Every so often, the electrical industry is able to add to our understanding of what makes a safe, well-designed electrical system. This knowledge can have a profound effect on the way we design, maintain, and operate electrical systems. Such is the case with the growing understanding of the arc flash hazards faced by the workers who operate and maintain these systems. As electrical engineers and designers, we have a responsibility to incorporate this new knowledge into our designs.
Three things contribute to an arc flash hazard once the arc is established — the current magnitude, duration of the arc, and distance the worker is from the arc. While there isn't much that can be done to manage the worker distance from the arc, the engineer can manage the other two factors through a thoughtful system layout, careful protective device coordination plan, and specification of new arc flash-resistant technologies.
The layout of the electrical system can have a significant effect on the arc flash hazard levels. By keeping the effects of arc flash in mind while laying out the system, you can help keep the arc flash hazard risk category (HRC) level to a minimum (see Hazard Risk Categories).
The initial layout of the electrical system begins with the service entrance. It's important to get an accurate model of the utility source connection. Because the HRC is dependent on the I2T energy contained in the fault, the amount of available fault current will affect the rest of the design. A stiff source (a high-capacity source with a low source impedance) will contribute high fault currents with associated high arc fault energies. Conversely, a weak source (a low-capacity source with a high source impedance) can result in long arc clearing times, especially at the end of long cable runs. It can also result in high arc energies. In any case, an accurate model of the power source is a cornerstone of good design.
At this point, you should think about how best to distribute the power throughout the facility and what kind of system grounding scheme to use. Because the majority of electrical faults begin as single phase-to-ground faults, an effective way to reduce the arc flash hazard level is to use a high-resistance grounded system. Design the system to serve 3-phase loads or phase-to-phase single-phase loads at 480V, and use a 480V-208/120V delta/wye step-down transformer for the single phase-to-ground loads. If 3-phase 4-wire loads at 480V are essential, you can use a delta/wye 480V/480V isolation transformer.
As you configure the power distribution system, keep in mind that the best way for operations and maintenance personnel to avoid arc flash hazards is to de-energize the equipment before working on it. Look for opportunities to group equipment and loads so that the equipment can be de-energized without disrupting other plant processes. Try to avoid a situation where a single piece of switchgear cannot be de-energized without shutting down a significant portion of the facility.
Grouping loads can also present an opportunity to manage impedance on the system since large, low-impedance transformers contribute high levels of fault currents that lead to high-energy arc flashes. By splitting up the loads among smaller transformers, you can cut the arc flash hazard significantly. For example, by using two 1,000kVA transformers in place of one 2,500kVA transformer, the HRC can be reduced by one or two levels. Splitting up the loads also reduces feeder sizes and can reduce fault currents on high-capacity systems.
Past transformer sizing practice often provided a generous capacity for future load growth and handle any harmonic currents that may be generated. These large transformers often spent their entire lives without ever reaching full load capacity. As mentioned above, if transformers have low impedance, coupled with a large capacity, they can be sources of high HRCs. When sizing transformers with arc flash in mind, pay more attention to the probability of future loads and harmonics. Realistically size transformers with the goal of having them operate close to their capacity.
Branch circuits can also be a problem. On a weak system, a long branch circuit can reduce fault currents down to a point where the branch overcurrent protective device reacts very slowly, creating a level 4 or higher HRC. Conversely, on a stiff system, a short branch circuit can result in very high short-circuit currents and associated high arc flash energies. By staying aware of the effect of the cable length on impedance and arc energies during the layout and overcurrent protective device (OCPD) coordination stages, you can minimize the arc flash hazards to maintenance personnel.
Although taps on feeders and branch circuits are allowed per the NEC, when it comes to arc flash they are not a good idea and should only be used when necessary. They can be difficult to protect and can lead to some very high HRCs. Sometimes, an upstream OCPD may respond very slowly or not at all to a fault on the end of a tap. Avoid them whenever possible.
Protective device coordination
Design that minimizes arc flash hazards requires fast fault clearing times. The short-circuit study and OCPD coordination phase of the project is a critical time when designing with arc flash in mind. This is also an opportune time to perform an arc flash hazard analysis in concert with these studies. Most commercial power system analysis programs now have arc flash analysis capabilities that use the same database and model necessary to run the other studies, making it easy to add value to your design by performing the study and presenting the client with the arc flash labels required by the NEC, NFPA 70E, and OSHA.
When creating the overcurrent protection schemes, use current-limiting fuses and circuit breakers where possible. Be sure to specify the fastest current-limiting devices available — the more the current is limited, the smaller the arc energy released during the fault. For instance, use UL Class J and RK1 fuses rather than Class RK5 fuses. Size these devices as small as possible rather than using the maximum size allowed by the NEC. The NEC maximum size dual-element RK1 fuses for protecting a 100-hp, 460V, 3-phase motor, for example, is 225A. However, a dual-element 175A RK1 fuse can be used here — it will let through less energy and will have a less destructive arc flash hazard associated with it.
A short-circuit calculation uses the bolted fault current to obtain the values necessary for determining equipment withstand capabilities and selective coordination of OCPDs. On the other hand, an arc flash hazard calculation uses arc fault current that includes the arc impedance. This is an important difference because the arc fault current is usually smaller than the bolted fault current, sometimes by as much as 40% or more. This reduced current can delay fault clearing time during an arc fault, resulting in high incident energies and higher level HRCs. Always check the clearing time of your protective devices at both the bolted fault and the arcing fault current values to ensure fast clearing times under arcing fault conditions.
Unfortunately, selective OCPD tripping and minimizing arc flash are goals that are often at odds. For instance, a main circuit breaker on 480V switchgear will be intentionally delayed to allow the feeder breakers to selectively trip for a close in feeder fault. However, if an arcing bus fault occurs, the main breaker will be slow to clear the fault, resulting in increased damage to personnel and equipment. The delayed tripping of protective devices required for selective coordination violates a fundamental principle of minimizing arc flash (i.e., not to delay tripping when an arcing fault occurs).
Perhaps the best (and most expensive) way to solve this problem is to use differential relaying on transformers and buses. Differential relaying is very fast at detecting and clearing any fault within the zone of protection. At the same time, it does not change the selective coordination of the OCPDs. However, the additional hardware required makes it an expensive option that is hard to justify, especially on low-voltage systems. A cost-effective alternative to differential relaying is a zone selective interlocking scheme.
Because the arc flash hazard is a safety problem when personnel are present, another solution to the delayed tripping problem is to change the main breaker settings when maintenance is performed. One way to do this is to use a circuit breaker with a switch-selectable instantaneous setting. The designer then has a variety of options available to use this switch to allow the instantaneous setting to be activated whenever maintenance is performed on the switchgear. A manual switch can be provided that changes the breaker settings when maintenance is performed, or the change in settings can be activated automatically by detectors that sense when a person has entered the flash protection boundary zone.
At least one manufacturer now makes a monitor that can detect the intense flash of light associated with an arc and instantaneously shunt trip the circuit breaker(s). Inadvertent trips due to sensing of sources of light, such as the flash from a camera, are prevented by including a permissive current detection interlock.
Motor starters and motor control centers (MCCs) are of particular concern when considering arc flash hazards. In an operating plant, it's not unusual for operating and maintenance personnel to visit an MCC several times a day to start and stop motors and perform troubleshooting tasks. This frequent exposure is coupled with the fact that the overcurrent protection on motor starters is often intentionally delayed to allow for inrush currents. Again, use of dual-element current limiting fuses will help minimize the hazard in these areas. Specify motor starter/OCPD combinations that have been tested and witnessed for Type 2 protection. These units are protected by Class J or RK1 fuses and are required to sustain no damage under short-circuit conditions.
There are a number of ways to write the specifications to either minimize the probability of an arc flash occurring or to reduce the worker exposure to the effects of the arc flash should one occur. One way to reduce the likelihood that an arc fault will occur is to specify finger-safe electrical components and an insulated bus for MCCs, panelboards, switchboards, etc.
If an arc flash does occur within the switchgear, high-temperature gases created by the arc will be vented from the enclosure. When specifying switchgear, pay attention to the location of the vents and keep them away from areas where people might be standing. Do not allow vents in the breaker cells. Where available, consider using arc-resistant switchgear that has been designed to withstand an arc flash and safely vent the gases.
Specify switchgear with remote racking devices to minimize the worker exposure to arc flash hazards during routine racking of the breakers. Specify quick-break, plug-in-type connectors for control wiring so that the circuit breaker can be easily removed from the arc flash boundary to an area where it can be safely worked on without workers having to wear cumbersome protective clothing and gloves. Also, consider specifying remote controls and metering so the worker does not have to stand in front of the circuit breaker when operating it.
As mentioned above, specify OCPD devices that will accommodate maintenance activities by changing to instantaneous tripping times on main circuit breakers whenever maintenance personnel are present. Specify differential relaying or zone selective interlocking as much as possible. Require fast-acting OCPDs, such as current-limiting devices, rather than other slower type devices.
Many facilities use infrared scanning as part of their maintenance programs. These activities require workers to perform tasks while equipment is energized and most likely exposed. Many times, this type of work requires the worker to open and remove covers, and then replace them when the task is finished. As the designer, you can reduce these hazards by specifying strategically located UL-approved infrared windows in major equipment and switchgear.
The arc flash hazard issues of today have been compared with the ground fault interruption issues of the 1970s. There is no doubt that arc flash hazard concerns are changing the way the industry thinks about electrical system design. As our understanding of arc flash hazards grows, new technologies will be developed and code changes implemented to provide you with better tools for designing the safest electrical systems.
Woods is a principal with Integrated Engineering Solutions, Grand Junction, Colo.
Sidebar: Hazard Risk Categories
The hazard risk categories (HRCs) noted in NFPA 70E are used to determine the proper arc flash personal protective equipment (PPE) required when working on energized electrical equipment. The higher the HRC, the more PPE required to protect the worker from the arc flash. The HRCs are based on the amount of incident energy the worker will be exposed to at a specific working distance from the equipment. For reference, 1.2 calories per centimeter squared (cal/cm2) can cause a second-degree burn, 4.0 cal/cm2 is the point at which cotton can burst into flame, and 8.0 cal/cm2 can result in third-degree burns.