Arc-flash boundaries need to be
established around electrical equipment such as switchboards, panel
boards, industrial control panels, motor control centers, and similar
equipment if you plan to work on or in the proximity of exposed
energized components.
Parts are considered exposed if they
are energized and not enclosed, shielded, covered, or otherwise
protected from contact. Work on these parts includes activities such
as examinations, adjustment, servicing, maintenance, or
troubleshooting.
Equipment energized below 240 V does
not require arc-flash boundary calculation unless it is powered by a
112.5 KVA transformer or larger. The arc-flash boundary is the limit
at which a person working on energized parts can be standing at the
time of an arc-flash without risking permanent injury unless they are
wearing flame-resistant clothing.
Permanent injury results from an
arc-flash that causes an incident energy of 1.2 calories/centimeter2
(cal/cm2) or greater and causes a minimum of second-degree burns.
This distance can only be effectively determined by calculating the
destructive potential of an arc.
First you must determine the magnitude
of the arc based on the available short circuit current, then
estimate how long the arc will last based on the interrupting time of
the fuse or circuit breaker. Finally, you will need to calculate how
far away an individual must be to avoid being exposed to an incident
energy of 1.2 cal/cm2.
It may sound like a lot of math and
factoring in of potentials, but believe me the extra time you take to
determine the arc flash boundary is well worth your safety and
well-being.
Calculating flash protection boundaries
for systems over 600 V requires performing a flash hazard analysis
coupled with either the NFPA 70E Hazard Risk Category/PPE tables or
the Incident Energy Formula.
Additionally, Section 4 of IEEE 1584
Guide for Arc Flash Hazard Calculations states that the results of
the arc flash hazard analysis are used to identify the
flash-protection boundary and the incident energy at assigned working
distances throughout any position or level in the overall electrical
system. The purpose is to establish safe work distances and the PPE
required to protect workers from injury. A flash-hazard analysis is
comprised of the following three different electrical system studies:
- A short circuit study
- A protective device time-current
coordination study
- The flash-hazard analysis and
application of the data
Arc flash hazard analysis
To perform an arc flash hazard
analysis, you need to start by gathering information on the
building’s power distribution system. This data should include the
arrangement of components on a one-line drawing with nameplate
specifications of every device on the system and the types and sizes
of cables.
The local utility company should be
contacted so that you can get the minimum and maximum fault currents
entering the facility. Next you will want to perform a short circuit
analysis and a coordination study. You will need this information to
put into the equations provided in NFPA 70E or the IEEE Standard
1584.
These equations will give you the flash
protection boundary distances and incident energy potentials you will
need to determine your minimum PPE requirements. In many ways an arc
fault analysis is actually a study in risk management.
You can be very conservative in your
analysis and the results will almost always indicate the need for
category 4 PPE. On the other hand, you can perform the analysis and
make adjustments to reduce the arc fault conditions resulting in
reduced PPE requirements.
However, use caution when adjusting
your calculations. Reducing the bolted fault current can reduce the
arc fault current, but it can actually result in a worse situation.
For example, if you reduce the current applied to a motor from 4000
to 1800 A, the arc fault energy is increased from 0.6 to 78.8
cal/cm2. This is the exact opposite outcome that you might expect to
achieve before doing the math.
Keep in mind that you are risking OSHA
violations and fines if you choose nominal compliance. On the other
hand, you can actually be increasing the risk of injury if you force
workers to unnecessarily wear cumbersome PPE.
This can also result in little or no
high voltage maintenance being performed, which will eventually
compromise safety and proper equipment operation. It might prove
beneficial to get a registered professional engineering firm to
perform arc flash hazard calculations on your behalf and have them
recommend appropriate actions and the lowest appropriate category of
PPE.
No comments:
Post a Comment