On Tue, 20 Jan 2004 09:54:16 -0000, "Andy Wade"
wrote:

"Matt Pearson" wrote in message
...

Do you think having a 2nd earthing source is good practice? My query
was do you feed house supply cable to earth rod or to the shed csu
first and then hook the earth rod into the shed csu.

This thread seems to be generating more heat than light and not much in the
way of signal-to-noise ratio.

Briefly, there are two main earthing options:

1. Exported house earth
-----------------------
Simply 'export' the house earth via the armour of the SWA cable (the
'submain'). This will be fine for a dry timber shed with its floor raised
well off the ground, so that it remains dry. With this option the shed
installation has TN-S earthing and RCD protection is only essential for
sockets likely to be used for feeding portable equipment outdoors. If
non-RCD protected circuits are used then you need to be sure that the earth
fault loop impedance (Zs) is low enough to operate the fuse or MCB at the
house end within 5 seconds in the event of an earth fault at the far end of
the submain cable, and to operate the protection for any socket sub-circuits
in the shed within 0.4 s.

If Zs is too high then the options a (a) use a larger size SWA cable,
and/or; (b) use a 3-core SWA cable with one core as a CPC (earth) in
parallel with the armour, or; (c) use an RCD at the house end (a 100mA
Type-S RCD if you want discrimination with any RCDs in the shed itself).

With this option there is absolutely no point in using a local earth
electrode; its resistance simply will not be low enough to do any good.

Do NOT use this option if the house earthing is TN-C-S (PME) and either of
the following apply:

- the shed floor is damp, or the 'shed' is in fact a greenhouse;

- you plan to use Class I (earthed) appliances outdoors on a regular
basis (most portable tools etc. are Class II ('double insulated).

If there are any metal service pipes entering the shed (water, etc.) then
they should be bonded to the incoming earth near the point of entry.

2. Separate 'TT' installation
-----------------------------
This option uses independent local earthing and is ultimately safer, but
only if the RCD(s) are correctly selected and regularly tested, and the
earth electrode is properly installed and maintained. The rationale is that
by limiting the current dumped into the local earth, its voltage will not
rise to a dangerous voltage relative to the ground.

The armour of the SWA cable is earthed at the house end, but MUST be
isolated at the shed end. This can be achieved by using a special isolating
gland (expensive) or by terminating the cable into a plastic housing via a
plastic cable gland. The armour should be treated as being live, so should
not accessible or connected to accessible metalwork.

ALL circuits in the shed must have RCD protection. For a comprehensive
workshop installation follow the advice in the OSG and use at least two
RCDs, or separate RCBOs, but for a simple garden shed a single 30mA RCD will
be quite OK. This can be at either the house or shed end, but it migh be
inconvenient to have to walk back to the house if it trips. If the RCD is
at the shed end then you still have to make sure that the submain cable is
fault and earth fault protected - see remarks about Zs, above.

An earthing system is required. In most soils a single 8 ft. earth rod (two
4 ft. sections screwed together) will be OK, but the earthing resistance
should always be measured. The BS 7671 requirement is that the product of
the earth electrode's resistance to earth and the highest rated RCD must not
exceed 50 volts - or 25 V if it's a 'horticultural installation'
(greenhouse). This allows quite high earth resistances to comply with
regs - 1,600 ohms with a 30 mA RCD, but note that the OSG recommends a
maximum value of 200 ohms. The connection to the earth rod should be in
16mm^2 wire (unless protected in conduit, etc.) and the joint should be
accessible, unless welded. Using a proper 'earth pit' from the electical
wholesaler is the professional way to do this.

~ ~ ~ ~ ~ ~ ~

Going back to the original enquiry: this is only two lights and 'small
power' in a shed, probably just for running portable power tools and maybe a
fan heater, with the outside socket being for a lawnmower or hedge trimmer,
etc. It's hardly a comprehensive workshop installation.

Design
------
The first step, as always, is to establish the maximum demand and I'd guess
that about 4kW would be adequate, or 5kW at the most. If that's the case
then a 16A (3.7kW max.) or 20A (4.6kW max.) circuit from the house is all
that's required -- unless the OP wants to provide for future expansion.
There's no real need for a separate consumer unit here. To keep things
simple the sockets could be connected directly to the circuit cable and the
lights fed via switched fused connection units, fitted with a 5A fuses and
used as the lightswitches. You could also add a 20A control switch to act
as a main isolator if desired. Something like this, perhaps (or re-arranged
to suit the desired physical layout):

DP SW FCU DBL SKT FCU
20A 5A 13A 5A
__ __ __ __
From house ----------|__|-----|__|-----|__|------|__|
+ | ^ *| |
| | |
| | |
Shed -- | -- PIR
light | light
| __
---|__|*
Outside
socket

RCD options (if no RCD at house end):
+ use RCD as main isolator (essential if using local TT earth option)
^ insert RCD here (for 'exported' TN earthing only), or
* use RCD sockets

For a tidy looking job use metalclad wiring accessories mounted side-by-side
and coupled together using 20mm conduit bushes and lock-rings.
Terminate the incoming SWA onto the first accessory (the DP switch in my
sketch). The only additional wiring is then that to the lights and outside
socket.

Cable size
----------
The max. circuit lengths in 2.5mm^2 cable are 33m for a 16A circuit and 27m
for 20A (voltage drop limited). This makes the use of 2.5mm^2 SWA rather
marginal and I'd strongly recommend using 4mm^2 to keep the voltage drop
down. The difference in cost is tiny.

HTH

Nice write up. As this comes up a lot, how about sending to Phil to
go into the FAQ....

I've used both of the described methods for different applications.

Do you have any particular suggestions where the provisioning is for a
workshop and heavier equipment is in use - say 30A single phase, where
the workshop is dry and the house supply is TN-C-S?





..andy

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