Garage wiring advice needed

[robgraham]

I vaguely remember my Dad replacing the catenary supply to his garage some 20 years ago with an underground supply, but a comment about long distance feeds and seperate earths rang a bell when it appeared somewhere here recently and has made me be a bit concerned about the security of this supply.

The details are that I can work out is that it is a 2.5 T & E that runs I reckon some 25 m from the CU to a shed en route to the garage which is some 8 m further on - at least the outer wall of it is. The CU fuse is 15A. There's a fused splitter in the shed (brick !), one feed supplying the lights and sockets in the shed and attached lean to greenhouse, and the other the garage, both I think are 15A.

The old man's memory isn't as good as it used to be - or more likely he is suspicious that I am going to condemn his work(!) as he is not being that forthcoming on how the cable runs underground from the house, but as far as I can see it is in a plastic pipe about 25mm dia. with an 8m u/g run to the brick shed from the house.

The loading on this possible guddle is light - some lights, a deep freeze, beer fridge and a wood turning lathe are the only things in regular use; the workshop is elsewhere and seperately fed (6mm, etc) and that's where the heavy loading is.

Condemning this garage supply would be met with severe objection, so how can I make it safer ?

Rob

[Andy Wade]

Is the greenhouse timber- or metal-framed?

Is the house earthing TN-S or TN-C-S (PME)? What's the floor like, especially near the fridge, freezer and lathe (Are these items in the brick shed, or the garage? Are the floors damp? Are there any metal service or waste pipes entering thro' the floors? Is there any RCD protection anywhere?

What aspect(s) are you actually worried about? The length limit for 2.5 mm^2 T&E on a 15 A fuse is 35 m, so there may be a concern there, especially if it's pre-1984 cable with a 1.0 mm^2 CPC.

[robgraham]

Many thanks for your answer, Andy

I've been over to his cottage on another matter and got the answers.

The earthing terminology doesn't mean anything to me but the overhead feed to the house is two wire and there's a none-to-heavy green and yellow wire coming out of the meter box and disappearing into the ground - the house was re-wired about 25 years ago I think.

The floor in both the initial small brick shed and in the wooden garage is concrete. Neither floor is damp and certainly the garage one has a dpc. The electrical equipment I referred to is all in the garage.

No metal pipes anywhere.

No RCD's

Concern - well it was something to do with earthing on long cable runs, and your comment about pre 1984 cable might back that up.

Your advice would be appreciated.

Thanks

Rob

[Andy Wade]

but the overhead feed to the house is two wire and there's a

Well that sounds as if it's TT. The wire you mention will go to an earth electrode - usually a buried rod.

Oo-err. No RCDs anywhere? Not even a whole-house 30 mA RCD, or possibly an older 30 mA earth leakage circuit breaker (ELCB) between the meter and the consumer unit, as would have been common practice when this was wired.

If the earthing is TT (i.e. with no metallic earth path back to the supply distribution transformer) and there is no form of residual current or earth-leakage device anywhere in the installation then there is certainly a very dangerous situation since it's highly likely that any earth fault would not be cleared (not enough current to blow the fuse) leaving exposed metalwork live. (In the jargon this is called risk of electric shock through indirect contact.)

Obviously, detailed advice can't be given without seeing the installation. If you are worried, and from what you've said there may be grounds to be, then I suggest getting a competent electrician (someone with a C&G 2391 certificate in inspection and testing) to do a Periodic Inspection. Done properly this will give your dad a comprehensive report on the condition of the installation, with recommendations for remedial work.

Remedial work, if it is TT, is likely to involve providing 100 mA RCD protection for the whole installation, together with 30 mA RCD protection for all socket-outlets on (at least) the ground floor and in the outbuildings. Another option might be to have the overhead supply upgraded to PME (TN-C-S) in which case the 100 mA RCD would not be required.