Electrical Fires: Origins Common and Unique. Factory Mutual Engineering
Abstract: This article provides some statistics of electrical
fires from 1965-1975. The statistics were compiled from Factory Mutual insured
plants. During these ten years 3, 882 fires of electrical origin occurred.
Fifty percent of the fires were a direct result of wiring and over seventy
percent of the fires were caused by the top six electrical causes. The article
summarizes these six most common electrical causes.
Wiring is the most common electrical cause of fires because they are
connected to almost every part of the electrical system. Grouping wires
together in steel enclosures or other similar devices is a common practice.
This allows fires to advance before discovery and makes them harder to extinguish
because they are inaccessible, which results in more property damage. To
prevent wire fires, cables with non-combustible insulation or self-extinguishing
cables should be used. Also, wires can be coated to prevent the spread of
fires, but this must be done carefully so additional heat is not generated.
Electric motors that are maintained improperly or are not equipped with
the proper electrical protection are the second greatest cause of electrical
fires. The most common electric motor cause is overheating due to single-phasing.
When a polyphase motor is operating without the proper overload relays,
it can overheat and ignite when one phase becomes de-energized due to a
fuse or the circuit breaker. Motors should be lubricated to the exact specifications
because too little, as well as excess lubrication are both dangerous. Motors
should be cleaned regularly because dirt can block air passages, allowing
heat to build up. Synchronous motors should be maintained properly because
there are many malfunctions that can cause overheating. Lightning surges
also present a hazard because the sudden increase in voltage can cause the
insulation to fail. To prevent this, lightning arresters should be installed
close to the electrical equipment.
Electrical elements, mostly resistance grids and heating elements, rank
third on the list of most common electrical fire causes. These fires occur
because these elements are not installed or protected properly. Sometimes
resistance grids are simply placed too close to combustible materials. Most
heating elements overheat because the temperature controls are not accurate
or they fail. The temperature controls of a heating device should be backed
up with an excess temperature indication device.
Controllers and switches are also a common cause of fires when they are
maintained improperly. Overheating and arcing will occur because contacts
carrying current become dirty, loose, or corrode. Overcurrent protection
for control centers is needed to prevent this from happening. Circuit breakers
that control electric arc steel melting furnaces are subjected to extreme
stress. In these instances, an air- circuit breaker should be used and maintained
Transformers fail frequently resulting in fires. Overloading, internal
failures, and lightning are some of the reasons transformers fail. The oil-insulated
transformer is the most popular because it is the most inexpensive; however,
it the most dangerous because internal failure can cause the expulsion of
burning oil resulting in massive damage. The national electrical code has
specific regulations for the installation of both indoor and outdoor transformers.
Generators are the sixth most common electrical fire cause. To prevent
oil-fire hazards, oil lines should be separated from steam lines and hot
turbine parts. Hot, exposed parts should be insulated to prevent oil from
coming in contact with them.
Most electrical fires are a result of minor defects that can be detected
in the course of frequent maintenance. Proper protection and installation
of electrical systems will also help prevent fires.
For more information, contact:
Factory Mutual Research Corporation
1151 Boston-Providence Turnpike
Norwood, MA 02139
Phone: (781) 762-4300