• Arc-resistant equipment—Rather than specifying and installing equipment such as switchgear and motor-control centers
with the traditional design, more arc-resistant equipment is
being used. This substitution of equipment reduces the exposure to the arc flash hazard by directing the energy away from
a worker that is performing a task, such as operating or racking a circuit breaker in or out with the doors closed.
• High-resistance grounding—The use of a high-resistance
grounding (HRG) scheme can reduce the likelihood of an
arc flash as a result of a line-to-ground fault. An HRG system
uses a grounding resistor to limit the ground-fault current
to a value typically around 10 amperes. Note that the HRG
system will not eliminate a fault that begins as phase-to-phase or three-phase event.
• Current-limiting devices—The use of current-limiting
devices, such as fuses and circuit breakers, can greatly reduce
incident energy by interrupting the fault in less than one-half of
a cycle when the device operates in its current-limiting region.
3. Engineering controls
If the hazard cannot be removed through elimination or
reduced to an acceptable level with substitution, the next
method in the hierarchy is to use engineering controls to
reduce the exposure or severity.
• Temporary device settings—This broad category includes
methods such as an arc-energy-reducing maintenance
switch, protective relays with multiple setting groups, and
similar devices and functions. This type of control method
provides the capability to temporarily change the speed at
which a protective device responds during an arc flash. By
reducing the device’s fault-clearing time, the severity of the
arc flash can be greatly reduced. If permanent device setting changes can be made, that is the simplest approach.
However, it is not usually possible, since miscoordination
between protective devices could lead to a more widespread outage during a fault.
• Remote operation—The farther a worker is from the arc
flash source, the less energy he or she would be exposed to.
Remote operation, such as remote racking and remote opening and closing of protective devices, permits the worker to
increase the distance from the arc flash source.
The three D’s
Most methods that are available to reduce the severity of the
arc flash hazard focus on three controllable variables, including
duration, distance and direction.
Table 1 provides a sample list of methods that can be used
to reduce the incident energy exposure by controlling one of
the three variables. Many other methods are also available,
and manufacturers continue to develop new ways to reduce
arc flash hazards.
ARC FLASH HAZARD—RISK CONTROL METHOD
Duration Distance Direction
Current-limiting devices X
Temporary device settings X
Circuit breaker type and settings X
Remote operation X
Longer breaker racking handle X
Hot sticks X
Arc-resistant equipment X