Dave Osborne wrote:
Toby wrote:
John Rumm wrote:
Toby wrote:
Hi,
I am going to be running SWA cable out to the garage soon (Total run
is a little under 20em from the house CU to the garage CU)
I will also be installing a generator point on the garage, which
will be connecting to a proper transfer switch in the house.
It will be easier and cheaper if I just run a single SWA cable with
4 cores, two for the generator and two for the feed into the garage.
Is this permitted, or do I have to have two separate cables?
The SWA will be terminated in the house and garage in a metal box,
then the two circuits taken from there to there respective places.
The main complexity sounds like it is going to be dealing with
earthing and possibly main equipotential bonding. What earthing
scheme do you have in the house?
The idea was to use the SWA armour as the earth for both and have an
additional earthing rod next to the generator point connected via
10mm cable (Generator is only 7.5KVA).
The generator transfer switch This one -
http://www.briggsandstrattongenerato...ertransfer.htm
does not have the facility to switch the earthing, so I am assuming
all the earth connections can stay connected all the time?
My main earth is supplied by the armour of the incoming supply cable
(TN-S)
Is there any reason the earth connection to this can't be left
connected in the event of a power failure with the generator running?
(I assumed it is OK as the transfer switch does not have the facility
to switch the earth connection)
The generator has one leg of it's output tied to ground, so there will
be no potential difference between neutral and earth in the system. If
live found it's way to earth, this would just create a short, tripping
the MCB, so, as far as I can see it, there is no risk to anyone
working on the power grid here (just like the power company ties their
neutral to earth everywhere)
Also any reason the earth rod can't stay "in circuit" during normal
operation (considering the lead water pipe is connected to the
earthing system 24/7 via 10mm cable and this will be acting the same,
if not better)
In reality the generator won't be running very often, as the mains
power supply should be reliable, but I already have the generator, so
am just making provision for it's use "just in case" - If I move out,
it will be coming with me too, so nothing to worry about later on
either, it will simply be disconnected.
Toby...
I think John is alluding to concerns about "exporting" the earth from
your TN-S main supply out to the garage and "re-importing" the generator
earth back from the garage to the house.
see here for more info:
http://wiki.diyfaq.org.uk/index.php?...i ng_an_earth
Given that you have a TN-S supply and are proposing properly buried SWA
cabling and this garage is presumably a building of substance rather
than a rickety shed, then I would probably be inclined to (permanently)
connect all the earths together and run an additional main-bonding
conductor alongside your SWA cables. You should consider getting a duct
to run your cables rather than direct burial, because you would need to
upgrade your main bonding earth wire further if it was direct buried.
e.g.
http://www.polypipe.com/polypipe/con...ategoryID=7598
Thanks for the reply.
The garage is indeed a proper building (brick) and is only about 6m from
the house, so this is why I am choosing to export the house earth there
- if it were down the other end of the garden, then I expect a separate
TT system would be better for the garage. - It will have an earthing rod
for the generator too, so it will be both TN-S and TT I suppose!
I was planning on using 4mm XPLE SWA cable, and having the feed to the
garage via a 32A MCB (Not RCD protected) and then have a 5 way CU in the
garage, with a main switch, RCBO's for sockets and exterior lights and a
MCB for the freezer circuit (All the wiring in the garage will be done
in surface mount conduit, so nothing will be buried in the walls)
How do I find out the CSA of the armour in the 4mm SWA cable - is there
a chart somewhere showing the values?, I am happy to use 6mm if I need
to, to get the CSA up on this cable, and save having to run another duct
(I will be running a green duct for comms cables anyway, but would
rather not run another duct just for a separate earth cable if I can
help it.
Toby...