High Voltage Arc
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A high voltage arc is a very destructive occurrence that
can lead to severe injury, death and very high monetary consequences. A arc is essentially a
breakdown of the resistance of air
created by the
initial fault that precipitated the arc.
When a high
voltage arc occurs not only is the fault energy transferred through from the
source but also the metal and parts of the source container such as the box
enclosure can melt and become part of the arcing material. The heat energy
released is very intense and depending on the factors that go into the fault
such as voltage, current and clearing times of the over current protection
devices such as fuses and circuit breakers it can be a catastrophe to any
person or object that is in the path of the arc blast
Studies conducted by Lee (1982) and others reveal that the heat released in
an electrical arc can rise to values of more than 35,000 degrees F This is
approximately four times hotter than the surface of the sun. The metal in
electrical panels will melt when heated to approximately 2,500 degrees F so
you can see that electrical panels can withstand electrical arcs for only a
few seconds before panel components will begin to disintegrate. Anyone
standing in the path of the arc blast would feel the effects of the IE along
with any accompanying metal.
There are a few methods of protecting yourself and your facilities which
include the use of the proper
arc flash PPE and system design and a good electrical
maintenance plan. The NFPA 70E provides a procedure that can be followed to
create a safe practice for computing an arc flash analysis in order to
select the correct rating of the arc flash protective clothing. There is
also system design of electrical equipment which can help in limiting the
amount of time it takes to clear a fault which would lower the risk factor.
Garments are designed and rated at a break open point that is higher then
the exposure that may result according to the arc flash analysis. The most
important safety precaution is that electrical equipment should always be
worked on in a de-energized state but of course even putting the equipment
into a de-energized mode calls for the correct PPE.
When putting equipment into a de-energized state one must consider that it
is sometimes assumed that because the “live” circuits are contained in a
metal enclosure that a person is somehow protected from the potential of an
arc flash. There is no guarantee that the metal enclosure is sufficient to
contain and arc fault and or whether all side walls of the metal cabinet are
the same thickness and strength.
Whether you are protected by system design or de-energizing there is always
the need for high voltage arc protection. The reason being that you can not
always be 100% sure that you are safe from an arc fault resulting in a flash
and or blast of electrical energy. When it comes to your safety and the
economics of the company you own or work for there is no substitute for a
good job task plan to protect yourself and your employees. This is why it is
so important to do a proper arc flash analysis so that you get the right
amount of protection and also are not being over protected. What I mean is
that I have spoke to people who just want to get the highest protection
available so they are sure to be in compliance. The problem with that is
that it is not only a higher expense then is required but also the garments
that are rated as class 4 for example are much heavier then a class 2
assemble that are available in
arc flash kits.
When deciding on what level of protection you require you can either follow
the NFPA 70E. Table 130.7(C)(9)(a) which lists certain common tasks that can
be looked up or perform an arc flash analysis. The point is to use either
method although the arc flash analysis is by far the best method to use it
is not always economically feasible for some companies. Also if a certain
task is not listed in the NFPA 70E Table 130.7(C)(9)(a) an arc flash
analysis must be performed.
Protecting workers
from the effects of arc blasts begins with proper equipment design and
installation to prevent arc blasts from occurring in the first place. This
includes following the design and installation practices outlined in the NEC
(NFPA 70) and National Electrical Safety Code (IEEE, 2007). At the end of
the day we all want to go home to our families as that is the whole point in
working in the first place.
