High tension leads or high tension cables or spark plug wires or spark plug cables, colloquially referred to as HT leads, are the wires that connect a distributor, ignition coil, or magneto to each of the spark plugs in some types of internal combustion engine. "High tension lead" or "cable" is also used for any electrical cable carrying a high voltage in any context. Tension in this instance is a synonym for voltage. High tension leads, like many engine components, wear out over time. Each lead contains only one wire, as the current does not return through the same lead, but through the earthed/grounded engine which is connected to the opposite battery terminal (negative terminal on modern engines). High tension may also be referred to as HT.
Spark plug wires have an outer insulation several times thicker than the conductor, made of a very flexible and heat-resistant material such as silicone or EPDM rubber. The thick insulation prevents arcing from the cable to an earthed engine component. A rubber "boot" covers each terminal. Dielectric grease can be used to improve insulation; a small amount can be applied in the inside of the rubber boot at each end of each wire to help seal out moisture. Printing on spark plug wires may include a brand name, insulation thickness (in millimeters), insulation material type, cylinder number, and conductor type (suppressor or solid wire).
The wire from each spark plug is just long enough to reach the distributor, without excess. Each end of a spark plug wire has a metal terminal that clips onto the spark plug and distributor, coil, or magneto. There are dedicated spark plug wire pliers, tools designed for removing the terminal from a spark plug without damaging it.
To reduce radio frequency interference (RFI) produced by the spark being radiated by the wires, which may cause malfunction of sensitive electronic systems in modern vehicles or interfere with the car radio, various means in the spark plug and associated lead have been used over time to reduce the nuisance:
Placing spark plug wires back into their separators or holders during replacement helps to keep them in place despite engine vibration, extending their life. A common problem with spark plug wires is corrosion of the metal end terminals. Better-quality spark plug wires usually have brass terminals, which are more resistant to corrosion than other metals used.
Older engines also have a wire connecting the ignition coil to the distributor, known as a coil wire. A coil wire is of the same construction as a spark plug wire, but generally shorter and with different terminals. Some distributors have an ignition coil built inside them, eliminating the need for a separate coil wire, e.g. GM High energy ignition system and some Toyotas and Hondas.
Many modern car engines have multiple ignition coils (one for each pair of cylinders) built into a coil pack, eliminating the need for a distributor and coil wire. Some car engines use a small ignition coil mounted on top of each spark plug, eliminating the need for spark plug wires entirely.