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Text - Tech - 11 Myths of Surge Protection.txt
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2003-07-16
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11 Myths of Surge Protection
11 Myths of Surge Protection
by Wendell H. Laidley
Myth #1. ANY SURGE PROTECTION IS BETTER THAN NO
SURGE PROTECTION.
This is perhaps the most reasonable, yet the most
misleading of all. With no surge protection at
all, incoming surges will hit only the computer's
power supply (which is considerably more surge
tolerant than the data line circuitry), and will
not affect the system ground level at all. Since
shunt surge suppressors divert power line surges
into data lines, using the wrong type of surge
suppression can actually cause failures throughout
the network. Thus the network is likely to be
better off if individual nodes have no surge supp-
ressor than if they have ones that shunt
power-line surges into data-lines.
Myth #2. A UPS WILL PROVIDE DEPENDABLE SURGE
PROTECTION.
Because a UPS costs far more than a surge
protector, it is often assumed to provide better
surge protection. However, virtually all UPS units
designed for microcomputers simply combine an
inexpensive MOV surge suppressor with a battery
backup power source. The MOV surge protection is
designed primarily to protect the UPS's own
circuitry, and diverts incoming surges to ground,
just like a common surge protector. Unfortunately,
once the surge hits the UPS ground, it will then
couple directly into the computer's data lines.
Since many microcomputer UPSes are used in the
context of local area networks, this problem must
be addressed or the UPS will endanger the network
data line. Some UPS makers show how surges which
encounter the UPS battery are effectively
eliminated. This is true for those surges which
actually reach the battery; but most are diverted
away from the UPS circuitry to ground before they
ever reach the battery. Thus the belief that the
battery in a UPS is an effective surge sink is not
entirely relevant or dependable. Just like the
basic surge suppressor, the UPS protects the
computer power supply; but in doing so, it
endangers the data lines. Another problem with
UPSes is the existence of an alternative surge
path around the battery and inverter. So called
standby UPSes normally provide direct utility
power to the computer, with only the MOVs at the
UPS power inlet offering any surge protection.
On-line UPSes generally have a bypass circuit to
enable utility power to flow directly to the load
in the event of UPS failure. Both these circuits
provide paths for incoming surges. In the case of
the standby UPS, the surge must pass through the
transfer switch. These switches are often
solid-state components with modest tolerance for
high energy surges, so they may not prevent a
surge from passing through.
Myth #3. TRANSFORMERS ARE THE BEST SURGE
PROTECTORS.
Transformers are designed to transmit power, not
to suppress it. The two major advantages of
transformers are their surge-absorbing mass (tech-
nically, their leakage inductance) and their
availability as a complete subassembly, which
eliminates the need to design a custom
surge-processing circuit. And while the
transformer's leakage inductance offers some surge
protection, it provides much less than would
inductors designed spec- ifically for surge
suppression. In fact, a transformer is far from
being an ideal surge suppressor, and it presents
significant disadvantages, including ringing,
regulation, increased source impedance, and
efficiency loss. Transformers also have
substantial parasitic capacitance to ground, and
this can couple surges to ground. Finally,
transformers used for surge protection often
incorporate MOVs, since the transformer itself may
be unable to handle the higher voltages in surges.
The often-cited benefit that isolation
transformers protect against common-mode surges is
some- what of a red herring when applied to
computers, as will be discussed under myth 5.
Myth #4. VOLTAGE-REGULATING TRANSFORMERS ARE
HELPFUL WITH COMPUTERS.
Most modern desktop computers use switch-mode
power supplies rather than older style, linear
designs. A switch-mode power supply draws from the
AC power line only as much energy as it requires
to maintain its output power. In this sense it
responds spontaneously to voltage fluctuations. If
the line voltage drops, the power supply draws
current for a longer period, until it replenishes
the energy it put out since the previous cycle of
the power wave. Because of this natural ability to
accommodate varying source voltages, a switch-mode
power supply gain no benefit from a voltage
regulating transformer. However, switch-mode power
supplies are more sensitive to source impedance
than source voltage, and the increased impedance
inserted into the line by the transformer may
actually hinder the power supply by restricting
the current available. A tap-switching,
voltage-regulating transformer may also introduce
noise if the tap switch hunts back and forth
between adjacent output taps. Computer switch-mode
power supplies often have a wider tolerance for
input voltage than do regulating transformers
themselves. Thus the primary benefit of a voltage
regulating transformer is its leakage inductance,
which is much less than that of an isolation
transformer, but the regulator introduces
offsetting disadvantages. Moreover, the
transformer's promary function, voltage
regulation, offers no material benefits.
Myth #5. COMMON-MODE SURGES CAUSE COMPUTER
PROBLEMS.
Just as modern switch-mode power supplies
compensate spontaneously for voltage variations,
they also naturally attenuate common-mode noise
(i.e., voltage differences between the neutral and
ground lines). Desktop computers have five orders
of magnitude of common-mode noise attenuation
built-in, from the EMI/RFI filter and the high
frequency isolation transformer in the power
supply. Low-voltage, low-frequency ground
potential differences will not cause disruption or
damage, because the primary cause of disruption is
coupling, which depends on frequency and
amplitude. Computers are inherently immune to
common-mode disturbances below a certain
threshold, but problems occur when high- enregy
incoming normal=mode surges (i.e., differences
between the hot and neutral lines) are converted
to common-mode surges by the action of shunt surge
suppressors. Surges originating from outside your
building are always normal mode, since neutral and
safety ground lines are tied together to an earth
ground at the service entrance. Keeping shunt
surge suppressors off circuits powering computers
will eliminate the conversion from normal mode to
common mode.
Myth #6. COMPUTER MODEM DAMAGE IS CAUSED BY SURGES
ON THE PHONE LINE.
The phone line is a high-impedance circuit which
cannot support high energy surges, so they die
away rapidly after the inducing source (e.g.,
lightning) disappears. In contrast, the
low-impedance power- line provides an ideal
propagation network for high-energy surges. Also,
the telephone service entrance is protected to
under 300 volts, while power-line surges can reach
6000 volts before they will arc over in 110-volt
fixtures. Most computer modem damage is caused
when high energy power-line surges are diverted to
the reference ground and coupled into the digital
side of the modem. This elevated voltage then
seeks the phone line ground reference on the
analog side of the modem and arcs through the
modem. As a corollary to this, phone- line
protectors which provide shunts to the power-line
ground (commonly found as cube taps which provide
two phone line jacks that plug into a 110-volt
receptacle) may introduce more disturbance to the
phone line than they relieve, creating more
problems than they solve.
Myth #7. SIGNAL GROUND IS ISOLATED FROM CHASSIS
GROUND.
Some manufacturers attempt to isolate signal
ground from frame ground, but all such isolation
configurations have coupling coefficients and
dynamic ranges which are likely to be exceeded by
high-energy surges. The effectiveness of most such
isolation circuits is generally limited to
short-duration, low-energy noise.
Myth #8. THE ONLY RISK FROM THE POWER LINE IS
HARDWARE DAMAGE.
Computers are vulnerable to data alterations as
bit streams pass through microprocessors. Stray
power surges can alter data or programs, causing
data errors or lock-ups which cannot be traced.
The consequential cost of such soft damage can be
very high, especially if errors are not found and
data files are contaminated.
Myth #9. SURGE PROTECTORS ARE PERMANENT DEVICES.
Most point-of-use surge protectors use metal oxide
varistors as their primary protection component.
Despite all its strengths, this inexp- ensive (15
cent) component wears out a little with each surge
above a very modest threshold... a threshold that
is exceeded mant times a day in most environments.
Unfortunately, the race among surge protector
manufacturers to provide the "best" protection
(i.e., the lowest let- through voltage) has led
them to use lower voltage MOVs which age faster
and fail sooner. The normal failure mode for an
MOV is overheating, and they have been known to
cause fires. Thus MOVs wear out and should be
replaced periodically. Unfortunately, equipment to
test MOVs is very expensive (on the order of
$20,000). Indicator lights purporting to show that
protection is operational ,are not always
reliable; in fact, those are sometimes wired
across the power line and thus only indicates that
the power line is live.
Myth #10. NOTHING CAN STOP LIGHTNING.
While this simple statement is true in absolute
terms, it is misleading for all but the rarest of
cases. Two important factors limit damage from
lightning. First, a direct lightning strike is
extremely rare, although in that event equipment
may be destroyed and people killed. But lightning
normally manifests itself in the power line as
induced currents caused by the lightning's
magnetic field. Thus we normally need only deal
with the induced surge, not the lightning strike
itself, and the energy of that induced surge will
be limited by the capacity of the conductor to
carry the surge energy. The second factor is that
surge voltages are limited to 6000 volts because
any higher voltage will cause 110-volt circuit
fixtures to arc. Thus surge protectors need only
deal with voltages up to 6000 volts, and currents
determined by the circuit imp- edance. There are
surge protectors available which suppress surges
up to 6000 volts and unlimited current to under
250 volts without degradation, and without
disturbing the critical reference ground.
Myth #11. YOU GET WHAT YOU PAY FOR.
The assumption that higher priced surge protectors
provide greater effect- iveness and reliability is
often not valid. Almost all surge suppressors
priced under $200 rely on the same fundamental MOV
components. Much of the supplementary circuitry is
actually peripheral to the surge protection
function, such as lights and switches, or it
provides a minimal level of noise filtering that
will be ineffectual in the face of an actual
surge. Many users would be just as well served
with a $3 hardware store MOV protector that they
discard and replace periodically, as they would
with an expensive protector using the same MOVs,
which will also wear out.
Written by Wendell H. Laidley, President, Zero
Surge Inc.
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