http://www.screwfix.com/p/prysmian-lv-armoured-6943x-3-core-cable-4mm-x-25m-black/67843
http://www.screwfix.com/p/mk-1-gang-32a-dp-switch-with-neon-metal-clad/31874
http://www.screwfix.com/p/bg-3-way-metal-garage-unit-2-mcbs/1926g#_=p
http://www.screwfix.com/p/tower-external-gland-kit-20-ip66-pack-of-2/56505#product_additional_details_container
BEFORE CARRYING OUT ANY WORK MAKE SURE YOU TURN OFF THE POWER, ELECTRICITY CAN AND DOES KILL!!!!!
The above list is for parts needed to supply power to a shed no more than 20 mtrs from the house. from a spare rcd protected way in the consumer unit, rated at 20 amps take a length of 4mm twin and earth into the 32a dp switch, making sure to use a grommet where cable leaves consumer unit and where cable enters switch, sleeve earth with green and yellow sleeving.
Next, having buried the armoured cable a minimum of 600 mm deep, with 100 mm of soft sand above and below cable, and the appropriate warning tape 300 mm above cable, bring both ends into the buildings, at shed end, secure a piece of marine ply, to the wall and attach the consumer unit to this. gland the armoured cable correctly and connect to the consumer unit, the earthing tag should be fitted to the gland before inserting into the knock out on the unit, drill a 6mm hole through unit and secure tag with a nut and bolt, on inside of unit attach a short length of 6mm green and yellow cable to bolt using a crimp lug and attach other end to earth bar in unit, connect armoured cores to incoming live terminals. I recommend using brown as live, grey (sleeved blue) as neutral and black (sleeved with green and yellow) as earth
Make sure all connections are secure and tightened down correctly.
Bring other end of armoured cable into the house and after glanding it off connect to load side of switch, remember also to attach short lead from earth tag to earth terminal. Secure cable where appropriate with cable cleats at no further than 400 mm spacings.
You now have power into the building and can connect sockets or lights to the unit in the garage/shed.
After carrying out the work, make sure you test for correct insulation resistance, polarity, earth loop impedance and correct trip times on the RCD, Also note this work is notifiable under part P of the building regulations in some parts of the united kingdom.
The maximum measured earth loop value for a 20A mcb type B to BS 60898 or BS 3871 as laid down in the current edition of the IEE wiring regulations is 1.74 ohms.
The above is based on using a 3 core 4mm pvc insulated SWA cable, clipped direct or buried underground, on the specified cable, based on the 20 mtr run the maximum load would be 6Kw, or 26.09 amps with a volt drop of 5 volts, which is 2.2%.
Whilst some will say that 2 rcd's in series is pointless, modern homes have a fully rcd protected consumer unit and I do not wish to encourage people to mess with it, while it cannot be guaranteed which rcd will trip first at least one will, thereby giving protection.
All described above are my personal views and are not intended as a definitive guide to electrical work, I take no responsibility for death or injury that may occur from carrying out the above works, my advice would always be to use a qualified electrician. however hopefully following the above method will prevent a lot of the poor installs we see on a regular basis.
While the above is suitable for earthing systems using TNS or TNCS (PME) I would not advise on a TT system. I am aware that many people have an objection to "exporting" earthing. however I believe that you cannot obtain a better earth with a rod than you can using the supplied earth.
As stated earlier I do not advocate people doing electrical work without the correct training, but if you are going to do it then the above method if followed should result in a safe installation.