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COVER STORY
Arc -Energy
Mitigation Techniques
B y J o h n Cad ick, PE Cadick Corporation
COVER STORY
OK – you’ve done the engineering study and you know the arc-energy levels at all
of your switchgear, panelboards, and other such equipment. Some of the locations
in your facility have unacceptably high incident energy levels. What this means to
any given facility depends somewhat on local philosophy; however, it is a safety
imperative to reduce incident energy levels to the lowest possible value and/or
eliminate the exposure of your workers.
Ideally you would like all incident energy
levels to be zero – not a realistic goal. Some
would like to have no incident energies above
8 cal/cm2 or some other low goal. Unfortunately,
your study shows some locations with incident
energy levels of well over 15 cal/cm2, and some
locations are over 40 cal/cm2.
This article describes several techniques
of reducing exposure of your personnel to
high arc-energy levels. The several methods
described all fall into the following three,
broad categories:
• Reducing the available arc-energy by
designing/redesigning your power system
• Reducing personnel exposure by
designing/redesigning your power system
• Reducing personnel exposure by
modifying – and enforcing – working
procedures.
You will probably find some of these techniques
to be useful in your system, building, or plant.
Others will not be feasible for you because
of work rules, physical plant limitations,
and/or economics. In any event this article
presents several common and not-so-common
techniques that should be considered by you
and/or your engineering consultant.
arc-energy mitigation techniques
Reducing the available
incident energy by modifying/
reengineering your power system
Take a close look at equation 1. This equation
is taken from the 2009 edition of NFPA 70E
– Standard for Electrical Safety in the Workplace.
Equation 1:
Where:
EMB = maximum incident energy received
from an arc occurring in a 20 in. cubic
box (cal/cm2)
DB =distance from arc electrodes in inches
(for distances 18 in. and greater)
tA =arc duration in seconds
F =short circuit current in kiloamperes
(for the range of 16 kA to 50 kA)
Equation 1 is one of several formulas that have
been developed by creating arcs in various
controlled environments and measuring the
resulting heat energy. Research in the area is
ongoing and will undoubtedly lead to changes
in the equations that we use; however, the basic
relationships will remain the same – available
incident energy decreases as:
• time of exposure (tA) decreases
• arcing short-circuit current (F) decreases
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Reducing the maximum arc-energy available
Reducing Time of Exposure
For protective devices with adjustable operating times, the engineer can often select a faster
tripping time, thus reducing the available incident energy. If the engineer is able to reduce the
tripping time from 2 seconds to 1 second, equation 1 shows that the available incident energy will
be cut in half.
Prior to about 1995, the time delay settings for protective devices were primarily based on two
criteria. The first criteria was the requirement that tripping times be fast enough so that noninvolved equipment would not be damaged by carrying excessive fault current for too long.
Transformers, for example, overheat and may be damaged if they are required to carry fault-level
currents for too long.
Figure 1
Refer to Figure 1 Short circuits that occur in zones 2 or 3 must be cleared by either the transformer
secondary breakers (Sec Bkrs) or other protective devices below the Sec Bkrs (not shown). Since
faster tripping times will provide better protection for the transformer, the ideal approach is to set
all protective devices in zones 2 and 3 to trip instantaneously. This solution will also decrease the
available incident energy – a seemingly perfect solution.
However, tripping times should also provide selective
tripping. Selective tripping is defined as tripping the closest
protective device upstream of the location of the short
circuit. A fault that occurs below one of the transformer
feeder breakers (Zone 3) should trip the feeder breaker – not
the main secondary breaker. This means the following:
• Pri Bkr should clear faults in Zone 1
• Th
e secondary main breaker should
clear faults in Zone 2
• Th
e feeder breakers should clear faults
in Zone 3
Prior to 1995, the criterion for time settings was to
set them as fast as possible to protect the transformer
while still providing selective tripping. Since 1995 the
reduction of incident energy has become a third important
element. In some cases the settings may have to be
based on the likelihood of a worst case short circuit. That
is, you may wish to set your tripping times to provide
selective tripping for the lower fault currents and sacrifice
coordination for higher current/higher energy short circuits.
Since the worst case short circuit is a very rare occurrence,
you will optimize your selective tripping at the same time
you reduce the available incident energy.
SPRING 2011
arc-energy mitigation techniques
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In low-voltage systems (less that 1000 V) the use
of current-limiting devices (fuses and breakers)
will also help to accomplish the desired goal.
Fault currents that fall into the current limiting
region of such devices can provide tripping
times, including arc quenching, of less than
17 milliseconds.
Reducing Available Fault Current
Reducing tripping time is much easier if the
engineering is done before the system is built.
This allows the engineer to select protective
devices that will fit the needed requirements.
But what about new or existing systems wherein
the devices cannot be adjusted to reduce the arc
energies to the lowest possible value?
In such cases the installation of current
limiting reactors is advisable. The reactance of
these devices increases the system impedance
and, correspondingly, reduces the magnitude
of the available fault current. While somewhat
expensive, this approach may be the only way
that a given system’s available incident energy
can be reduced to acceptable levels.
There are at least two side effects that
must be taken into account when adding
additional impedance.
First, a reduction in available fault current will
cause the protective devices to operate more
slowly. This must be taken into consideration
because slower operating times increase
available incident energy offsetting the effect
of reduced current. This situation is especially
pronounced if the reduced fault current
causes the protective device to trip in a slower
time characteristic.
Assume that the calculated fault current
without the reactors will cause a low-voltage
power circuit breaker to trip in its STD (short
time delay) of less than 0.5 second. Further
assume that the calculated fault current with the
reactors will trip the same device in a long time
delay characteristic. This could increase the trip
time to several seconds (or more) resulting in
a large increase in incident energy. Clearly, the
selection of current limiting reactors must take
reduced tripping times into account.1
This same problem can
crop up when engineers
try to determine the worst
case scenario. Many assume
that the maximum fault
current will coincide
with the maximum
incident energy. For the
same reasons as those
given above, this is not
necessarily true.
1
Second, increasing the system impedance
will reduce the system voltage even when
operating normally. If the reduction in
voltage is objectionable to operations,
transformer voltage taps may have to be
changed. If transformer taps are not available a
second method is to contact the electrical
supplier to ask if they can increase their
supply voltage slightly. If neither of these
methods work, and the voltage reduction is
unacceptable, other incident energy reduction
approaches must be used.
Reducing personnel exposure
by designing/redesigning your
power system
Experience shows that a significant percentage
of arcing incidents occurs during equipment
operation. Activities such as opening,
closing, and racking put physical stress on
the equipment that is not normally present.
This can lead to equipment failure and an
arc-flash event.
There are at least two ways of reducing
personnel incident energy exposure by design
and/or redesign.
• Install remote control rooms to allow all
switching to be performed remotely
• Install or retrofit arc-resistant switchgear
arc-energy mitigation techniques
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2
This is why workers
should always be taught to
stand to the side of
a breaker or switch when
it is operated.
3
Many are surprised to
learn that in classic designs
an arc blast is usually not
taken into account for
doors and/or panels.
Remote Control Rooms
Some facilities choose to install enclosed control
rooms that allow remote electrical operation
of switching equipment. This approach is
especially suitable for new installations, but
many companies are retrofitting the rooms
in existing systems. Unfortunately not all
equipment can be operated electrically.
Manually operated protective equipment
requires either a remote control appliance
(discussed in the next section) or physical
presence of the worker.
Arc-Resistant Switchgear
Prior to the advent of arc-resistant switchgear,
virtually all gear was incapable of enclosing the
worst case arcing event. That is to say, if the
worst case arc were to occur, the switchgear
could sustain a rapid, noncoherent disassembly.
It could blow up. The failure point is usually
the front and/or back of the gear.2 Sometimes,
it didn’t even take the worst case fault to cause
a failure.
Modern arc-resistant gear incorporates at least
three major features that greatly reduce the
possibility of such an explosive failure.
1. The internal bus structures are better
insulated to reduce the probability of a
sustained arcing event.
2. The construction materials and methods are
much sturdier than older designs. Stronger
metals, thicker walls, interlocking corners,
and heavy duty fasteners and hinges3 are
all employed. Figure 2 shows the door
construction used by one manufacturer for
their arc-resistant gear.
3. The gear has an internal venting system
that directs the by-products of the arc away
from the personnel. Usually it is directed
up and back.
Figure 2
Shows the door
construction
used by one
manufacturer
for their
arc-resistant gear.
Permanently installed remote control rooms
are very effective and strongly recommended
because they put distance and at least one
wall between the worker and the arc blast.
While an excellent way to reduce exposure
during opening and closing, remote control
rooms only cover half of the hazard. After
the breakers or switches have been opened,
personnel must still approach the gear.
A viable solution to this exposure is available in
the form of arc-resistant switchgear.
Of course, arc-resistant gear is effective only
when it is closed with all fasteners and latches
in place. After the breaker or breakers have
been opened, the gear must be opened to
perform any work that requires access to the
internal equipment, thus exposing the worker
to the arc-flash hazard.
Unless the bus has been de-energized, the
system is still hot; consequently, a failure can
occur if the breaker or switch is racked in our
out with the door open. External (door closed)
racking systems greatly help in the reduction
of this hazard. However, even then, if the bus
is energized there is a possibility of an arc
occurring.
The solution to this problem lies in the use of
appropriate safety procedures.
SPRING 2011
arc-energy mitigation techniques
COVER STORY
Reducing personnel exposure
by modifying – and enforcing –
working procedures
Remote Switching and Racking Appliances
Increasing distance from the arc location
will greatly reduce the incident energy.
From equation 1 we see that an increase in
distance will reduce the incident energy by the
1.4738 power.4
Figure 3
and the equipment.
Figures 3 and 4 show two
types of equipment that can
and should be employed
so that workers can operate
gear from a remote distance.
These types of mechanisms
are less expensive than
installing separate control
rooms, and they provide a
very good level of protection.
Of course, nothing can be
better than having a block
wall between the operator
Figure 3 is the Chicken Switch.5 This was one
of the earliest and most popular of the remote
operating mechanisms. As you can see from the
photo, the operating mechanism is mounted
on the door of the breaker to be operated. The
control box is connected to the mechanism via
a long, spiral control cable. Use of this device
allows the operator to stand at a safe distance
(up to 25 feet or more) while opening or
closing the breaker.
Figure 4
Figure 4 shows the CBS ArcSafe® remote
switching device. This somewhat more
elaborate equipment includes not only open
or close operation, but also can be set up to
physically rack circuit breakers and other such
equipment remotely. Although more expensive
than the many open/close devices that are on
the market, such equipment adds enhanced
safety by allowing opening, closing, and
racking from a remote location.
Employee Safety Training
There are many, many electrical safety
training options in the marketplace today.
They range from classroom/laboratory
presentations that require participants to
demonstrate their ability to work safely around
electricity to simple video-tape/computerbased training packages.
Although employers are always looking for
ways to get the greatest bang for the buck,
I recommend that more comprehensive training
methods be used. Workers should certainly be
required to take cognitive skill enhancement
training, but they also should be required to
include demonstration of their safety skills.
A person remembers something they have done
much more completely than something they
have heard or read.
arc-energy mitigation techniques
Theory predicts an inverse
square relationship for a
point source of energy. The
slower energy reduction
in the real world is caused
by the focusing effect
of switchgear and arc
geometry.
4
Chicken Switch is a
registered trademark of
MarTek, Ltd.
5
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Figure 5
Some are even more
conservative and draw
the line at locations over
8 cal/cm2. If this is
feasible in the given
situation, I concur.
6
Safety Procedures and PPE
Even in late 2010, I am amazed at how many
companies are still unaware of the potential for
arc injuries. Unfortunately, such companies
very often are unaware of their obligation to
provide the necessary worker protection.
Simple, easy-to-enforce procedures can go a
long way towards preventing electrical accidents.
Methods such as:
• Stand to the side when operating a
breaker (See Figure 5).
• Always test before you touch (test the
meter, test the circuit, retest the meter).
• When testing, wear all appropriate PPE.
Remember that it isn’t dead until it’s
measured.
• Always wear and/or use properly rated
PPE (See Figure 6).
• Observe the shock and flash boundaries.
Do not cross them without proper PPE.
• Put signs, barriers, or safety watch
personnel in place to keep out unqualified
personnel.
• Whenever possible apply equipotential
safety grounds in the work area.
• De-energize the system from a location
that has relatively low incident energy.
SPRING 2011
Figure 6
Conclusion
I believe that any location with incident
energy over 15 cal/cm2 should be reviewed for
possible mitigation solutions.6
Usually solutions exist to reduce the maximum
incident energy at an exposure location in a
power system. For example, some say that it
is impossible to reduce the incident energy
of a fault occurring between a transformer
secondary and the first secondary protective
device. In fact the normally high, incident
energy in these locations can be mitigated.
Protective schemes such as transformer
differential or zone-interlocking protection
will provide instantaneous tripping for
transformer secondary faults. The cost of
mitigation may be high, but the cost of a
single injury or fatality will be much higher.
Some consultants, as a normal part of an
arc-flash study, will recommend changes in
protective device settings in areas where high
incident energies are present. In fact, system
owners should insist on this as a part of a
contracted study.
However, major redesign efforts are usually
beyond the scope of a contracted study.
Adding current limiting reactors, designing
remote control rooms, and checking the effects
of different protective devices are examples of
such out-of-scope efforts.
arc-energy mitigation techniques
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If a study reveals locations where a simple
setting change is not sufficient, the system
owner should commission an engineering
review to determine which of the several design
options will provide the optimal incident
energy mitigation.
Procedural and PPE considerations should
be based on the results of an arc-flash study.
The use of PPE tables in NFPA 70E should be
limited to those locations where a study has not
been performed. However, for a given location,
this should be a temporary fix. The best and
safest approach is the arc-flash study.
John Cadick is a registered professional engineer and the
founder and president of Cadick Corporation, John Cadick
has specialized for more than three decades in electrical
engineering, training, and management. His consulting
firm based in Garland, Texas, specializes in electrical
engineering and training and works extensively in the areas
of power system design and engineering studies, conditionbased maintenance programs, and electrical safety. Prior
to creating Cadick Corporation and its predecessor Cadick
Professional Services, he held a number of technical and
managerial positions with electric utilities, electrical
testing companies, and consulting firms. In addition to
his consultation work in the electrical power industry he
is the author of Cables and Wiring, The Electrical Safety
Handbook, and numerous professional articles
and technical papers.
Finally, do not forget electrical shock hazards.
In the midst of focusing on arc flash it is
sometimes easy to forget that shock hazard
should also be investigated. Voltage level is
certainly the first criteria in selecting proper
insulating equipment. However, the overall
shock hazard should be investigated to properly
assess the need.
arc-energy mitigation techniques/NETA News
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