Detached Garage Supply

12 Jun 2012
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United Kingdom

I have bought a new house with what was described as a "properly installed power supply" to the garage (used as a workshop.) On further investigation the power to the garage is 3 Core + Earth (1.5mm) plugged in in the conservatory, then buried under concrete for about 40m to the garage. In the garage are 2 double plug sockets (one RCD protected.) Into the RCD socket is a 500w floodlight with PIR, and the other socket has a florescent lamp.

I know from looking at it this supply is "rough"!

The house supply this is taken from is a TT system protected by a 80A 30ma RCD.

I'm loathe to leave the supply connected whilst I'm out incase it trips the house RCD. Is there any way to fit a faster acting RCD to the supply to the garage to ensure the house RCD doesn't trip if the outside lamp has a problem?


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You could connect one that trips at a lower value than 30mA.
But if it's plugged in to the conservatory, remove the plug when out!
Thanks Prentice. Is this best as a device replacing the plug at the conservatory end so that everything is protected by one device?
If the cable powering the garage is plugged in to a socket outlet in the conservatory and you are worried about trip hazards whilst out. The most logical thing is to remove the plug from the wall socket.
But in answer to your question, you can changed the socket outlet plate, to a 13A DP RCD socket passive 10mA, which will only protect what ever is plugged in to it.
They are not a standard item and may take some sourcing but Ultra do them.
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Even RCDs with a lower rated trip current won't guarentee discrimination between it and an upstream normal RCD.
The only way to ensure discrimination between devices is by having time delayed devices upstream which you can't as it is protecting sockets.
Your only other real option is to have the garage connected to it's own RCD back by the house consumer unit hence not relying on the RCD for the house sockets.
Thanks, I had thought about that. I guess if i go down that route I should probably use SWA down to the Garage and wire it with it's own Consumer Unit?
Thanks, I had thought about that. I guess if i go down that route I should probably use SWA down to the Garage and wire it with it's own Consumer Unit?
That would be the ideal solution, then you would require RCD protection at the garage rather than at the CU.
Time to rack up the overtime, talk nicely to the sparky we use at work and see if he's happy for me to do the donkey work while he does the clever bits!
Also think about running a duct along side the SWA. This will allow you to pull in wiring for the burglar alarm, internet, TV, telephone etc as and when you decide you'd like them.
When we moved to our current house 3 years ago the garage supply was one of the many things on my list as it was 2.5mm t&e buried in the garden!

I earth looped it and found I had a silly high earth resistance.....

So best replace t&e with a proper cable, 4mm sounds ok at a guess, 10 or 16 would be overkill

My garage supply is only a 6mm and my welder causes no issues with voltdrop

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