All lightning arresters are designed to become more con […]
All lightning arresters are designed to become more conductive when the applied voltage becomes sufficiently high. Arresters are connected between the electrical device to be protected and a solid Earth ground. Under abnormal voltage stress, they’re designed to break down and become heavily conductive, safely conducting most of the lightning transient to ground. In a coordinated insulation power system, arresters break down at lower voltages than the more expensive equipment they protect.
Early arresters were simple spark gaps. These were useful for protecting low-voltage communication systems, such as antennas, telephone lines, and telegraph lines. However, when used with power systems, spark gaps continue to conduct, causing an AC power arc to ground. This requires power to be shut off, a fuse to blow, or a circuit breaker to trip in order to extinguish the arc. Small spark gaps are still used today within many communication, telecommunication, and signaling equipment since they provide little capacitive or resistive loading in their normal (OFF) state but can handle thousands of amperes of surge current when necessary. Large spark gaps can also be found today as "arcing horns" in electric power systems. These are placed across insulators and insulator strings to protect them after a lightning strike causes them to flash over. They’re designed to stretch out and extinguish the power arc after the initial insulator flashover, preventing the hot power arc from damaging the insulator.
Early "self-resetting" AC power arresters used water and oil-filled aluminum-electrolytic arresters in series with a spark gap. These were similar in principle to modern aluminum electrolytic capacitors and were connected in series to handle higher operating voltages. Once a day, the spark gap was manually closed to temporarily connect the arrester to the power lines. This was necessary to "form" a thin aluminum oxide layer on the submerged aluminum plates. During a lightning transient, the gap would fire, and the oxide layers in the arrester would temporarily break down, safely conducting the transient to ground. After the transient was over, the devices would rebuild the oxide layer as the spark gap arced. As the oxide layer was rebuilt, current flow to ground was reduced, allowing the spark gap to extinguish the power arc. Unfortunately, these devices required daily maintenance and were quickly replaced by maintenance-free silicon-carbide arresters.
Silicon-carbide arresters were solid-state semiconducting devices that would break down under high voltages. Because the OFF current was often quite high, a series of spark gaps were often connected in series with the SiC arrester elements to prevent normal AC voltages from passing through the SiC element. Since these devices required little maintenance, were physically smaller, could handle wider temperature swings, and were quite robust, they rapidly displaced electrolytic arresters by the 1930's.
Modern arresters use sintered zinc oxide as the non-linear resistive element. These devices are called Metal Oxide Varistors (MOV's). They pass little AC current under normal operation, easily pass abnormal lightning surges to ground, and then rapidly recover to prevent follow-through power current from flowing to ground. Larger-diameter MOV's can be made to handle large current, and they can be stacked in series to increase total operating voltage. MOV-type arresters are critical to insulation coordination and protection in modern electrical power systems. Smaller MOV’s can also be found in whole-house surge protectors, corded-outlet strips, and inside many electronic devices.
Unfortunately, MOV's have wear-out mechanisms. Under repetitive breakdown stress, their leakage current increases. This can eventually lead to heating and thermal runaway even at normal operating voltages. This results in excessive self-heating, catastrophic failure of the device, and even fire.