What you have is basically OK; all the rambling that follows is in the
realm of minor tweaks towards unattainable perfection. That said... the
cascade of RCDs and MCBs is not best practice - should you have a fault in
the garage, you could well end up with not only the garage power being cut
(which we want) but also the shed (pain) and the house sockets
(double-pain). What you want to get to is to have the cables to shed and
garage protected against short-circuit and overcurrent, while leaving
protection of the loads they feed to a dedicated RCD and closer-rated MCB
in each place (shed and garage). To achieve this, you want to feed the
'submain' - the supply to shed and garage - from sthg like a 20A *fuse*
(not MCB: you actively *want* the slower reaction to overloads of a fuse
here) on the non-RCD side of the house CU. That should feed into the
little shed CU, and 'daisy-chain' (i.e. not passing through any way of the
shed CU) on to the garage. In both shed and garage your small CU has its
own RCD (or if you're often working at night, separate RCBOs for lights
and power in the garage, so that earth leakage fault from power tool
cable-slicing oopsie ;-) doesn't plunge you into darkness).
Well, the garage is basically for the car (at the moment!)
The shed is my "work room" for fixing PC's (My job) so no real risk of me
wielding a huge angle grinder in there, then getting plunged into darkness!
I have installed an emergency light in the shed anyway, as I had one going
spare ;-)
As to earthing - again, what you have sounds about right (shed close to
house shares its earth, more distant garage has its own rod). If your
supply is PME (TN-C-S) there's an argument for making even the shed a TT
installation, but it's not an overwhelming one. The reason PMEness matters
here is that (in handwavy outline) PME creates one bigass 'equipotential
zone' (kinda like supplementary bonding does in one bathroom, on the
whole-house scale), so we stop caring about 'true' earth potential; and
with metallic incoming services bonded to, the installation earth is going
to be v. close to 'true' earth anyway. Once you're 30m away, like your
garage is, and given that a single fault in your buried feed could lose
the bonding between N and E, it makes increasingly little sense to rely on
the (you-hope-it's-been-)exported house earth; so making the garage its
own little TT installation - as you already have - is the Right Answer.
Excellent!
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?
Should the feed from the house to the shed be via a 100mA RCD, separate
to the rest of the house?
It'd certainly be better for it not to share the house sockets' 30mA RCD -
doing that increases the probability of nuisance trips in the house (both
from full-on faults Outside, and just from outside appliances adding a few
mA of their legitimate leakage to 'preload' this shared RCD close to its
tripping point). As I whittered above, it's OK for the supplies themselves
not to have any RCD protection, provided the socket circuits they feed do
end up with RCD protection. If you feel happier running through a 100mA
RCD, you can, but you can't rely on that necessarily discriminating with
the garage RCD. (The strength of that argument again differs with what
you're doing in the garage: if you've one light in there and one socket
where you plug in a vac to clean the car, a single whole-garage RCD is
fine; if it's a uk.d-i-y.hardcore garage, you've got no room for a car,
but a couple of benches, two pillar drills (one of which works and the
other's going to Real Soon Now), a boatload of other power tools, some
unreasonably dangerous electroplating lashup in the corner, oh and the
feed to the combination Thunderbirds launchpit-and-burglar-pit, and so
on - in which case you'd want the whole garage TT to have its own 100mA
time-delay RCD, and either a splitload 30mA-RCD CU or individual RCBOs for
(at minimum) the socket and fixed-equipment circs... In that case, you
want the house end of things to protect *only* the feed cable, leaving all
the RCDing local to the garage.
I have been planning on adding a second, smaller CU to the house, with just
an isolator and a few MCB's for the freezer and alarm system, I think I will
just add the outside stuff to this new CU (On a 32A Type1 MCB)
Another question is, why do I need to TT the supply in the garage? Can't
I just use the house earth?
(My logic is, the earth provided by the electricity company is at least
500m long (That's where the substation is!)
Hope I've outlined the reasoning above. For TN-S, the exporting issue is
less sharp than for TN-C-S, but for a 'remote' garage 30m away you're
increasing the earth loop impedance a fair bit from what the supplier's
big-ass incomer gave you as you pass it down your wimpy little 4mmsq SWA
to the end of the garden. And with PN-C-S becoming more widespread, even
as a 'retrofit' as suppliers meet increased local demand from all that
good brownfield development, it makes sense to make new/upgraded
installations 'PME-aware' ;-)
Fair point, makes sense to future proof these things as much as possible, as
there is already a rod, I wont take it away!
HTH - Stefek
Thanks for taking to trouble to explain this, I did think it was basically
there, just needed a few minor adjustments!
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
Installation method: clipped to a surface
Uncorrected nominal current: 36 amps at 30 degrees celcius
Rated full load temperatu 70 degrees celcius
Ambient temperatu 30 degrees celcius
Current rating temperature correction factor: 1
Semi-enclosed fuse correction factor: 1
Grouping method: no grouping; cables widely separated
Cables in group: 1
Grouping correction factor: 1
Ring correction factor: 1
Length in thermal insulation: 0
Thermal insulation correction factor: 1
Corrected nominal current: 36 amps
Specific resistance of power conductor: 0.00461 ohms/metre at 20 degrees
celcius
Specific resistance of earth conductor: 0.0121 ohms/metre at 20 degrees
celcius
Temperature cofficient of resistance: 0.004 K-1
Specific resistance of power conductor: 0.005532 ohms/metre at 70 degrees
celcius
Specific resistance of earth conductor: 0.01452 ohms/metre at 70 degrees
celcius
Voltage drop:
For a total cable length of: 25 metres
Volt drop at end of cable: 8.8512 V when carrying 32 amps and cable
temperature is at maximum value of 70 degrees celcius
Basic compatibility of overcurrent device with cable and design current
Selected over-current device: 32-amp type-1 MCB
Nominal current rating of overcurrent device is compatible with design
current
Nominal current rating of overcurrent device is compatible with corrected
cable current rating
Current required to disconnect device in 5 seconds: 128 amps
Effective worst-case earth conductor resistance of cable: 0.363 ohms at
maximum rated temperature
Worst-case shock voltage after five seconds: 46.464 volts
Disconnection times
Total fault resistance: 1.3013 ohms
Fault current: 176.746 amps
Over-current device will disconnect in 5 seconds
Over-current device will disconnect in less than 0.4 seconds
This cable/device combination is suitable for all allowed applications,
including bathroom and outdoor systems
Now from the shed to the garage (Assuming it's on a 20A MCB in the shed):-
Shed to Garage
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
Installation method: clipped to a surface
Uncorrected nominal current: 36 amps at 30 degrees celcius
Rated full load temperatu 70 degrees celcius
Ambient temperatu 30 degrees celcius
Current rating temperature correction factor: 1
Semi-enclosed fuse correction factor: 1
Grouping method: no grouping; cables widely separated
Cables in group: 1
Grouping correction factor: 1
Ring correction factor: 1
Length in thermal insulation: 0
Thermal insulation correction factor: 1
Corrected nominal current: 36 amps
Specific resistance of power conductor: 0.00461 ohms/metre at 20 degrees
celcius
Specific resistance of earth conductor: 0.0121 ohms/metre at 20 degrees
celcius
Temperature cofficient of resistance: 0.004 K-1
Specific resistance of power conductor: 0.005532 ohms/metre at 70 degrees
celcius
Specific resistance of earth conductor: 0.01452 ohms/metre at 70 degrees
celcius
Voltage drop:
For a total cable length of: 30 metres
Volt drop at end of cable: 6.6384 V when carrying 20 amps and cable
temperature is at maximum value of 70 degrees celcius
Basic compatibility of overcurrent device with cable and design current
Selected over-current device: 32-amp type-1 MCB
Nominal current rating of overcurrent device is compatible with design
current
Nominal current rating of overcurrent device is compatible with corrected
cable current rating
Current required to disconnect device in 5 seconds: 128 amps
Effective worst-case earth conductor resistance of cable: 0.4356 ohms at
maximum rated temperature
Worst-case shock voltage after five seconds: 55.7568 volts
Disconnection times
Total fault resistance: 1.40156 ohms
Fault current: 164.103 amps
Over-current device will disconnect in 5 seconds
Over-current device will disconnect in less than 0.4 seconds
This cable/device combination is suitable for all allowed applications,
including bathroom and outdoor systems
All looks OK to me, and that would be exporting the earth to the garage via
1.5mm cable (I have 4mm)
Sparks...