Plato, the Greek philosopher, had it right almost 2,500 years ago. He is widely credited for the quote, “Necessity is the mother of invention.” When it comes to arc flashes, many creative methods have been developed out of necessity to reduce or eliminate these potentially deadly hazards.
Arc flash study results
The story is familiar: You have just completed an arc flash hazard calculation study. The results confirm that the prospective incident energy is less than 8 calories per square centimeter (cal/cm2) for about 85 percent of the electrical equipment that you reviewed. Protective clothing and personal protective equipment (PPE) with an arc rating of 8 cal/cm2 will be sufficient for these locations. But what about the other 15 percent? One solution would be to recommend using PPE with a greater arc rating. It seems simple enough—unless you are the one wearing the PPE! Sound familiar?
Too often, the study results focus only on PPE recommendations rather than addressing the hazard itself. A better approach is to identify methods that can be used to reduce or eliminate the hazard rather than just increasing the PPE level. Such methods typically will reduce the duration of the arc flash, increase the distance between the arc flash and the worker, or isolate the worker from the event.
Equipment such as switchgear and motor control centers can be constructed in an arc-resistant manner. The enclosure is designed with heavier gauge metal and special reinforced doors for each cubicle. The relays and control equipment are isolated in a separate compartment. Using an arc-resistant design, the operator can stand in front of the equipment while performing the racking operation, opening and closing the breaker, or working on the control and relay equipment. If an arc flash occurs, the energy and debris will be contained within the equipment and vented through a channel away from the worker.
A noncurrent-limiting circuit breaker or fuse will interrupt the short-circuit current at a naturally occurring zero crossing in the waveform. This means the peak current will occur at least once, and there will be at least one half-cycle of thermal energy. However, current-limiting devices can greatly reduce the amount of incident energy by reducing the duration of the current, its magnitude and resulting thermal energy.
To achieve selective coordination and minimize the extent of an outage in a power system, adjustable overcurrent devices are often set with a significant time delay to permit downstream devices to operate first. However, a long delay can significantly increase the amount of prospective incident energy.
Many protective devices can be specified with an arc flash-reduction switch. In many cases, it also can be installed as an aftermarket device. Before performing energized work, the maintenance switch can be enabled to provide faster protective device settings that can reduce the incident energy. A person familiar with the coordination study must determine the maintenance settings to avoid nuisance tripping under normal conditions.
Putting as much distance between the worker and the prospective arc flash hazard can greatly reduce the incident energy exposure. Although a relatively rare event, a catastrophic arc flash can occur when performing operations such as racking a circuit breaker in or out of switchgear (connecting and disconnecting). The remote racking process uses a motorized unit mounted on a movable frame that engages the racking mechanism on the switchgear. A remote unit may be operated from a long extension cord or even by a wireless remote device.
If an arc flash occurs, the worker should be well beyond the fireball and blast. Devices also are available for remote insertion/-removal of motor control center buckets, circuit breakers and many other types of devices and equipment.
Thru-door voltage sensing
Knowing if equipment is energized before the enclosure door is opened can help determine if an electrical hazard exists. Devices such as a hard-wired LED indicator can be mounted in the equipment door and provide thru-door voltage detection, keeping the worker away from the hazard. These devices can be used to provide continuous status of phase-to-phase and phase-to-ground voltage.
Optical detection and high-speed clearing
High-speed breaker tripping can greatly reduce arc flash duration and incident energy. Sensing both the ultraviolet light and resulting overcurrent can be used to identify an arc flash occurrence. The separate signals from both light and current can more accurately detect an arc flash, and then the protective device within the arc flash zone is operated in its high-speed instantaneous region to reduce the arc flash duration and incident energy.
Infrared viewing windows
As part of a preventative maintenance program, infrared (IR) thermography is often used to identify potential hot spots on electrical equipment. These hot spots may indicate problems. Those who perform IR thermography traditionally have had to open energized equipment, exposing the worker to an electrical hazard. IR viewing windows can be installed in the equipment’s doors, allowing the thermographer to scan the equipment with the doors closed, eliminating the possibility of worker-initiated electrical hazards during inspection (see next page for an example).
Convert arcing to bolted faults
A bolted short circuit occurs when there is no impedance at the point where conductors make contact—as if the connection was “bolted” together. Unlike an arcing fault, when a bolted fault occurs, there is no air gap, so no arc occurs. This concept is being used to reduce the incident energy from an arc flash. When an arc flash is detected by using a current and optical sensor, a high-speed electromechanical switch closes to provide a low-impedance parallel path in a separate cubicle. This creates a three-phase bolted type of fault with no arc, which is then cleared by the normal overcurrent protection scheme.
Transformer blind spot
If an arc flash occurs between the secondary terminals of a transformer and the first protective device, the primary device must respond to clear the fault. Depending on the size and/or setting of the primary device, the magnitude of the arcing short-circuit current, and the winding configuration of the transformer, the primary device will either respond very slowly or not at all, creating a potentially dangerous situation.
There are two common solutions to protect the transformer’s secondary blind spot. One solution is the addition of secondary protection as close as possible to the secondary bushings or terminals (there will still be a small blind spot). Another solution is the addition of current transformers on the secondary side, interfacing them with the primary relaying (if available) to trip the primary breaker upon sensing an abnormal current on the secondary side.
Differential protection is a scheme that produces high-speed fault clearing. The concept of differential protection is simple: current transformers are placed on the line side and the load side of the protected zone, containing the equipment to be protected. This can include a substation bus, transformer, large motor, generator or similar equipment.
Under normal load conditions, current flows into the protected zone through the line side current transformers and out through the load side current transformers. A differential relay monitors both sides. If there is a mismatch in current between the two sides, it is interpreted as current flowing into the protected zone that is not flowing out, indicating a short circuit within the zone. In this case, the differential scheme can usually be set to trip much faster and reduce the arc flash duration.
Protective relays—multiple setting groups
If a short circuit occurs on a transmission or distribution line close to the substation, the protective relay may trip instantaneously and minimize the incident energy exposure. However, as the arcing short circuit occurs farther away from the substation, the impedance of the line reduces the magnitude of current, and it could ultimately be lower than the protective relay’s instantaneous tripping function.
Many digital relays can be programmed with multiple setting groups. In this case, the primary setting group can be used for normal operation and a second setting group for maintenance settings. This second setting group should be sensitive enough to respond quickly to reduce the incident energy to a lower value for faults at the end of the line. Before any energized electrical work is performed on the line, the second setting group can be enabled to reduce the incident energy. Care must be used when determining the second group of settings to ensure they do not create any nuisance tripping problems.
The best method
The best solution is to only work on equipment that is placed into an electrically safe working condition as outlined in NFPA 70E. This means the equipment has been de-energized, locked out, tested for the absence of voltage and has safety grounds installed if necessary. Only then is it truly safe to work on.
Plato had it right. The necessity to continually improve both safety and reliability of electrical power systems will continue to challenge equipment designers to develop newer and better methods for reducing or eliminating the electrical hazards. Necessity really is the mother of invention!
PHILLIPS, founder of www.brainfiller.com and www.ArcFlashForum.com, is an internationally known educator on electrical power systems and author of “Complete Guide to Arc Flash Hazard Calculation Studies.” His experience includes industrial, commercial and utility systems, and he is a member of the IEEE 1584 Arc Flash Working Group. Reach him at firstname.lastname@example.org.