60amp garage supply

[Rich]

Evening all, need some advice, I have got a 60amp supply to my garage which is 40mtr's from the house mains position the SWA is 16mm 2-core using the wire armour as the earth which is connected to the house earth via a 60amp 1 way fuse box wired in to the out going meter tails via phase blocks, in the garage there is a 80amp 30mA RCD.Using an RCD tester the trip times are well under the 200ms max, think it was about 0.05ms time on fast trip but is this a true test in the event of someone coming in to contact with a live wire, i.e. doing an RCD test you test between live and the earth ( which goes back up to the house earth ) so would not simulate live to the ground of the garage ???? is there another type of test you can do to simulate this.

Richard

[Mike Harrison]

RCDs sense imbalance in the live and neutral current, so it shouldn't matter which earth the current is leaking to.

[Chris Oates]

matter which earth the current

& the armour is no longer no longer acceptable as an 'earth' should be an internal conductor or an external (min) 10mm

[Christian McArdle]

That's a new one to me. When did it change?

Christian.

[Christian McArdle]

What do you mean by ground of the garage?

Is there any structural metalwork, or metal services, such as water supplies? If so, they should be main bonded back to the house, which will be very difficult being 40m away.

Your garage might be better rewired as a TT system. Isolate the earth at the garage end and install an earth rod. Keep the RCD and ensure the incoming cable goes straight in and the consumer unit is a plastic insulating type, not metalclad. Check the earth loop impedence is nice and low.

Christian.

P.S. you can test the RCD against any earth by getting a 13A socket with flying leads. Connect the live to the circuit you wish to test. Connect the earth to the earthing point you wish to test. Plug your RCD tester into the socket. Perform test.

[Chris Oates]

Dunno but our NIC inspectors don't allow it now and 10 years ago when I ran an Amusement Park we had to run separate protective conductors.

Also when I did multistory bedsit conversions sep earth was required.

Used to be common to use bonding clamps on cut back armour to export an earth

[Rich]

I mean the actual floor of the garage that you would be standing on.

No services or exposed metalwork

What is the point of having an earth rod if you have a fault and the current flows through you to earth, surley the earth rod does not do anything ?? or is it there just for the metalwork in the garage ie:- metal sockets light switches and strip lights etc:

What is the point of doing an earth loop test if there is an RCD protecting the garage??

so I could test from a live wire in the fuse box to the earth rod for example and get a reading??

Rich

[Rich]

I thought that you could use the SWA as an earth but over 16mm the SWA csa was not big enough to comply so you have to run a separate earth along side it!!!!!!!!!!!!!

Rich

[Christian McArdle]

The earth rod is connected to any main equipotential bonding (in your case none) and the earth pins of the sockets and all the protective conductiors in the circuits.

You must ensure the earth loop impedence is low enough so that the RCD will trip before the earth potential reaches 50V. This requires an earth loop impedence below a certain value (around 1600 ohms). However, I believe the recommendation is to have one below 200 ohms, which is well within the 50V requirement when using a 30mA RCD. (It will drop 6V by the time it reaches guaranteed trip current). It allows for changes in ground resistance and is very easily achievable, so accepting less is asking for trouble.

Yes. The testing will attempt to draw a predetermined current (usually 20A). It then measures the voltage drop. If the earth loop impedence is high, the voltage drop will be greater. If you are working in low current circuits, such as lighting, it might blow the MCB, though. Lower current testers are available for this reason, but you should use the full current to test when possible.

Christian.

[Rich]

So as the earth for the garage is using the SWA of the cable from the house this is surley much better than the earth rod in the mud which I imagine must alter its resistance wildly between the very wet times and the very dry times of the year if a regular RCD test proves ok surley this is good????

Rich

[Christian McArdle]

The earth would certainly be better in terms of loop impedence. However, the problem arises because the earth potential of it could be different from the earth potential of the physical garage ground.

Christian.