John Rumm wrote:

It looks rather like the RCD covers both ways on that. Depending on how
you plan to use the garage you may wish to opt for some arrangement that
will not lose the lights at the same time you lop through a power cord
with a still spinning circular saw! (i.e. switched two way CU with a
RCBO for the sockets, or perhaps the one pictured with a non maintained
emergency light)

Yes, for TT a main switch and two RCBOs would be a good option, or main
switch with one MCB (lights) and one RCBO (power) for TN. I'd be
inclined to make the 'power' RCBO a 20 A one - RCBOs only come as Type B
and 16 A Type B might tend to trip on motor starting currents, or the
inrush current of a 230 - 115 V tool transformer. (Alternatively keep
the 16 A circuit for sockets only and provide separate motor circuits
for any machines.) You can buy all the MK consumer unit parts
separately from a wholesaler, including TLC if you want to buy on-line.

On the same theme as above you may need to bear in mind is
descrimination of the protective devices. You want to try and ensure
that a trip on the power circuit in the garrage only takes out the local
MCB and not the upstream one in the main CU as well hence losing your
lights.

That would probably require the feed to be via a separate switch-fuse
rather than an MCB in the CU - certainly a better arrangement, but does
involve teeing in to the meter tails with a service connector block.

That is a different cable head end from any I have seen in use in this
area, so can't give you a definitive answer. However the connection of
the (undersized!) earth connector to the side of the main cutout like
that would suggest TN-C-S.

Looks like TN-C-S to me, with old earthing and bonding conductors which
are undersized by current standards. These ought to be upgraded if you
intend to use the exported PME earth.

Where does that other earth wire go that drops down from the CU?

One of the main bonds?

What is that little box at the bottom of the frame?

Looks a bit like an old GPO telephone wiring junction box (a 'block
terminal' something-or-other)?

* Use 3-core SWA cable, and use one core (6 mm^2) as the earth - and
join it onto the end of the T&E's 2.5mm^2 protective earth conductor -
thus exporting the earth from the domestic CU

This is slightly more reliable that the next option in that there is
less chance of the earth becoming poor due to corrosian etc at the gland
- but as you later say these are internal connections and a new garage
is unlikly to be damp so there is not much in it. If doing this I would
tend to use one core *and* the screen.

* Use 2-core SWA, and use the steel armour as an earth, again joined
to the T&E's earth conductor - again exporting the
earth. [There will be no external connections to be subjected to
'weather', so corrosion shouldn't be a problem]

One slight advantage with this one is the cable will be a little thinner
and easier to handle.

Using a core as CPC gives you a 6 mm^2 conductor with a resistance of
about 3.7 milliohm/m. Using the armour gives you a CPC equivalent to
9.8 mm^2 copper, but with a resistance of around 7 milliohm/m. I'd use
the 2-core unless the CPC resistance (R2) is a problem in the design.

The weakest link in this design though is the 2.5 mm^2 CPC in the T&E
cable. Note that it would not be acceptable to use this if there are
any metal services rising in the garage that would need main bonding -
this because the CPC of the submain also acts as the main bonding
conductor, so would need to be a minimum of 10 mm^2 Cu equivalent.

* Use 2-core SWA and an earth spike at the garage end - thus providing
the garage with its own earth - independent of the house supply

If the house end is not already TT then the arguments for a separate TT
supply in an outbuilding usually come down two one of two reasons -
either it is a long distance (obviuosly not the case here), or, when
there will be potential difficulty extending the house's equipotetial
zone into the outbuilding (of most concern with a PME house).

As I usually say here, the TT/TN decision depends on the use of the
outbuilding and the practicality of creating an equipotential zone
there. At one extreme is a dry workshop building where there will be
little use of electrical equipment outdoors (TN OK here) and at the
other extreme is the metal greenhouse (TT essential). Any use extensive
of portable Class 1 (earthed) equipment outdoors again signals the need
for TT earthing.

* Some combination of the above. [Is there a down side to exporting
the earth *and* having an earth spike?]

No, and there is an advantage to doing that - but understand that it's
still TN-C-S with its attendant risks. You are adding another earth to
the M (multiple) in PME, so you'll help reduce the touch voltage in the
event of a broken supply neutral. You do need a damn good earth
electrode to do much good though, and it should be connected to the main
earth terminal in the house, not at the garage end via a weedy 2.5 mm^2 CPC.

[...]
SWA would go straight out through the wall from the back of the box -
with its gland fitted to a knock-out in the box. Does anyone see any
problems with this?

That's completely impractical because you've go no access to the back of
the box to tighten the gland nut. It would also be extremely fiddly to
do this with a single-gang accessory box. Better as John said to
connect the SWA in the loft, alternatively take the T&E through the wall
to a box on the outside, and run the SWA straight down from there.

[...]
Do I need to earth the SWA's armour even if it isn't being used as
the protective earth?

Yes, definitely, to provide protection in case the cable is damaged.

--
Andy