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Written by
Jim Pharr
Fire Marshall
Gaston County, North Carolina
Preventing electrical surges from reaching and destroying delicate electronic equipment
is a vital concern for businesses and homeowners. As use and versatility of electronic
components increases, so goes the demand for something to assure information is not
corrupted and devices work when desired.
For more than a decade inexpensive devices designed to prevent damaging voltage from
harming electronics such as computers and entertainment centers have been marketed
under the general category of surge suppressors. Most of these suppressors resemble
multiple power taps that feature housings providing four to six connections for electrical
plugs. This enables the user to expand fixed outlet capacity while receiving overcurrent
and over voltage protection.
In the late 1990’s the Consumer Products Safety Commission issued warnings that
multiple power tap devices were subject to overheating that could result in fire. These
fires were categorized as improper soldering of connections and improper use, i.e.
overloading. It should be noted that these taps were not equipped with devices to prevent
over voltage but were outfitted with integral circuit breakers to preclude excessive current
within the conductors.
Recent fires involving multiple outlet devices toted as surge suppressors raised attention
at the Gaston County Fire Marshal’s office primarily when one such fire occurred in a
fire station. Investigation of a fire that started behind a desk in an office revealed the
ignition source was a surge suppressor. Immediately prior to the fire workers were
attempting to connect an emergency generator to the firehouse’s electrical system to
provide an alternative electrical source. Workers noted problems with the system and
later, after the fire, discovered the generator was shipped from the factory with a loose
neutral (a condition that causes voltage differences between legs of a 120 volt electrical
system). Fire fighters noted fluctuations in their radio and other electronics thus started
to disconnect all electronics from the system. In the office area they discovered a small
fire burning behind the desk. A portable fire extinguisher was used to suppress the
flames and an investigation was initiated.
Within that firehouse, three separate surge suppressors were recovered and examined.
Each had failed, the one caught on fire, another suppressor ceased working, while the
third continued working but later was found to have failed internally. These findings,
coupled with suspicion of suppressor involvement in other fires, prompted in-depth
examination of possible reasons.
To understand how a surge suppressor is capable of generating a fire, one must
understand the technology engaged in most low cost units. Prevalent technology
incorporates a Metal Oxide Varistor (MOV) across conductors to redirect excessive
voltages ahead of connection with appliances. Less expensive units typically have a
single MOV bridging the hot and neutral conductors so that when over-voltage occurs on
the hot leg it is dissipated through the neutral conductor and not experienced by
connected appliances. More expensive units have multiple MOVs arranged in various
configurations while even higher priced units have more sophisticated features to
supplement MOVs.
A MOV is a small disk with two lead wires that serve as conductors positioned on
opposite sides of a Zinc Oxide disk encased in epoxy. During normal operation with line
voltage (120 volts) no voltage passes through the Zinc Oxide disk, however when voltage
exceeds the MOV’s ‘clamping voltage,’ energy passes from a conductor on one side of
the disk through the Zinc Oxide to the opposing conductor. Typically this causes no
occurrence, however when excessive voltage or repeated spikes are experienced, the
MOV can fail. Failure occurs in one of three ways:
1) Conductors inside the MOV weld together forming a short circuit. This action
should cause the integral circuit breaker to open preventing the unit from
working. When this happens, the user must replace the suppressor.
2) Conductors inside the MOV completely separate preventing electrical flow
across the MOV’s components. Though the outlet strip will continue to
function, no surge suppression capability is present. Unless the unit is
equipped with an indicator light, the user could remain uninformed that no
protection is present.
3) Conductors form a circuit with high resistance. In this case a heater is created.
Depending on characteristics generated, this is the result that initiates fires.
Figure 1 – Circuitry from a simple surge suppressor
Metal Oxide
Cut Out
More modern surge suppressors are manufactured with a Thermal Cut Out mounted near,
or in contact with, the MOV that is intended shut the unit down overheating occurs.
It should be noted that most surge suppressors house MOVs and other components inside
plastic housings that ignite and contribute to fire development.
When fire investigators examine fire scenes where surge suppressors are involved in the
ignition few know what patterns indicate failed MOV’s. If not properly collected,
suppressor parts cannot be carefully examined to determine involvement, thus fire cause
is improperly categorized in reporting systems.
Alternatively, fire investigators my correctly determine the suppressor was involved in
ignition but improperly categorize the cause as overloading or other related failure
initiated by the user.
As one digs through a fire scene, when stranded conductors are discovered the
investigators should carefully excavate the area while examining all debris. Examine and
collect all debris that may be connected with the device. Document electrical arcing or
other indicators of over voltage within the device or, if the device is in sufficiently good
condition, which parts first burned. Examination electrical wiring, rails and MOV
components are primary concerns.
MOVs exposed to exterior fire will exhibit smooth surfaces and intact lead wires while
those which have been exposed to surge and internal fire are more likely to have irregular
surfaces and lead wires that are diminished. Sifting the debris with a fine screen may be
required to find MOV components. Zinc Oxide disks are very fragile thus are easily
damaged or destroyed by fire suppression operations.
Figure 2 - MOV exposed to external flame during testing
The epoxy coating burned away exposing the fragile Zinc Oxide Disk and lead wires.
Note the disk’s smooth surface and lack of distortion on the lead wires.
Photographs in figures 3-6 are from a Belkin F5CO47V360 surge suppressor that was
used in police department bomb truck. Officers involved in a training exercise noted
smoke emitting from the unit disconnected it from the receptacle, thus prevented the
possibility of fire. Note the irregular patterns on zinc oxide disks. Lead wires were
absent or melted into the disks.
Figure 3 – MOV exposed to electrical energy and internal fire
Figure 4 – MOV exposed to electrical energy and internal fire
Figure 5 – MOV fragment – side A
Figure 6 – MOV fragment – Side B
Determine the device manufacturer through markings on plastic covers and on remaining
electrical cords if possible. When damage precludes identification of writings, engineer
familiar with this manufacturing process may be able to identify the product through tool
markings and manufacturing methods, therefore assure all components are collected for
later examination. When units are not fully destroyed, use non destructive methods for
examining components not visible.
As with many appliances, more is learned from those slightly damaged than ones that are
completely destroyed by fire. Report all incidents where surge suppressors ignite even
when damage does not progress beyond the housing.