In uk.d-i-y, RichardS noaccess@invalid wrote:

In my case, it's a wooden shed about 12m from the house, with portable power
tools being used in it...

For the equipotential bonding issue your answer implies that the SWA would
have to be run right back to the CU and terminated there, rather than at the
point of entry into the house, or are we talking about a bonding earth cable
completely separate to the SWA sheathing here?

Yes - one or the other ;-) You really have to do the earth-loop-impedance
calcs to be sure. The "unbroken" requirement, though, does mean that
if you can get away with using the steel (note, steel, not copper, so
a rather higher resistance) armour as the bonding conductor, the SWA needs
to run unbroken to the CU and have its sheath made off directly at your
main earthing terminal - possible but messy (weird pigtailing of the
sheath, wrapping the wrapped-and-collapsed sheath in green-and-yellow
heatshrink, and bringing it through the CU to the main earth terminal).
And at the garage end, full adherence to the "unbroken" requirement
if you need to cross-bond structural metalwork or water/gas pipes
makes it impractical. Hence the more common route is to end up with a
hefty separate bonding conductor, likely thicknesses off the top of my
head being 10 or 16mmsq, cable-tied to the outside of the SWA, doing
the bonding-in-the-outbuilding-thing, "visiting" (looping through) the
earthing point in the garage, continuing along the route of the SWA,
and then running direct to the PME earthing point. Heavy...

All of which sounds, thank goodness, like it's going to be unnecessary
*for* *your* *particular* *installation* (no generalisations to
bottom-of-the-garden brick-built workshops with 63A submains and
a water main connection, please!): you've a *wooden* shed, so presumably
no fixed metalwork which would need equipotencial bonding; and with a,
what, 20A? 30A? breaker in the CU which feeds your 12m of, what, 4mmsq?
6mmsq if you want minimum voltage loss and lower earth-loop-impedance
figures, you're likely (but I haven't done the calcs!) to be in good shape
to meet the 0.4s disconnection time requirement even without an RCD for
the whole shed. (You may find it sensible to use 3-core SWA with its own
4mmsq/6mmsq earth - sorry, circuit protective - conductor, rather'n
using the sheath for the main earth; you'd still earth the sheath at
the house end, but now you're using the sheath for mechanical protection
only. It'd be nicer to keep the shed lights off the 30mA RCD you'll want
for the shed sockets; that way you'll still see what the spinning power
tool is doing after you slice through its supply cable or whatever ;-).

Hope this helps rather than confusing... Stefek