Hardware Reference
In-Depth Information
to 6,000V—considered the maximum level of surge that can be transmitted to a system through an
electrical outlet. Any higher voltage would cause the power to arc to the ground within the outlet.
None of the systems sustained permanent damage in these tests. The worst thing that happened was
that some of the systems rebooted or shut down when the surge was more than 2,000V. Each system
restarted when the power switch was toggled after a shutdown.
The automatic shutdown of a computer during power disturbances is a built-in function of most high-
quality power supplies. You can reset the power supply by flipping the power switch from on to off
and back on again. Some power supplies even have an auto-restart function. This type of power
supply acts the same as others in a massive surge or spike situation: It shuts down the system. The
difference is that after normal power resumes, the power supply waits for a specified delay of 3-6
seconds and then resets itself and powers the system back up. Because no manual switch resetting is
required, this feature might be desirable in systems functioning as network servers or in those found
in other unattended locations.
The first time I witnessed a large surge that caused an immediate shutdown of all my systems, I was
extremely surprised. All the systems were silent, but the monitor and modem lights were still on. My
first thought was that everything was blown, but a simple toggle of each system-unit power switch
caused the power supplies to reset, and the units powered up with no problem. Since that first time,
this type of shutdown has happened to me several times, always without further problems.
The following types of power-protection devices are explained in the sections that follow:
• Surge suppressors
• Phone-line surge protectors
• Line conditioners
• Standby power supplies (SPS)
• Uninterruptible power supplies (UPS)
Surge Suppressors (Protectors)
The simplest form of power protection is any one of the commercially available surge protectors—
that is, devices inserted between the system and the power line. These devices, which cost between
$20 and $200, can absorb the high-voltage transients produced by nearby lightning strikes and power
equipment. Some surge protectors can be effective for certain types of power problems, but they offer
only limited protection.
Surge protectors use several devices, usually metal-oxide varistors (MOVs), that can clamp and shunt
away all voltages above a certain level. MOVs are designed to accept voltages as high as 6,000V and
divert any power above 200V to ground. MOVs can handle normal surges, but powerful surges such
as direct lightning strikes can blow right through them. MOVs are not designed to handle a high level
of power and self-destruct while shunting a large surge. These devices therefore cease to function
after either a single large surge or a series of smaller ones. The real problem is that you can't easily
tell when they no longer are functional. The only way to test them is to subject the MOVs to a surge,
which destroys them. Therefore, you never really know whether your so-called surge protector is
protecting your system.
Some surge protectors have status lights that let you know when a surge large enough to blow the
MOVs has occurred. A surge suppressor without this status indicator light is useless because you
never know when it has stopped protecting.
