User's Manual

3 Fluke Corporation Concerned about arc-flash and electric shock?
The current flow is therefore “comparatively”
low but the explosive effects are much more
destructive and potentially lethal. Unlike a
bolted fault, it is difficult to predict exactly how
much energy will be released by an arc fault. In
particular, it is difficult to predict the duration
of an arc fault as this depends on many factors,
feedback mechanisms and the response of the
over current protection devices.
When an arc fault occurs
NIOSH (National Institute for Occupational Safety
and Health, Pittsburgh, USA) has published
the results of a survey of electrical accidents
reported by MSHA (Mine Safety and Health
Administration, US Department of Labor) in the
mining sector over the period: 1990-2001. In
more than two-thirds of the cases of arc flash
injuries, the victim was performing some form
of electrical work such as troubleshooting and
repair. More surprisingly, 19 % of the accidents
arose from the direct failure of equipment during
normal operation. Overall, 34 % of the accidents
involved some form of component failure.
The key components involved in the acci-
dents where: circuit breakers (17 %), conductors
(16 %), non-powered hand tools (13 %), elec-
trical meters and test leads (12 %), connectors
and plugs (11 %). Of the cases that reported the
arcing voltage, 84 % occurred with equipment
at less than 600 V and only 10 % with equip-
ment at more than 1000 V.
Arc fault make-up
When an arc-fault is triggered, a plasma
arc—the arc flash—forms between the shorted
components. Once established, the plasma arc
has a virtually unlimited current-carrying capac-
ity. The explosive energy release causes:
•
A thermoacoustic (dynamic) pressure wave
•
A high intensity flash
•
A superheated ball of gas
The thermoacoustic wave is a dynamic pressure
wave caused by the instantaneous expansion of
gas local to the fault. It causes panels to rupture,
flying debris and barometric trauma. The wave
front travels outwards, away from the fault, and
as it impacts surfaces it increases in energy: an
effect known as “pressure piling”.
A common misconception is that an arc flash
will always result in panel rupture. However, by
incorporating high-speed interrupt devices and
additional protection systems, an engineer can
reduce the arc flash energy to a level where the
thermoacoustic wave front does not have suf-
ficient energy to rupture the panel.
Although the thermoacoustic wave resulting
from an arc fault can be very destructive, it is
not the only characteristic of an arc flash. Unlike
a chemical explosion, the energy of an arc flash
converts primarily to heat and light energy.
Temperatures at the epicenter of an arc flash
can reach 20,000 °C (four times hotter than the
surface of the sun) within a millisecond. Such
high temperatures are capable of explosively
vaporizing metals such as copper. The presence
of vaporized metal can then feed and sustain
the plasma arc and exacerbate its power.
An arc flash essentially lasts until the over-
current protective devices open the circuit. A
fast-acting fuse may open the circuit as quickly
as several milliseconds.
The consequences of an arc flash
Arc faults are potentially fatal to any person-
nel in the vicinity. The intense heat of the arc
flash can severely burn human skin and ignite
the clothing of anyone within several feet of
the incident. Treatment for arc flash burns can
involve years of skin grafts.
Without proper eye protection, projectiles
and molten debris can cause eye damage. The
intense UV radiation associated with the flash
can cause retinal damage. Superheated vapors
can injure lungs and impair breathing. The
thermoacoustic blast can damage hearing with
ruptured eardrums, cause collapsed lungs and
damage other internal organs. The blast can
knock personnel off their feet; falls may result in
broken bones or lead to electrocution or further
injuries on other parts of the system.
19 % of the accidents arose from the
direct failure of equipment during
normal operation