Arc Flash

Document Sample
Arc Flash Powered By Docstoc
                   A FLASH REHASH

An electrician screws a circuit breaker onto a mounting plate, working under
no tag-out restrictions. One of the screws makes contact with the line-side
wiring behind the circuit breaker, penetrating the wire insulation. The
resulting short circuit vaporizes metal and ionizes the air--an arc flash--
causing a phase-to-phase fault. The electrician receives second-degree burns
from the arc flash. Because there is no prejob analysis, neither the worker's
proximity to potential danger nor the necessity for appropriate Personal
Protective Equipment (PPE) is established.

Such an arc-flash explosion in and around electrical equipment that requires
burn treatment is said to occur five to 10 times per day in the U.S., costing
as much as $15.75 million per case in direct and indirect outlays.

The basic distinction to be made between arc flash and ordinary short-
circuit, or bolted, contact is the heat and force, or incident energy, generated
by the arc. Temperatures as high as 35,000 degrees Fahrenheit -- four times
hotter than the sun's surface -- can be achieved. Vaporized metal can expand
40,000 to 1, producing a shattering blast. The physical conditions that
produce arc faults -- voltage x current x time -- along with distance from the
arc, which can be reduced by protective clothing, are the basic quantities that
are controlled and regulated to better insure protection from an arc flash.
Most regulations for improving worker safety change over time as
requirements become more familiar and economic conditions rise and fall.
Thus, safety regulations governing arc flash are in a state of transition with
regard to applying effective protection.

Here's the bottom line: According to Occupational Safety and Health
Administration (OSHA) Standards 29-CFR, Part 1910 -- the electrical
industry's defining set of rules for dealing with arc flash -- equipment must
be de-energized, that is, put in an electrically safe condition, whenever
possible before work begins. That includes disconnecting
every power source over 50 volts as well as all control power; application of
lockout/tag-out procedures; discharge of all stored energy sources; and
testing for safe conditions by qualified personnel wearing appropriate PPE.
This means, practically speaking, that de-energization must occur when the
bus voltage is greater than 120 volts, since arc faults generally need in
excess of 120 volts to be sustained.

Some energy sources or live lines are not obvious, as in the case of the
worker described above. Potential danger should be identified beforehand as
part of a flash hazard analysis that uses one-line drawings and nameplate
specifications to determine all areas where de-energization must occur. De-
energization is not always possible. However, for example, life support
systems at some facilities could shut down with an outage, and functional
testing of energized systems must, of course, be done with at least part of the
system online.

Further, the need to maintain a high level of production increasingly
demands that systems be worked hot. Those needs are countered by
expenses incurred through accidents, including worker injury and unplanned
outages from equipment damage, as well as the industry's desire to keep its
electricians safe. OSHA requires system calculations based on the one-lines
mentioned above as well as details of the line sizes involved, along with
minimum and maximum fault currents supplied by the provider utility, to
determine estimated short-circuit energy and the flash protection boundary,
or approach distance, throughout the system. Those results then determine
the extent of PPE -- specifically, protective clothing -- that must be used.
The equations needed for those calculations are found in either National Fire
Protection Agency (NFPA) regulation 70E-2000 or IEEE Standard 1584-
2002. These calculations are not simple and are usually achieved by a
software program. Once the necessary values have been established, OSHA
requires labeling, that is, field markers placed prominently on equipment
that warn electrical workers of potential arc flash hazards.

What follows next in the flash protection process is different from place to
place, depending on the nature of the equipment involved and the governing
facility policy. This is the coordination of plant protective
equipment -- breakers, disconnects, relays, fuses -- with the energy situation
at hand. How quickly and efficiently this protective equipment responds to a
short circuit to shut the energy down can greatly minimize harm to personnel
and damage to equipment.

Note that the proper selection and adjustment of protective devices won't, as
is often thought, prevent an arc flash from occurring. But the process of
defining the appropriate time and current-limiting response of protective
devices can provide distinct advantages. It can limit an outage to the portion
of the system downstream from the protective device, preventing the main
breaker from tripping. Further, and most critical to worker protection, it can
better assure that the necessary NFPA 70E category of protective clothing is
chosen. Overly conservative flash-hazard analyses that emphasize
preserving system reliability have traditionally established protective
equipment settings on the high side, with slower response times tolerated
and higher currents allowed. But slower reacting breakers and fuses allow
more arc energy to be produced before they trip. That dictates using the
most protective as well as cumbersome gear, corresponding to NFPA
Category 4.

More recent coordination studies have considered that arc fault current is
usually less than bolted fault current, allowing faster equipment response
and reducing the potential heat and force that protective clothing must
withstand. Such an evaluation might allow electricians to wear a lower
category of PPE that is also much more comfortable and work friendly.
Current-limiting circuit breakers and lower rated fuses, coupled with other
protective measures like long-handled tools and infrared inspection windows
on cabinets for closed door inspection, can provide significantly improved
protection for electricians who must work with live equipment. Thoughtful
preventive measures -- de-energization when possible, equipment selection
and settings to minimize accidental arc exposure -- can make a big
difference in providing a more effective measure of electrical worker safety.

Shared By: