In a vehicle’s ignition process, ignition leads are utilised to distribute the high voltage from the ignition coils to the spark plugs.
Also known as high tension leads, these leads connect each spark plug to the distributor or ignition coil, transmitting the voltage to where it’s needed. If the leads are in bad condition, the spark may be intermittent, resulting in a misfire. They have a thick outer coating that is heat-resistant and an excellent insulator.
While “high tension” is an old phrase for voltage, it indicates a very high voltage. As the spark must jump an air gap, the voltage required is incredibly high.
In older cars, the HT ignition leads carried the high voltage from the coil to the spark plug. However, this high voltage could interfere with the car radio and electrics. Modern cars have a coil pack per spark plug, so the leads aren’t strictly high tension. Instead, they carry the 12V charge to the coil pack, where it’s stepped up to jump the air gap and generate a spark. This has resulted in smaller, lighter leads. They’re no longer referred to as high tension leads, so people call them ignition leads, spark plug cables, and even spark plug wires.
The outer casing is typically made from silicone, rubber, or a similar compound that must keep out moisture, allow the lead to bend and flex, and prevent the current from arcing out. You’ll notice that each end has a rubber boot to keep out moisture, which also helps maintain a good contact and insulate the internal wire.