Apparatus and materials
Twin mains socket, 13 A e.g. MK781
Mounting box, plastic, e.g. MK2172. This must be about 40 mm deep
Terminal block, standard screw terminal 6 A, e.g. RS Components 423-166
Flexible mains cable, 2 m, 6 A, 3-core, double insulated
Mains plug, 13 A
6 A green/yellow insulated flexible conductor, 1m (the earth core stripped from a length of cable similar to that listed above)
6 A blue insulated flexible conductor, 15 cm (the neutral core stripped from a length of cable similar to that listed above)
Plywood, chipboard, etc. slightly bigger than the mounting box and about 10 mm thick, one piece of
All the components can be obtained from local electrical suppliers. MK is a trade name and other makes will be suitable. However, the plastic mounting box must be deep enough to allow the terminal block to fit under the sockets.
a Cut off a length of terminal block containing four terminals, using a small saw or a knife.
b Make marks for holes on the front and bottom of the mounting box to give access to the outer two terminals when the length of terminal block is fixed upside down inside it. (Figure 4a)
c Drill the holes carefully as the plastic is often brittle. The two holes in the side should be about 1/4” (6 mm) in diameter; it is sensible to drill smaller pilot holes first. The two holes in the bottom should be about 3/16” (5 mm).
d Drill an extra hole in the bottom so that the terminal block can be fixed to it; a 4 BA or 3 mm metal screw is usually appropriate.
e Wire up components as in above. Secure the mains cable with the clamp in the box.
f Fix the terminal block in place. Screw the mounting box to a base slightly bigger than itself. Holes must be made in its base to give access to the two holes in the base of the box.
g Test the completed device with an earth-bonding and insulation tester (e.g. a Clare tester) with the earth clip attached in turn to the earth pin in a plug put into one of the sockets and to the green/yellow wire. Label the device ‘Not for use as an extension lead’.
The safety details of this experiment are published with permission of the Assocition for Science Education. It originally appeared in Appendix 5, Topics in Safety (2nd edition).
NOTE: The latest edition of Topics in Safety is the 3rd edition, published in 2001. Details of these experiments were not included in the 3rd edition because the safety warnings were then available in the DfES publication Safety in Science Education and the details in CLEAPSS publications.