ARC FLASH

Don’t get too complacent when working on electrical
equipment.
A major short circuit or ground fault at the main
service panel or at the meter cabinet or base can
deliver a lot of energy. On large electrical installations,
an arc flash (also referred to as an arc blast)
can generate temperatures of 35,000°F (19,427°C).
This is hotter than the surface on the sun. This
amount of heat will instantly melt copper, aluminum,
and steel. The blast will blow hot particles of
metal and hot gases all over, resulting in personal
injury, fatality, and/or fire. An arc flash also creates
a tremendous air pressure wave that can cause serious
hearing damage and/or memory loss due to the
concussion. The blast might blow the victim away
from the arc source.
Don’t be fooled by the size of the service.
Typical residential services are 100, 150, and 200
amperes. Larger services are found on large homes.
Electricians seem to feel out of harm’s way when
working on residential electrical systems and seem
to be more cautious when working on commercial
and industrial electrical systems. A fault at a small
main service panel can be just as dangerous as a
fault on a large service. The available fault current
at the main service disconnect for all practical
purposes is determined by the kVA rating and
impedance of the transformer. Other major limiting
factors for fault current are the size, type, and length
of the service-entrance conductors. Available fault
current can easily reach 22,000 amperes, as is evident
by panels that have a 22,000/10,000-ampere
series rating.Don’t be fooled into thinking that if you cause
a fault on the load side of the main disconnect that
that main breaker will trip off and protect you from
an arc flash. An arc flash will release the energy that
the system is capable of delivering for as long as it
takes the main circuit breaker to open. How much
current (energy) the main breaker will “let through”
is dependent on the available fault current and the
breaker’s opening time.
Although not required for house wiring, NEC
110.16 requires that electrical equipment, such as switchboards, panelboards, industrial control
panels, meter socket enclosures, and motor control
centers, that are in other than dwelling units,
and are likely to require examination, adjustment,
servicing, or maintenance while energized shall be
field marked to warn qualified persons of potential
electric arc flash hazards. The marking shall be
located so as to be clearly visible to qualified persons
before examination, adjustment, servicing, or
maintenance of the equipment. More information
on this subject is found in NFPA 70E and in the
ANSI Standard Z535.4, Product Safety Signs and
Labels.
TIP: When turning a standard disconnect
switch “ON,” don’t stand in front
of the switch. Instead, stand to one side. For
example, if the handle of the switch is on the
right, then stand to the right of the switch,
using your left hand to operate the handle of
the switch, and turn your head away from the
switch. That way, if an arc flash occurs when
you turn the disconnect switch “ON,” you will
not be standing in front of the switch. You will
not have the switch’s door fly into your face,
and the molten metal particles resulting from
the arc flash will fly past you.