Suppressor Technology: Surge Amps, Clamping Volts and Equipment Protection
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Transient Voltage Surge Protection
Copyright © 1996 Francis J. Martino
The Need For Protection
Transient Voltage Surge Suppressors protect against transient voltage spikes that normally
occur in less than one nanosecond (.000000001 second). Transient voltage surges are caused by:
a) switching of lighting and the starting and stopping of motors.
b) electrical fault conditions (equipment failure which passes high currents to ground or from
phase to phase).
c) power failure and the subsequent return of power.
d) lightning strikes that hit the electrical system in your nearby geographical area.
e) lightning strikes that induce transients through radiation of electric-magnetic fields (without
hitting the electrical system).
Transient voltages with levels above twice the standard peak voltage of your electrical system
will cause degradation of electronic components and insulation. Repeated occurrences will
shorten the life-time of your equipment.
More transient voltages are generated from within your building than from outside your building.
In households, the most common sources of transients are from air conditioners, circulating pumps
on a furnace, fans, washing and drying machines, refrigerators, freezers, vacuum cleaners, hedge
cutters, and pool, sump and submersible well pumps.
The suppressors will perform well with electrical switching and distant ligtning strikes. However,
if a lightning strike should occur on the electrical system within a few feet of your building, your
hope is that the suppressor will pass the surge to ground before both the suppressor and your
equipment are damaged.
Peak current for lightning strikes generally range from 2000 to 400,000 amps. The strongest units
for three phase systems will pass currents of 320,000 amps per phase and up to 1,200,000 amps total.
It is important to have surge protection if your equipment is:
a) in an industrial environment.
b) your equipment must be operated throughout a lightning storm.
What Kind of Protection is Available?
Suppressors are available for use with standard three prong wall receptacles, coaxial connectors,
and for hard-wiring to a distribution system.
The most important rating of surge suppressors is the peak Let-Through Voltage. The true
effectiveness of a suppressor is given by the Let-Through Voltage rating, which is the maximum
peak voltage of a transient that is allowed to pass on to your equipment by the suppressor. The
rating gives the peak voltage that occurs within 100 microseconds following the application of a
test impulse. It is the maximum voltage to which the protected equipment will be exposed due to
The Category B Impulse Test as defined by ANSI/IEEE C62.41 will include a 6000 volt, 3000 amp
pulse which will rise to 90% of peak volts in 1.2 microseconds and will decline to 50% of peak in
It is desireable to limit transient voltage peaks to no more than twice the normal system peak
voltages. The normal peak voltages of the standard household electrical service is:
120 volts times 1.414, or, 169.68 volts peak and
240 volts times 1.414, or, 339.36 volts peak.
Twice the normal system peak values are:
169.68 normal peak x 2 = 339.36 volts on a 120 volt circuit.
339.36 normal peak x 2 = 678.72 volts on a 240 volt circuit.
There are two types of suppressors available. The most common and inexpensive utilize one or more
MOVs (Metal-Oxide Varistor). The effect of the MOV is to clamp the maximum voltage value at a
specified level above the peak of the rated RMS voltage. The clamping voltage will increase in a
non-linear fashion with an increase in peak surge amperes.
The other type of suppressor has an electronic tracking of the incoming sine wave so that the
clamping voltage will be a fixed value above the instantaneous value of the sine wave. Such an
arrangement will give an average peak Let-Through Voltage that will be lower than that allowed
by an MOV. The response time will also be faster than that of the MOV.
One suppressor is commercially available for use with 120 volt circuits. The manufacturer has
rated the device at 192 volts peak Let-Through Voltage per the 6000 volt impulse test, and
159 volts peak Let-Through Voltage per the 1000 volt impulse test.
Thus, a transient that is greater than 6000 volts will have a peak Let-Through voltage that is
higher than the rated 192 volts. A transient that is less than 6000 volts will have a peak Let-Through
voltage that is lower than 192 volts.
The unit bears an Underwriters Laboratory listing per UL 1449 specifying its rating to be 330 volts
peak Let-Through. UL 1449 allows no listing lower than 330 volts. A manufacturer may therefore
state in published literature test results that exceed UL 1449 specifications, but to bear the UL1449
label the manufacturer must label the device in accordance with UL specifications as per UL
A three wire device for use with a 120/240 system is available for hard-wiring to single phase
service locations that have amperage capacity ratings of up to 250 amps. It is rated at 370 volts
peak Let-Through Voltage on the 120 volt circuits and 630 volts peak Let-Through Voltage on the
240 volt circuit per the 6000 volt impulse test.
What Does All This Mean For Your Computer or Other Equipment?
If the two above mentioned devices are installed in your home or ofice, incoming transients will be
reduced in two stages.
With the 120/240 panel mount unit installed at the circuit breaker panel, a 6000 volt transient
which comes from either another circuit or outside the building will be reduced to 370 volts as it
makes its way toward your computer equipment on the 120 volt circuits.
If your computer is plugged into the typical 120 volt desk top unit as previously mentioned, the
maximum peak Let-Through voltage of the incoming 370 volt transient that was passed by the panel
mounted unit will then be passed by the desk top unit at a level below the 159 volt rating of the
1000 volt impulse test, which is well below our ideal maximum peak level of 339.36 volts as
Effects of Wire Length
Due to the capacitive effects of the high frequency content of the transient voltages, for every inch
of lead wire there will be an increase of 9.5 volts in Let-Through Voltage.
One manufacturer tests and rates its product at the six-inch lead length as provided with the
smaller sizes and at the sixteen inch lead length as provided with the larger sizes.
Thus the panel mount unit mentioned above is rated at 370 Let-Through Volts on the 120 volt circuit
with the 6000 volt, 3000 amp impulse test and with six inches of lead length.
In calculating the effects of “six inches of lead length,” note that there are six inches in each of
the two lead wires. Thus, the effects of lead length have been determined based on twelve inches total.
If the six-inch leads are extended to ten inches, the Let-Through Voltage increases by 9.5 volts for
each of the additional eight inches of lead length (four additional inches in each of the two lead wires), or, 76 volts, raising the Let-Through Volts to 446.
When comparing units of different manufacture, you must take into account the point at which the
manufacturer has rated the unit. If a rating is defined as "at the module" or "at the lugs," you must
add to the rating the additional 9.5 volts per inch of lead wire required. To compare such a unit with
the above mentioned unit which is rated at the six inch lead length as supplied, you must add to the
manufacturer’s “at the lugs” rating 9.5 volts for each of twelve inches of lead wire to determine the
true Let-Through Voltage capability of the unit when it is installed with six inch lead length.
The above lead length calculation is for hard-wired panel mount units only. A suppressor with a
cord and receptacles will have its suppressor effectively wired at the receptacles. Therefore, it
will provide its rated protection at the suppressor’s receptacles. The 9.5 volts per inch does not
apply to either the cord on the suppressor or the cord on your computer or other protected device.
What Other Ratings Are Used For Suppressors?
a) Joules - indicates the amount of energy that the suppressor can dissipate. If the incoming
energy exceeds the suppressor's ability to dissipate, the suppressor will be damaged and the excess
energy will be passed to the protected equipment.
b) Peak Surge Current in Amperes - maximum current allowed to pass without damage to the
suppressor for a single standard test impulse. Surge currents exceeding that value will be passed
to the protected equipment.
Both of those ratings are measures of the physical strength of the suppressor. A typical rating for
a two wire plus ground suppressor may indicate both 720 Joules and 45,500 Surge Amps.
A panel mount unit with three wires plus ground may be rated 720 Joules Total and 60,000 Amps
Total, and have an additional rating of 360 Joules per phase and 30,000 Amps per phase.
Appropriate Branch Circuit Protection
A surge suppressor that is hard wired into a distribution panel must have adequate circuit breaker
or fuse protection. The breaker or fuse will be sized based on the lead wire size as per the standard
branch circuit protection required by the National Electrical Code.
The suppressor may be placed directly in parallel with any branch circuit that is protected by a
breaker or fuse that is adequate for the suppressor's wire size.
Keep in mind that the time span of a transient surge, which is a short duration, will not cause the
heating effect of a steady state current. As such, the wires to any surge suppressor do not need to
be sized for the surge ampere rating. Follow the manufacturer's recommendation.
Power Quality and Drives LLC