Feed through varistors with thermally-activated override

A varistor (50) comprising:

• a feed-through conductor (52) comprising input (56) and output (58) terminals at opposite ends thereof (52), which terminals (56, 58) are electrically connectable, in use, to an incoming power supply or signal line and to a power or signal input of a device to be protected, respectively;
• and a varistor disc (72) interposed between, and electrically connected to, conductor layers disposed on opposite surfaces of the varistor disc (72), the conductor layers being electrically isolated from one another;
• wherein
• the varistor disc (72) comprises a through aperture (60) through which the feed-through conductor (52) extends;
• a first one of the conductor layers is electrically connected to the feed-through conductor (52);
• a second one of the conductor layers is, in normal use, permanently electrically connected to ground (64);
• the varistor (50) further comprising:
• a thermally-activated override adapted, in use, to disconnect the varistor disc (72) from the feed-through conductor (52) once the temperature of the varistor disc (72) has exceeded a predetermined temperature.