"Stefek Zaba" wrote in message
...
Sparks wrote:
Is there any value in having the house earth in the garage as well, and
also connecting this to the rod? - or is this a definite no?
A definite no. I believe you wrote you had the SWA armour connected to the
house earth at the house end, but isolated from the garage CU -
Yup, you belive correclty :-)
that's just what you want to make the garage a pukka TT system. If you did
connect the house earth to the garage rod, under fault conditions (not
necessarily in your own house, but for example another place close by on
the same phase of the same substation) you might end up with quite
substantial fault currents coursing through your earth wiring; not really
desirable...
Yea, but, when I earth my metal gas pipe and metal water pipe, like the good
little boy I am, does this then make my house installation partly TT, thus
potentally unsafe as you describe!
(This is why I asked :-) )
I have just done some calculations for the house - shed run... using
http://www.kevinboone.com/cablecalc.cgi
House to Shed
Generated by CableCalc V1.0 (c)2000 Kevin Boone, all rights reserved
Basic cable properties:
Cable type: PVC-insulated two-core-and-earth 4 mm2 with 1.5 mm2 earth
conductor
Liar, liar, pants on fire ;-) You said in your original post it was SWA.
Yep, I did, however, the site doesnt have an option for SWA, so I assumed if
it was OK with the 4mm+1.5mm earth, then is must be OK with a 4mm earth!
If 3-core (L,N,E all get an inner core of their own, armour earthed
Because It Should Be rather than to carry fault currents) then the earth
conductor's the same size - 4mmsq - as the L & N. If it's 2-core with the
sheath being used as the E conductor, you need to use the right Table (or
hope that CableCalc has access to its contents) to use the impedance of
the armour.
I am using three core SWA, the armour is connected only at the house end.
However the thrid earth conductor from the shed - garage is not connected
(as it is earthed via a rod)
Voltage drop:
For a total cable length of: 25 metres
That's the second time you've said that ;-) I take it, then, that the CU
is more or less in the opposite corner of the house to the shed, so that
although the shed's only 4m from the nearest bit of house, the total SWA
run is 25m as you say...
Yea, the CU in under the stairs, in the middle of the house - the run was a
pain in the arse, goes up to the loft, then back dowm - however I should be
able to shorten this soon as the kitchen is being ripped out.
(New SWA will now be run under the patio, not overhead with 8 coax, 5UTP and
a couple alarm cables (It looks crap, but it was the only way without
digging uo the crazy paving (Yea, I know, its all been dug up now!)))
Volt drop at end of cable: 8.8512 V when carrying 32 amps and cable
temperature is at maximum value of 70 degrees celcius
So at (unrealistic) full load, that's just about your entire voltage drop
budget eaten in this first 25m (conventionally we allow up to 4% as cable
loss - for the 240V supply that's 9.6V). Unlike the earth impedance calc,
this isn't affected by SWA-versus-T&E, since the live conductors are the
same size (4mmsq).
Then you take another 25m (and you lie again about the cable construction
;-)
Same excuse as before ;-)
for the segment from shed on to garage. But the calcs from CableCalc
believe this is a 25m segment from the origin of the supply; so you need
to (a) add the voltage drop for this segment to the already-established
8.8V drop for the first run - putting you over the voltage drop limit for
drawing a heftier load in the garage; and (b) establish clearly what the
earth fault path actually is for an L-to-E-conductor fault right at the
end of the garage run. If that path relies on the house CU breaker, you
need to factor in the full 50m of the combined run to do your fault-path
calcs (which I now notice you mention, at the end of your message, do
assume the reduced-cross-section protective conductor of T&E, while you
know you have 4mmsq
.
Yea!, See! I was paying attention :-)
All looks OK to me, and that would be exporting the earth to the garage
via 1.5mm cable (I have 4mm)
Though we're not actually 'exporting' the house earth to the garage, are
we? (The garage end is TT).
Nope, we are not, but the calcs were assuming I was, so I thought with a
better earthing arrangement, it was even better than OK!
But the house earth is what an L-to-E fault at the end of that supply
cable will go back through (and you've got the worst-case 0.8ohm earth
impedance of your TN-S supply to allow for in calcultating the fault
current, touch voltage, and disconnection time, therefore...)
Okay, if I was to connect the garage to a 20A MCB, connected to a 30mA RCD
in the shed (No not Ideal I know!), would I still need to calculate from the
house?
If not, what would you suggest I do (I Really don't want to have to replace
the cables!!)
Sparks...