Chapter 23: Electrical Power Issues^605
the level of line conditioning performed by a separate line conditioning unit, but a
high-endsurgesuppressorshouldbeabletohandlemostofthenormallinenoisethatcan
be found on virtually every electrical line.
A surge suppressor uses a component called a metal-oxide varistor (MOV) to sup-
presspowersurgesontheelectricalline.Anytimethevoltagegetsaboveaspecifiedlevel,
even if only for a millionth of a second, the MOV redirects the current to the ground cir-
cuit and is not passed on to the device plugged into the suppressor. The specified level of
the surge or spike that the varistor can handle is somewhat limited (a surge suppressor is
not designed to handle the surge caused by a lightning strike). If the level is exceeded, the
MOV is destroyed and, from that point on, the surge suppressor is nothing but an expen-
sive plug strip and all spikes and surges are passed on to the PC’s power supply.
What to Consider When Buying a Surge Suppressor
When purchasing a surge suppressor, remember that a good unit typically costs a bit
more. Don’t let price be your only guide, but a quality surge suppressor will not be the
least expensive one on the shelf. There are $30 surge suppressors that promise complete
protectionthatwillbeworthlessaftereventheslightestspikeontheline.Herearethefea-
tures you should consider when purchasing a surge suppressor:
Amount of energy absorbed You are buying a surge suppressor to have it
absorb over-voltage events and continue to protect your equipment. A surge
suppressor is rated by the amount of energy it absorbs, which is stated in a
quantity of joules. The higher the number of joules the surge suppressor is rated
at, the better the unit. The rule of thumb is that 200 joules is the minimum pro-
tection, 400 joules is average protection, and 600 or more is excellent protection.
Voltage let-through Underwriters Laboratories (UL) has defined a standard
for surge suppressors, UL 1449, that rates surge suppressors by the amount of
electricity they pass through to the equipment plugged into the surge suppressor.
This rating states that if a big surge or spike hits the surge suppressor, itwill be
able to absorb all of the energy except a certain amount that will be passed on
to its outlet plugs. A low number in this specification indicates a better quality
unit. There are three levels defined in the UL 1449 standard: 330 volts, 400 volts,
and 500 volts. When you consider that the device is tested with over 6,000 volts,
330 volts doesn’t sound so bad, but that’s three times more than the typical
electrical outlet nominally supplies.
Clamping voltage and speed Clamping voltage is the voltage at which the
suppressor begins to protect the computer. In other words, it’s the high-voltage
level at which the surge suppressor begins redirecting the over-voltage portion
of the electrical circuit to a ground. The clamping speed is the time delay before
the surge suppressor begins providing its protection or how much time elapses
between detection and protection.