Sparks wrote:

Should the garage feed simply piggyback the feed TO the shed CU, eliminating
the RCD and MCB there?

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).

That's the outline of the 'best' solution. However, to install it and be
sure you've met required disconnection times for the supply cable will
need Real final-circuit design calculations - they're not hard, but do
require reference to tabulated values of cable characteristics.

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.

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.

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' ;-)

HTH - Stefek