Surge protector

 

 

A surge protector (or spike suppressor, or surge suppressor, or surge diverter) is an appliance or device designed to protect electrical devices from voltage spikes.
A long term surge, lasting seconds, minutes, or hours, caused by power transformer failures such as a lost neutral or other power company error, are not protected by transient protectors. Long term surges can destroy the protectors in an entire building or area. Even tens of milliseconds can be longer than a protector can handle. Long term surges may or may not be handled by fuses and over voltage relays.
Underwriters Laboratories (UL), a global independent safety science company, defines how a protector may be used safely. UL 1449 became compliance mandatory with the 3rd edition in September 2009 to increase safety compared to products conforming to the 2nd edition. A measured limiting voltage test, using six times higher current (and energy), defines a voltage protection rating (VPR). For a specific protector, this voltage may be higher compared to a Suppressed Voltage Ratings (SVR) in previous editions that measured let-through voltage with less current. Due to non-linear characteristics of protectors, let-through voltages defined by 2nd edition and 3rd edition testing are not comparable.
Every time a MOV shorts, its internal structure is changed and its threshold voltage reduced slightly. After many spikes the threshold voltage can reduce enough to be near the line voltage, i.e. 120 vac or 240 vac. At this point the MOV will partially conduct and heat up and eventually fail, sometimes in a dramatic meltdown or even a fire. Most modern surge protectors have circuit breakers and temperature fuses to prevent serious consequences. Many also have a LED light to indicate if the MOVs are still functioning.
Lightning and other high-energy transient voltage surges can be suppressed with pole mounted suppressors by the utility, or with an owner supplied whole house surge protector. A whole house product is more expensive than simple single-outlet surge protectors and often needs professional installation on the incoming electrical power feed; however, they prevent power line spikes from entering the house. Damage from direct lightning strikes via other paths must be controlled separately.
Most modern surge strips and house protectors have circuit breakers and temperature fuses to prevent serious consequences. A thermal fuse disconnects the MOV when it gets too hot. Only the MOV is disconnected leaving the rest of the circuit working but not protected. Often there is a LED light to indicate if the MOVs are still functioning. Older surge strips had no thermal fuse and relied on a 10 or 15 amp circuit breaker which usually blew only after the MOVs had smoked, burned, popped, melted and permanently shorted.