Surges aren't anything new, of course. In fact, the power coming into our homes today is more consistent than ever before. What's changed is that most gear is now transistor-based (tubes are far better able to handle extra voltage), with delicate surface-mounted parts and microprocessors that can't stand up to surges and spikes. The computer microchips that control many components these days have made them smarter but also wimpier.
That's why many A/V enthusiasts today use a surge protector - more accurately called a transient-voltage surge suppressor, or TVSS - between the wall outlet and their big-screen TV and home-theater receiver. Many are also opting for units that combine surge protection with both line conditioning, which can eliminate noise and "dirt" in AC power lines, and an uninterruptible power supply (UPS), which provides battery backup in the event of sudden power loss so you can safely power down your gear. (For an overview of the different types of surge suppressors, line conditioners, and UPS devices, see Powerhouse.)
Surge suppressors vary widely in both price and performance. Chances are that $20 suppressor you bought for the extra outlets has already outlived its protective usefulness. That's because it probably uses a metal-oxide varistor (MOV) as its primary source of protection, which degrades with use. All suppressors deal with surges by diverting excess energy away from your gear - typically either by shunting it to ground or by absorbing it, most frequently by storing it in capacitors. By far the most common suppressors are so-called shunt-mode models, such as those that use MOVs, which are pancake-shaped components constructed from pressurized zinc-oxide fragments. MOVs have wires on either side, one connected to the "hot" wire, the other to the ground. An MOV is a variable resistor that acts like a pressure valve. When the voltage is below a certain level, the MOVs are inactive. But once it gets too high, they become conductive, taking excess voltage from the "hot" line (which goes to your gear) and bleeding it off to the ground. Once the level drops, the MOVs once again become inactive, so they don't drain needed voltage from your gear.
A number of companies, including APC, Panamax/Furman Sound, and Monster Power, use MOVs in well-regarded products, often in conjunction with other technologies. (For a look inside an APC surge protector, see Inside a Surge Protector.) MOVs react quickly to surges, can handle large currents, and are inexpensive. But they're "sacrificial" components that degrade after every surge, since the zinc-oxide particles weaken after conducting current, so their ability to continue protecting your gear declines with use.
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