ISTR from discussions in the past that there are various alternative
earthing schemes used with regard to mains earthing - some of which use an
earth provided by the electricity supplier, and some of which don't.
Can some kind soul please either remind me of the salient characteristics of
each scheme - or point me to an on-line source of said information, with
particular reference to:
* how I determine which sort I've got
* which schemes (if any) continue to provide an earth in the event of a
Many thanks for the links. Having studied them, I'm pretty certain that my
system is TN-S.
I have what looks likes SWA cable coming out of the ground and connected to
the main fuse. There is a thick black cable (presumably containing L & N
conductors) from the top of the main fuse to the meter. On the side of the
main fuse block are screw terminals to which several earth wires are
connected - to the consumer unit and to pipes etc.
Is it fair to assume that earthing is provided via the armour of the SWA
cable - which is connected both to phase neutral and to a true earth at the
local sub-station? If so, is it fair to assume that this true earth
connection will continue to exist even during power cuts?
On Sun, 11 Jan 2004 21:57:22 -0000, Set Square wrote:
Seems reasonable from that description.
No, the power cut could be due to a JCB slicing through the cable...
I don't think you can rely on anything provided by the electricity
supplier during a failure. This is where it gets tricky when
installing a generator and its earth... B-)
Dave. pam is missing e-mail
A more common method would be TN-C-S but they both effectively present
themselves at the cutout in the same manner.
During a power cut it is quite possible that the earth could become
disconnected either by the original problem of a line coming down for
instance or in the process of repairing a fault and having to dig a
Quite a few people I have worked with in the past have prefered to use
a TT system in order to be able to provide an earth themselves.
I could go into quite a bit of detail but if you really wanted to know
I could probably find some info for you somewhere.
Oh, and FYI the cable coming in from the REC is more than likely
concentric, not SWA.
When you say thick cable, do you mean like 15-20mm dia. or 25-30mm
dia. region. If the latter it could be TN-S, the former more likely
The cable coming out of the ground is about 15mm and the cable from the fuse
to the meter is about 13mm.
Yes, perhaps it is TN-C-S because there is no clamp with an earth connection
around the incoming cable - the earth connection is higher up, alongside the
You may have seen my other thread about using a genny as an emergency power
supply for central heating electrics and freezers in the event of a power
cut. I am still trying to bottom out what to do about earthing such an
arrangement - which is why I wanted to know whether I can rely on any sort
of earth from the mains installation.
It looks like I can't. My current thoughts are that I should plant a metal
spike in the ground and connect the genny earth to it - and also power all
the critical items via an RCD when I unplug them from the ring main and
connect them to the genny instead. Does this sound reasonable?
I personally just chuck the genny in the back garden and plug a lead
in to power the important stuff, TV, digibox, PC etc... but that's
Depending what size genny you use and what you're connecting to some
come with a earth stud on the frame, connect this to an earth spike a
la TT method. Also switch to TT on the main earth at the mains
position. I have done this in the past with the installation of earth
links to be able to bring in and out the two different methods of
earthing, (that is TT and TN-C-S).
If you're just running the CH and a couple of lights its probably not
really required so yes, your original idea of supplying what you want
via leads would suffice, as I said, it works fine for me.
On Mon, 12 Jan 2004 00:06:17 -0000, Set Square wrote:
And ensure that what the genny says is "neutral" is also connected to
the genny frame and you spike. Do a bit of googling on the hows of
planting a spike as well. It's not quite as straight forward as buying
an earth rod and hammering it into the ground, though it can be.
As you are going the unplug and plug to generator extension wiring
method it seems very reasonable and side steps that knottty earth
Dave. pam is missing e-mail
"Set Square" wrote
| ISTR from discussions in the past that there are various alternative
| earthing schemes used with regard to mains earthing - some of which use an
| earth provided by the electricity supplier, and some of which don't.
| Can some kind soul please either remind me of the salient characteristics
| each scheme - or point me to an on-line source of said information, with
| particular reference to:
| * how I determine which sort I've got
TT - earth rod
TN-S - supply protective conductor / cable armour - often a clamp round the
supply incomer before it goes into the service fuse.
TN-C-S - supply neutral used for earth (also called Protective Multiple
Earth) - an earth wire will come out of the service fuse along with the
meter tails. TN-C-S can have an earth rod as well sometimes.
| * which schemes (if any) continue to provide an earth in the event of a
| power cut
Actually, in the event of a powercut if there was no power there
wouildn't be a need for the earth. Having said that, if the earth was
required for the purposes of the generator you would need to reference
the generator earth to the same potential as the TT arrangement.
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