Exporting a PME earth

[ac1951]

I noticed on a recent thread about cable size to an outside detached Garage, that there where some issues mentioned regarding whether or not you where exporting a PME earth.

The points raised that interested me related to the size of earth conductor and usage of the supply. In my case I have, already in-place, a 2.5mm twin SWA cable connecting my garage which converts to a 2.5mm T&E cable on the outside of my house. This is connected to a 6A RCBO in the CU. (Total distance circa

40M)

In the garage is a 2 way CU with 2x5A mcb's, one feeding the lights and the other feeding 2xDouble 13A sockets. I use the sockets for basic power tools (drill, planer, sander etc) and also electric lawn mower. All the tools are double insulated and therefore do not have an earth.

Is this installation legal ?

What would you do differently ?

Where can I find a concise explaination of do's/don't regarding the exporting of a PME.

thanks Andy

[Pete Verdon]

I can't answer that, but to help those who can you will also need to say what, if anything, is connected to the armour of your SWA. The actual connection to the armour is in the form of the brass gland(s), but what is connected onwards from here?

Pete

[John Rumm]

Did you really mean 6A or was that typo for 16A?

What is the imbalance trip current rating for the RCBO?

That means you are not at risk of taking the PME earth into the garden etc. Which is good. ;-)

What is the construction of the garage like? Any other services in there like water or gas?

Probably make the garage a TT install with its own RCD and local earth.

Well you could say BS7671 is "concise" to the point of being terse at times. I don't recall seeing any particularly details articles or coverage in books on the subject - although there are plenty of items that cover at least a part of the requirements.

This one is ok:

that, our very own:

on a number of salient points (even more so since I added some today!)

[robgraham]

A timely posting and I hope my questions do not get taken as a highjack !

I'm about to deal with a similar situation. The garage is fed with a

6mm T&E from it's own mcb in the house CU - earthing is TN-S and I had already considered that a TT earth for the garage would be a good idea (30m plus run from the CU) with the earth to the house disconnedted.

Two questions - as the TT earth is relatively high resistance compared to the supplier earth, and hence the fault current will not be high, does the earth cable size have to be any bigger than 2.5mm ? If it is to be bigger, what is the justification for this?

Secondly, although there is no water pipes, etc, the garage roof is steel frames; should these be bonded ?

Thanks

Rob

[Andy Wade]

By "earth cable" do you mean the earthing conductor - i.e. the connection to the earth electrode? If buried for part of its length this needs to be 16 mm^2 on grounds of mechanical robustness. If not buried then 4 mm^2 is OK. In both cases the size can be reduced to 2.5 mm^2 if the conductor is mechanically protected by being run in conduit, etc.

There is no need to bond structural steelwork unless it's in contact with the ground.

[John Rumm]

You mean the one in the 6mm^2 T&E at the house?

At a quick glance, no; because the earth fault protection here is provided by the TN-S earth at the house and not your TT earth. (hence why you don't need RCD protection at the head end).

Might be worth checking the disconnect time though. You have not said what proportion of the run is T&E and what is SWA, so we will check the worst case - all T&E with a round trip resistance phase to CPC of 10.49 mOhms/m. If we take the TN-S external earth fault loop impedance as 0.8 ohms we get 0.8 + 30 x 0.01049 = 1.1147 total. That gives a prospective fault current of 206A. That should open a 32A on the instant part of its trip response. For a 40A MCB you will be getting closer to the lower end of the fault current required[1] to open within five secs, so you will need to watch the fault withstand time of the CPC - at 5 secs it would be under sized.

[1] However we are making pessimistic assumptions here, so real life will probably be better.

If you are talking about the main earth connection to your earth stake, then that should be in 6mm^2 as per Fig 4c in the OSG.

No need since you are TT and your frame etc is attached to the same local earth as the CU. There is also no risk that a fault in the supply could leave your earth floating at a high voltage - as might be the case with a PME head end.

[ac1951]

Thanks John see answers to your questions below:-

This is connected to a 6A RCBO in the CU. (Total distance circa 40M)

No definately a 6A but I had thought of changing it to 10A. I seem to remember when I did some calculation this was OK but I need to double check unless you know differently.

30mA

No Water or Gas..concrete block construction timber frame roof with normal flet and roofing tiles, wooden up-over door, concrete floor.

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