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