Arc Flash

Arc Flash LabelWhat is an arc flash?

Arc Flash is a very fast explosive discharge of energy caused by a short circuit, similar to a magnesium flash bulb in an old style camera. Before the circuit breaker has a chance to trip (and clear the fault), a current of several thousand amps can flow for a few milliseconds. The metal conductor is converted to high temperature plasma which can cause permanent burns, shrapnel, shock wave trauma, blindness, and even death.

Introduction to arc flash:

Arc flash is the explosive delivery of hot plasma energy, with the effect being burns and bludgeoning. The energy is the product of the available power (voltage x current) and the time that it takes to clear the fault (open the circuit breaker or fuse). At the main panel of your home, the available energy (240 VAC x 7000 amps x 0.1 seconds) is equivalent to a stick of dynamite. But at an outlet, the available energy (120 VAC x 200 amps x 0.1 seconds) is more like a loud pop. For outlets, the energy is limited due to the fact that 12 gauge wiring has a resistance of about one ohm per 500 feet.

Arc flash is mitigated by reducing the current (adding resistance) and with faster clearing time. In addition, 1) avoid hot work, if possible, 2) wear appropriate PPE, 3) hazard warning labels, and 4) company policy and training. Lack of LOTO (lock-out tag-out) is the #1 Cal/OSHA citation for industrial factories.

In the US, there are ~8000 arc flash events per year and ~18 serious injuries per year. Over half of all electrical injuries involve burns. Historically, OSHA (and NFPA 70E) has been more concerned with electrical shock than with arc flash. The new NFPA 70E (2012) will emphasize arc flash, and will likely be adopted into 29CFR1910. Worker’s Comp insurance carriers (such as Chubb) are pushing for this.

Arc flash will require site-wide analysis of available energy at every panel, and energized work permits for activity > 50 volts, excepting only diagnostic and testing. (source: www.BAESG.org monthly meeting, July 20, 2011, arc flash presentation).

This video demonstrates the type of fatal accident that an arc flash can create

Typical scope of work

1. Survey and Verification of the Single Line Drawing

Together, Client and A/E will determine all places (nodes, panels, machinery, etc) where a Client QEW (qualified electrical worker) would be doing energized work (installation, testing, troubleshooting, calibration, repair, maintenance, etc). The result would be an accurate single line and panel list.

2. Data Collection

Together, Client and A/E will obtain the model numbers of all of the up-stream circuit breakers, and from that, obtain the data sheets that show the clearing times as a function of the current. These are log-log plots of clearing time vs. current. We will also collect the information for transformers, disconnect switches, automatic transfer switches, UPS, and back-up generators.

From the single line drawing and the survey, we will estimate the amount of up-stream circuit resistance in both the wiring and the transformer. This determines the available fault current.

3. Development of the Distribution Model

The distribution model will be loaded into ETAP or Easy Power.

4. Coordination Study

Based on the circuit breaker plots, we will determine if any down-stream faults could cause any up-stream trips other than the one immediately up-stream. This prevents the entire building from being knocked off. Generally speaking, coordination is assured if the up-stream circuit breaker requires ~4x the amount of trip time, compared to the down-stream circuit breaker, for the expected fault current TBD.

5. Calculation for Arc Flash Labels

Using the model, various fault conditions (line to ground, line to line, etc) will be introduced. Panels will be analyzed one-by-one.

The calculation for arc flash hazard warning labels typically includes: 1) available short circuit current, 2) fault clearing time at that level of current, 3) hazard risk category, 4) shock protection boundary, 5) arc flash protection boundary.

6. Printing Arc Flash Labels

Arc flash labels will be printed on high quality contact label paper. Then the finished labels are separated with a paper cutter. These labels have already been checked for resistance to most cleaning solvents including IPA.

7. Energized Work Permits

Energized work permit forms will be created in PDF for each panel one-by-one. To use a form, Client would select and print one sheet as appropriate, fill in the name, then sign and date.

Deliverables

  • One nicely printed arc flash label per panel
  • A large PDF file containing an image of every label
  • A large PDF file containing energized work permits
  • The ETAP model on a CD, for future maintenance

 



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