POWER April 2015 - 78

FUNDAMENTALS
Understanding Electrical Fire
Hazards at Electric Generating
Stations
Minimizing the impact of electrical fires in power plants requires a combination
of prevention, compartmentalization, detection, and suppression strategies.
But first, everyone in a plant needs to understand the hazard.
Dominique Dieken, PE, CFPS
F
ires at electric generating stations are
rare-but not as rare as one might think.
Loss history at hydroelectric facilities,
for example, shows that fires involving electrical
equipment other than generators and
transformers account for the highest likelihood
of experiencing a fire.
Although the majority of electrical fires
are small and extinguished quickly, some
have had severe consequences. Occurrences
at Thermalito Power Plant (California) in
2012, Detroit Dam (Oregon) in 2007, and
Watts Bar Hydroelectric Plant (Tennessee)
in 2002 resulted in major electrical fires. In
almost all cases, the fires caused forced shutdowns,
some for a year or more. The loss of
generation as the result of an electrical fire
often outweighs the actual fire damage.
If damaged cables cannot be spliced, large
sections of cables must be replaced. If the
fire is severe, concrete buildings can be damaged
through spalling. Even after a relatively
small fire, smoke and soot removal can cost
several millions of dollars in facilities with
large open spaces. For example, cleanup
costs at Thermalito were reported at approximately
$90 million, with a total cost of the
fire around $200 million.
The Hazard
Electrical equipment carries energy and comprises
combustible materials in the form of
insulation. Electrical fires typically follow a
fault and result in a smoldering slow-growth
fire that can eventually become self-sustaining
while growing exponentially. Products of
combustion usually involve thick dark smoke
(Figure 1).
Electrical fires are Class C fires; however,
the term " electrical fire " is something of a
misnomer, as it is the combustible insulation
that burns, rather than the conductor or
the electrons. Once the energy source is removed,
electrical fires become Class B fires.
Unlike flammable/combustible liquid
78
fires, electrical equipment fires are typically
slow-spreading. Breakers usually fail suddenly
and catastrophically. In many cases,
there is no ensuing fire, and the damage is
contained to the breaker and its housing.
However, when there are nearby combustibles
(such as cable trays), the extreme heat
can cause a major fire.
Electrical hazards are different than " special
hazards " (such as turbines, conveyors,
hydro generators, and the like) because electrical
wiring, motors, and breakers proliferate
throughout the plant. It is feasible and common
industry practice to provide fixed fire
protection equipment for the special hazards,
but not necessarily for common hazards. In
other words, it is usually impractical to provide
fire suppression systems throughout the
entire powerhouse.
Establish Risk Tolerance
It may seem obvious that it is every owner's
goal to prevent and protect against electrical
fires, but it is not quite that simple. The
reduction of fire risks has a price-both in
monetary cost and in equipment accessibility.
Owners must therefore establish their risk
tolerance before fire protection solutions can
be applied. In other words, the lower the risk
tolerance, the more elaborate the fire prevention
and protection solutions will be.
The nuclear power industry, being at the
vanguard of risk assessment, has driven
methodologies for determining an owner's
risk tolerance. This tolerance defines fire
protection objectives. The four common categories
are summarized below:
■ Category I: No electrical faults or cable
damage are acceptable.
■ Category II: Limited cable damage such
as charring is acceptable, but faulting is
not. A fire event would not result in a plant
shutdown, and damaged cable will be replaced
at the next scheduled outage.
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POWER | April 2015
■ Category III: Both cable damage and faulting
are acceptable, but fire propagation is
not. Cables can be repaired with a minimal
plant outage.
■ Category IV: The total loss of all electrical
equipment within the fire compartment is
acceptable; however, the fire should not
spread beyond the compartment.
There is no " right " or " wrong " category;
the important thing is that owners realistically
determine their risk tolerance. In the
author's experience, most generating stations
fall into Category II or III.
Flame-Retardant Cable
The bottom line is that almost all cable in1.
Where there's smoke... This photo
shows a full-scale fire test of a cable tray arrangement
typically found in power plants.
Courtesy: Mike Mowrer, HSB Professional
Loss Control
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POWER April 2015

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