Simplifying Arc-Flash ArithmeticFind more articles on Arc Flash/Arc Fault
The electrical industry's acknowledgement of arc flash as a serious danger to service and maintenance workers has been a double-edged sword. While standards and requirements for determining hazard levels have increased safety for the electricians who regularly work hot on switchgear and electrical panels, they've also created mountains of paperwork for the engineers and contractors who have to run the calculations.
Recognizing that for some contractors all that extra work could be a deterrent to taking the proper safety precautions when working on a single point in a system, SKM Systems Analysis set to work in early 2004 to develop a calculator that would take the strain out of the process. And eight months later, the Manhattan Beach, Calif.-based software company released ArcCalc, a program for calculating incident energy levels, determining flash boundaries, and recommending the appropriate PPE according to NFPA 70E and IEEE 1584 requirements.
The program is actually a stripped-down version of the company's Power Tools for Windows suite, which runs the same calculations on a much larger scale. In this case, though, smaller is better. Steve Potter, a technical sales and marketing representative for SKM, says ArcCalc's strength is that it doesn't require a lot of prep work or system modeling — input a couple values and you're done (Sidebar below). “For simplified systems where you're not doing a full-blown engineering study and only need to spot-check certain areas, it's not necessary to go through all of those other calculations,” he says. “This is meant for the contractor who doesn't know what the entire system looks like but needs to perform an arc-flash calculation on one piece of equipment.”
Glenn Walls, a private consultant and the president of Virginia Beach, Va.-based Professional Power Systems, started conducting arc-flash studies for larger industrial facilities last year, and he shudders at the thought of doing it by hand. Although he uses the larger version of the software for full system analyses, he understands the advantages of a single-point calculator. “If you're out in the field and you need quick answers, you can get them,” he says. “Anyone could take that to a small service call where they had to replace a panel and within a matter of minutes finish their hazard analysis.”
Sidebar: How to Prevent Disaster in 10 Minutes or Less
To simplify the process of calculating arc-flash values, ArcCalc is built on a basic one-line model with six components. Click on the tab that corresponds to each one, fill in your values, and the program does the rest.
Fault source — Enter the starting voltage and the available 3-phase short-circuit current for a fault at the source location. Since the exact fault current may not be known, you can enter low and high tolerances.
Transformer — Enter the connection type (grounded or ungrounded), primary and secondary bus voltages, transformer type, nominal kVA, nameplate impedance, X/R value, and impedance tolerance.
Cable — Enter cable type, size, length, number of conductors per phase, and impedance tolerance. If there are multiple cable runs in a series between the transformer and the equipment bus, enter the total length of cable and average the size.
Protective device — This is the heart of the program. Choose your protective device from among thousands in this database developed by SKM. Trip times will automatically populate.
Equipment bus specification — To specify the equipment type, use the drop-down list and choose from MV switchgear, LV panel/switchboard, cable, or open air. For equipment protected by special instantaneous protection, you can specify the instantaneous trip time that will override the overcurrent protection trip time.
Motor contribution — To specify the motor contribution back-feeding the fault, enter the rated size, voltage, and locked-rotor code. Typical values for the motor short-circuit impedance will be based on the motor size and locked-rotor code.