shelving.

Is that any help?

 

[ban-all-sheds]

If I use 4mm 3 core SWA, will the resistance of 1 of the cores, over 45m back to house, be low enough to provide an acceptable earthing arrangement?

Why don't you look up the R1+R2 values for 4mm cable?

Please don't say that you're trying to do circuit design work without any reference data to hand....

 

[RF Lighting]

Basically it is any metal work which is in contact with mother earth, which is usually water pipes or structural steel work etc.

4.0mm² is a too small to be of any use really.

Due to the length of the run, volts drop will limit you to a MAXIMUM of 3kW with a 4.0mm² supply.

This really is not alot by time you have got a light in there, and if you ever wanted a kettle or heater, you have reached your limit.

 

[daz3770]

If using 4mm and assume that R1+R2 = 9.8, then 9.8 x 45m / 1000 = 0.44ohms. This seems high as Ze at CU is 0.35 ohms.

So, using 6mm and R1 + R2 of 6.53, = 6.53 x45m / 1000 = 0.29 ohms. This IMO sounds better.

Am I worrying to much about the earthing resistance? If looking at it purely from a voltage drop perspective, then 4mm would do the trick.

 

[ban-all-sheds]

Am I worrying to much about the earthing resistance?

Will Zs be under the limit for the protective device you'll be using?

If looking at it purely from a voltage drop perspective, then 4mm would do the trick.

Only if the load is pretty small.

Will it always be small? Can you be sure it'll never change and be more than 16A? Do you like digging 45m trenches? Relative to the overall cost of the entire project, including building and equipping the shed, how significant is an extra £30-40?

Install 10mm² and be done with it.

 

[daz3770]

Now that you put it like that, it would seem silly not too.

Many thanks

 

[ericmark]

To your original question TN-S or TT if you are using tools outside i.e. lawn mower the consideration is will the circuit open before you have 50 volts between you and the ground?
Assuming a TN-C-S supply to the house if your garden is surrounded by other gardens then using a TN-S supply will likely be OK as the PME system is likely to have rods in the ground all around you as it supplies other houses.
However if behind your house is fields then the ground may have a voltage gradient and you may have a problem getting a RCD to trip should you run over a lawn mower cable.
Although TT may seem way forward if you approach the house with the lawn mower you have same problem if there is a gradient.
Assuming you use a lawn mower now from the house then wiring the shed TN-S is no different to what you already have.
As long as the earth to your shed is big enough to take current should the TN-C-S go faulty there is not problem in having an extraneous-conductive-part connected to the TN-S earth so fitting an earth rod as well would ensure the trip would work and with 10mm SWA you have a big enough earth from the house.
May seem daft but the earth rod is not counted as an earth rod in this case it is only an extraneous-conductive-part and all tests are done as for a TN system.
You will hear people say you can't have an earth rod and use house earth which is correct as it's not called an earth rod it's an extraneous-conductive-part so you can fit an earth rod its just not called an earth rod. If the cable from house was only 4mm then regulation 544.1.1 would not allow you to use duel system as min size for non PME (TN-C-S) systems is 6mm and for PME it's 10mm this is to ensure under fault conditions it can't melt the earth cable.

 

[ban-all-sheds]

Why would you export a perfectly good supply earth and then deliberately introduce an unnecessary extraneous-conductive-part which served only to complicate matters?