SWA Wiring to shed from existing outdoor socket

More info needed. 
1) how are the existing sockets supplied - own circuit, or spurred off the house ring, or just hung off the back of the light switch  ? You need to know, and to be sure they are in fact supplied correctly to begin with, i.e. from a circuit that can take the load, and the supply cable is co-ordinated with the rating of the fuse or breaker, and there is RCD protection.
If they are spurred off an existing ring, it is a very good idea  to add a fused spur or RCD spur with a switch inside the house to protect just the outdoor bit, than you can isolate it when you go on hols or if water gets in.

2) This SWA - what is it's copper cross section? - for sockets over that distance I'd expect at least 2.5mmsq if the circuit fuse or breaker was 20A or less. Perhaps 4mmsq. Again, the cable size needs to co-ordinate for the rating of rest of the circuit. Look up how to terminate the IP rated glands unless you already know what you are doing.
3)Digging in  - if it is not under a concrete path, I'd suggest aim for half meter down or more, in case the garden gets dug at some point.

4) Earthing. How is your house earth, TT, TNS or TNC-s . Also is the shed floor timber and dry, or cement or earth or possibly damp, and is it possible you may plug something in and take it outside ?
Please confirm all these things, as it affects what is recommended for the earthing at the shed end of thing

5) Using a fused spur off the power cct.  for the light switch at the shed end may be OK - but you do need to think about what happens if lights all trip off due to a faulty appliance  - emergency lights or any critical loads.

So in summary it may well be OK, but it depends.





 

I have just undertaken a consumer unit renewal. The house was previously owned by a builder. The new owners are having lots of work done including a new kitchen. The builder had run an S.W.A. cable from the back of a downstairs' room socket through the wall to a pond pump position. The cable did not have the armour earthed, well it was not even connected. The outdoor socket was an ordinary indoor type on a pattress mounted on a low wooden stake. No R.C.D. protection at all. The circuit was protected by a B.S. 3036 30 Amp fuse.  The socket was protected from the elements by a removable wooded "hat" that wobbled a bit on the socket stake. Needless to say I disconnected the cable.

Z.

Zoomup:
The cable did not have the armour earthed, well it was not even connected.

Yes, that is exactly it Chris. That is why unqualified people should not dabble in the sacred art of electrical engineering. They should leave it to us, the professionals, don't you think?

Z.

Judging by the horrors under my daughter's floorboards electricians should not dabble in the sacred art of electrical engineering. 😯

The interesting thing is that virtually all, if not all, of the defects have been revealed on inspection rather than testing.

Exactly, you (experienced electrical engineer person) know what to look for in the electrical dept. whilst playing with the spiders under the floor boards. 

Z.

Thanks guys, back to the more info required qs - I’ll come back with some more info. I fitted SWA previously for some outdoor sockets onto appropriate junction provided by my electrical contractor - and earthed it 😀👍

Good. You pretty much always should connect  the armour to the earth at the supply end, for all the common house earthing arrangements.
However the earth at the load end may or may not be taken from the armour, it may instead be that the armour is insulated, and earth taken from a local electrode, depending if it is better to create a TT island for the load - examples where it is better to have a local earth rod and not use the suppliers earth at  the load end, when the suppliers earth is PME /TNC-s include for car charging points, swimming pools, areas with earth floors and livestock, caravan pitches, and a few other situations. Examples where you would use the house earth are if it has a dry wooden floor and all the electrical items are domestic, and essentially indoors.
Between those extremes lies a large  area requiring some thought.