By Grainger Editorial Staff 4/26/17
Reduce the frequency, severity and harmfulness of arc flash incidents with these safety tips.
From analysis to equipment, you can reduce the exposure to a dangerous event.
Arc flashes — the fiery explosions that can result from short circuits in high-power electrical devices — kill workers in the U.S. every year and permanently injure many more.
They can also wreak financial havoc in the form of fines, lawsuits and damage to expensive equipment.
Given the dangers they pose, arc flash incidents merit serious attention from engineering professionals. Here are six of the most effective strategies for reducing the frequency, severity and harmfulness of arc flash incidents.
Every arc flash mitigation program should begin with a hazard analysis aimed at calculating how much energy an arc flash could release at various points along the power chain. Accuracy is essential with such measurements, so plant managers who lack direct and extensive experience with arc flash incident energy assessment should always seek assistance from a qualified power systems engineer.
To ensure employees are always aware of potential arc flash hazards, companies must place warning labels on any piece of electrical equipment that poses an arc flash risk. They must also mark arc flash hazard zones on the floor so workers not wearing appropriate personal protective equipment (PPE) can clearly see how far away from electrical equipment they must stand to avoid serious injury.
Note that the NFPA 70E standard explicitly requires employers to post signage notifying employees of potential arc flash dangers. Organizations that ignore this requirement dramatically increase their chances of paying substantial fines and exposing themselves to significant liability.
Though not applicable to environments protected by fuses and current-limiting breakers, facilities using non-current limiting breakers (NCLBs) can reduce the amount of incident energy released during arc flashes by reducing the amount of available fault current. The following three strategies can help plants with NCLBs significantly reduce available fault current.
Operate with an open tie during maintenance. When maintaining dual electrical sources, current limiting devices above current values can increase available fault current and reduce incident energy. Sometimes, however, opening the tie between dual power feeds during maintenance procedures reduces arc flash dangers by cutting available fault current in half. Of course, opening ties during maintenance also temporarily renders your power scheme less redundant, exposing equipment to heightened risk of failure.
Employ high-resistance grounding. During ground faults, high-resistance grounding (HRG) systems provide a path for ground current via a resistance that limits current magnitude — dramatically reducing the size of line-to-ground faults and associated arc flashes. HRG can be used on systems that service only three-phase loads. The U.S. National Electrical Code prohibits using HRG on distribution systems serving loads that are connected line-to-neutral.
Use current limiting reactors. Current-limiting reactors act as a bottleneck on electrical flows, restricting current during faults. For example, low-voltage motor control centers can be supplied with three single-phase reactors that limit available short circuit current, resulting in smaller energy releases when faults occur.
Just as smaller arc flashes release less energy, shorter ones do as well. To shorten arc flash events by decreasing fault clearance times, you should:
Utilize zone selective interlocking. Zone selective interlocking (ZSI) is a protection scheme that uses an “inhibit” signal transmitted from downstream breakers that detect a fault to the next breaker upstream. The upstream breaker detects both the fault current and the inhibit signal and therefore delays tripping, allowing the downstream breaker to clear the fault. Should a fault occur between the downstream and upstream breaker, however, the downstream feeder doesn’t detect the fault or send an inhibit signal to the upstream breaker. That causes the upstream breaker to bypass any intentional time delay settings, significantly reducing arc flash incident energy.
Implement a bus differential scheme. These are coordinated zones of protection within an electrical system. When a fault occurs within a given zone of protection (e.g., between the main and feeder breakers), protective devices trip instantaneously, limiting arc flash durations while also confining arc flash damage to specific portions of your infrastructure. Bus differential systems are typically faster and more sensitive than ZSI, but require additional current transformers and relaying equipment, making bus harder to implement and more expensive than ZSI.
Deploy an Arcflash Reduction Maintenance System. An ARMS shortens faults by bypassing all time delays in the trip circuit any time current exceeds an ARMS preset maximum. That enables faults to clear even faster than a circuit breaker’s “instantaneous” function makes possible. Technicians must manually enable ARMS circuits before doing maintenance work and then disable them when that work is complete, employing familiar lockout/tagout procedures.
Executing potentially dangerous procedures remotely can help protect personnel from injuries. Here are two ways to limit maintenance operations performed in range of arc flash events:
Install remote monitoring, control and diagnostics software. Modern power management systems equip administrators with the ability to perform many administrative tasks remotely. They also equip companies to remotely de-energize electrical equipment before staff members come into contact with it.
Employ remote racking devices. Traditionally, technicians have had to stand close to equipment with live, electrical connections when racking and unracking breakers. Remote racking devices enable operators to perform these extremely dangerous tasks from a safe distance.
One of the most effective ways to prevent arc flashes is to anticipate and eliminate the conditions that cause them. The following three solutions help spot potential arc flash dangers before they have a chance to do harm and keep personnel safely away from live connections.
Monitor insulation integrity. Deteriorating insulation is the leading cause of arc-producing electrical failures. Identifying and repairing compromised insulation before it fails can help avert arc flash explosions. Predictive maintenance systems provide early warning of insulation failure in medium-voltage switchgear, substations, generators, transformers and motors.
Monitor pressure junctions. Most electrical equipment contains pressure junctions, such as shipping splits, load lugs, and compression fittings. Over time, vibration and thermal cycling can loosen these connections. When current flows through a loosened connection, it can cause overheating and eventually produce an arc flash. Non-contact thermal sensors called pyrometers, however, can monitor pressure junctions continuously and provide advance notification of loose connections before they create an arc flash explosion.
Use infrared (IR) windows. Using contactless IR thermography technology, IR windows enable technicians to perform IR scans without removing switchgear side panels, lessening the likelihood of arc flash events caused by accidental contact with live bus.
Equipment that directs arc flash energy away from personnel is called “arc resistant.” Arc resistant switchgear, for example, utilizes sealed joints, top-mounted pressure relief vents and reinforced hinges to contain the energy and heat released by arc flashes and channel them via ducts to an unoccupied area inside or outside the building.
When all else fails, arc-resistant switchgear offers vulnerable employees a critical last line of defense from the explosive power of arc flash incidents. However, its protective qualities are effective only when equipment doors are closed, so companies should train their technicians to fasten doors securely during normal operation.
Arc flash events can cause serious harm, ranging from disabling or fatal injuries to heavy fines and expensive lawsuits. Though no combination of countermeasures can totally eliminate the risks, utilizing the solutions and strategies discussed in this article can help organizations make arc flash incidents less likely to happen and less harmful when they do.
Article courtesy of Plant Engineering, David G. Loucks, PE, Eaton 11/18/2013
The information contained in this article is intended for general information purposes only and is based on information available as of the initial date of publication. No representation is made that the information or references are complete or remain current. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.