how to wire a plug for a sky box

Mains Plugs in the U.K.

Mains plugs have been responsible for many, many receiver faults and have also contributed to death and injury.

Some years ago, a friend was drilling a hole in his kitchen wall when the metal casing of his old power drill became "live". The shock he received caused his muscles to contract and he fell from the metal step ladder, still clutching the whirring drill.

13 Amp plug inside viewThe continuing shock made him dance like a puppet, leaving a trail of gouges around the kitchen, until finally he managed to kick the switch on the plug socket. It transpired that the earth wire inside the plug had come loose and touched the live pin, thus passing the live mains through the "earth" wire to the drill body.

You may laugh at this little anecdote because my friend survived, but the story serves to illustrate the importance of correct and secure mains plug wiring. So, even if you think you know how to wire a plug, please read on.

Your choice of plug is very important from the point of view of safety and ease of fitting. There are several cheap plugs on the market which are at best fiddly to assemble and, at worst, downright dangerous. The plugs which have been approved as safely designed and manufactured now carry the designation BS1363 (or BS1363A for the type which will withstand knocks), so avoid those which do not.

Don't be ashamed to take a plug apart in the shop to inspect it. Ask to borrow a screwdriver, if necessary, and tell the shopkeeper where to put his plug if he is unhelpful. (However, if he is serving someone who is buying a 500 electrical gizmo, be patient and wait a couple of minutes).
The features to look for in a plug are as follows:
1. Fuse clips which are rivetted or welded securely to the other metal parts. Beware flimsy rivets and screws which can work loose and cause serious overheating. Fuse clips which are silver plated will be more reliable than those which are not.

2. Cable sheath clamp which will hold the cable firmly. The best ones use a springy plastic flap which bites into the cable sheath and prevents it from pulling out. The worst are those with a thin fibre bridge held with two screws. Bridges moulded from plastic with tubular ends for the screws are good but fiddly; they are often reversible to cater for thick or thin cable. Be sure to fit them correctly.

3 A captive cover screw. While this feature is not essential, it saves much scrabbling on the floor!

4 Correct value fuse. Sometimes the shop will swap the fuse for a more suitable value. (Sometimes they will offer to sell you a pack of ten).

Fitting the Plug

Use a knife to remove about 40mm of sheath from the end of the cable. Do not saw with the blade but bend the cable over your finger so that when you touch it with the blade the stretched sheath splits. Continue the split around the circumference by turning and bending the sheath while touching it with the blade. Avoid nicking the individual coloured wires, or your own pinkies!

Once the end portion of sheath is removed, fit the cable into the plug, securing the remaining sheath with the clamp. Allow 10mm extra for connection and cut the brown and blue wires to reach the Live and Neutral terminals, respectively.

Any green-yellow wire should be cut with at least 25mm to spare.

Use pliers or wire strippers to remove 10mm of insulation from the end of each wire. (You may need to remove the cable from the plug while you do this).
The strands of each wire should be twisted tightly and doubled over to fit into those brass pins with a hole and clamp screw or else hooked clockwise around terminals which use a screw and clamping washer.

The green-yellow earth wire (if present) is deliberately left long so that, if someone should trip on the cable or tug at it in such a way as to pull the sheath out of the clamp, the safety earth will always be the last wire to be disconnected, thus ensuring that the appliance is earthed while there is a possibility of its becoming "live".

13 Amp plug inside view

Ensure that no loose strands of wire are left, fit the fuse and replace the cover.

Since many appliances are "double insulated" you will find that only two core cable is used, with no green-yellow. Wiring the plug is simpler but do still make sure that the brown wire goes to the live pin via the fuse and the blue wire goes to the neutral pin. Since the cable will be thinner than three-wire cable, be sure to check that the clamp is tight and holds the sheath firmly.

Britain is one of the few countries in the world which uses mains plugs fitted with a fuse. They are also used in Cyprus and most ex British colonies, Malaysia, Singapore, HK, Malta, Gibraltar, Seychelles, Kenya, and so on. Exceptions are Australia, NZ, Canada, & S Africa.

Other countries rely on the fuses at the main fuse-box or, sometimes, on a fuse in the socket. The fuse in the mains plug, therefore, while not being absolutely essential, does give an added degree of protection from the risk of fire. If the appliance itself has a fuse then the fuse in the plug will prevent the mains lead from catching fire in the event of a short circuit in the lead. It is also a useful backup in case the appliance fuse does not melt quickly when a fault occurs.

Fuses for use in mains plugs are manufactured to a British Standard BS1362 which should always be printed on the fuse cartridge.

Some appliances are now fitted with moulded plugs. The fuse is accessible for replacement but the wires are not. You should not remove this plug unless it is essential to do so. If you do need to remove the plug, take it out of the mains socket and cut the cable as close to the plug as possible. Remove the fuseholder and fuse. Destroy the fuseholder clip and dispose of the plug so that no child might plug it into a mains socket and receive an electric shock.

This information is given in good faith, based on several years of workshop experience involving thousands of repairs. No responsibility will be accepted for any death, damage or injury caused either directly or indirectly by the use of this information. The reader should check the facts himself. It is assumed and stressed that the reader will be familiar with good, safe, electronic workshop practice and will be familiar with all tools, components and terms used. If there is any doubt about the reader's ability to carry out such work competently and safely, the work should be referred to a specialist engineer.

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