Arc-resistant switchgear has been available since the early 1990s, first seen in Europe and Canada and later specified with increasing frequency in the United States. At first, no American National Standards Institute/National Electrical Manufacturers Association (ANSI/NEMA) standards existed to define the test procedures that the switchgear sold in the U.S. had to undergo. In 2001, the Institute of Electrical and Electronics Engineers (IEEE) created the C37.20.7 “Guide for Testing Medium-Voltage Metal-Enclosed Switchgear for Internal Arcing Faults.” This guide was updated in 2007 and is now called the “Guide for Testing Metal-Enclosed Switchgear Rated Up to 38 kV for Internal Arcing Faults.”
As safety has become a top priority in the industry, and acceptance of arc-resistant technology has increased, arc-resistant testing has expanded to include medium-voltage metal clad switchgear, low- and medium-voltage motor control, and recently medium-voltage adjustable frequency drives.
The C37.20.7 guide for testing metal-enclosed switchgear defines a successful outcome of an internal arcing fault test using the five criteria summarized below:
- Properly latched or secured doors, covers and so on do not open.
- No fragmentation of the enclosure occurs within the time specified for the test.
- That arcing does not cause holes in the freely accessible front, sides and rear of the enclosure (Type 2).
- No indicators ignite as a result of escaping gases (cotton indicators are placed 4 inches from the sides of the equipment being tested to simulate cotton clothing worn by an operator).
- All the grounding connections remain effective.
Since the introduction of arc-resistant electrical equipment, the design paradigm has been to create a method of containing the energy from an arc flash event and redirecting it away from the operator. These methods typically include heavily reinforcing the electrical equipment enclosure using thicker-gauge steel, reinforced hinges and multipoint door latches to contain the arc energy and prevent enclosure fragmentation and burn-through. Mounted on top of the gear, plenums connected to ducts serve to redirect the arc gasses through the top of the equipment and out of the room.
Such designs benefit from simplicity and acquisition costs. These passive arc-resistant designs require no electronics or electrical power to function. Minimal technological advancements were required to develop these designs. All that was needed was a more robust equipment enclosure. The incremental costs of such arc-resistant equipment remain relatively low, requiring only the addition of more steel and more durable hardware to the existing design.
However, there is a distinct set of drawbacks to passive arc-resistant construction. First and foremost, arc-resistant designs do nothing to prevent the propagation of arc faults and, therefore, do not protect the internal components of the equipment. Small arc flash events may damage individual components inside the equipment, while catastrophic arc flash events can destroy the equipment entirely, requiring a complete replacement.
In addition to the cost of repairing and replacing damaged or destroyed equipment, there can be substantial revenue lost due to the downtime of critical processes that relied on the damaged equipment.
Arc-resistant equipment also increases installation and construction costs when compared to standard equipment. Arc-resistant equipment often requires that electrical rooms be constructed with higher ceilings to provide clearance above the gear either for direct venting into the space or for the plenums. Installing the ducts through walls to redirect the arc gasses out of the space adds more cost, as does fencing off or otherwise restricting access to the area where the ducts vent the arc gasses into the environment.
And, there are some industries that cannot tolerate venting arc gasses into the environment. Hazardous areas such as those classified as Class I, Division 2 by the National Electric Code (NEC) Article 500 may contain flammable gases, so venting into these environments is not permissible.
The efficacy of arc-resistant designs hinge on operation and maintenance personnel following relevant administrative controls at all times. The safety afforded by arc-resistant construction depends on the equipment’s ability to contain and redirect the energy during an arc flash event. Arc-resistant equipment, by itself, does not reduce incident energy under any circumstances. If doors are left open or improperly latched and if bolts are removed or panels are missing, the equipment may not adequately contain and redirect the energy.
Therefore, it is incumbent upon the operations and maintenance personnel to ensure that the equipment enclosure remains completely intact at all times while the equipment is energized. And if maintenance activities require opening doors or removing circuit breakers while the equipment is energized, personnel must be educated to understand that the arc-resistant rating will be temporarily compromised.
For decades, the industry has relied on a variety of electronic means to reduce incident energy for the purpose of improving safety. These methods include zone selective interlocking, bus differential relays, arc detection relays and energy reducing maintenance switches. However, none of these technologies are able to reduce the incident energy enough to provide the equivalent level of protection provided by passive arc-resistant equipment. This is primarily because these technologies rely on tripping the main device in order to clear a fault.
In the case of low-voltage switchgear, the main device is a large power circuit breaker, which can take up to four cycles or 67 milliseconds (ms) to clear a fault.
Since incident energy is related to available fault current and clearing time, there is a finite limit to which these solutions can reduce the incident energy at a given fault current. In order to reduce the incident energy further, a faster-acting device must be used.
Systems that apply a bolted fault to reduce incident energy of low-voltage switchgear have been around for many years. However, as of this writing, none of these systems have been tested to the C37.20.7 arc-resistant test guide and shown to reduce the incident energy far enough to provide the equivalent protection of passive arc-resistant equipment. In addition, applying a bolted fault to electrical equipment causes the equipment to draw the peak available fault current. Drawing peak available fault current induces electromagnetic forces on the upstream equipment, which can damage the transformer windings, cable terminations, bus bracing and even lead to unmitigated arc flash events upstream.
Current-Limiting Arc-Quenching Systems
Considering all of these technologies, a gap exists for systems than can mitigate an arcing fault fast enough to reduce the incident energy level sufficiently to provide equivalent arc-resistant protection while not putting unnecessary stress on the upstream electrical equipment. This gap is being filled by technology that can be generally called current-limiting arc-quenching systems.
Typically, arc-quenching systems use a combination of current transducers and light detection to identify the initiation of an arcing fault. Upon arc detection, these
signals are sent to a device that acts to transfer the arc to a contained location.
Underwriters Laboratories (UL) describes these devices in their UL 2748 standard for “Arcing Fault Quenching Equipment” as “equipment intended to quench arcing faults by creating a lower impedance current path, located within a controlled compartment, to cause the arcing fault to transfer to the new current path.”
While this standard leaves the door open for systems that use a bolted fault to create a lower impedance current path, the most advanced arc-quenching systems create an arcing fault inside a controlled compartment. The benefit of creating an arcing fault is that it is higher impedance than a bolted fault. It draws less fault current and, in some cases, greater than 25 percent less peak fault current than bolted faults. The result is greatly reduced stress.
Sometimes this is greater than 44 percent less stress on cable terminations, bus bracing and the secondary windings of the upstream transformer. Such systems are called current limiting arc-quenching systems.
Arc-quenching systems that operate fast enough and result in a large enough reduction of incident energy may be able to meet the C37.20.7 test criteria. Testing revealed that the pressure wave from an arc flash event in low-voltage metal enclosed switchgear peaks around 7 ms after arc initiation. Therefore, an active system deployed in similar switchgear must have a total operation time that is significantly less than 7 ms to prevent enclosure doors from opening and/or enclosure fragmentation.
During an arc flash event, the peak of the pressure wave precedes the expulsion of arc gases and molten copper. Arc quenching systems that react fast enough to cut off the peak of the pressure wave will also drastically reduce or nearly eliminate the amount of arc gases and molten copper produced, thereby satisfying the remaining C37.20.7 criteria.
If the arc-quenching system is able to reduce the incident energy far enough, it is possible to exceed the C37.20.7 testing requirements. Passive arc-resistant equipment, for which the C37.20.7 guide was developed, can pass the testing only when all of the primary doors are properly closed, latched and all enclosure panels are in place. But depending on the quenching time of the arc-quenching system, it may be possible for these systems to pass the arc-resistant test with circuit breakers or enclosure panels removed and equipment doors open. As a result, the protection afforded by these systems can exceed that of passive designs, since administrative controls are not necessary to ensure the enclosure remains intact at all times while the equipment is energized.
The benefits of such arc-quenching systems include reducing the incident energy of the equipment in which they are installed. This results in the equipment being able to meet or exceed the rigorous C37.20.7 arc-resistant test guide. In addition, with reduced arc flash risk, this may permit modifications to personal protective equipment (PPE) requirements and the reduction of the arc flash boundary.
Depending on the design of the arc-quenching system, the system may also not require any venting into the environment. In that case, no ducts or plenums are necessary and the required clearance above the equipment can be reduced, leading to lower ceilings and reduced installation and building construction costs.
Arc-quenching systems of this type are ideal for use in Class I, Division 2 environments where venting arc gases into hazardous locations is not permissible.
Since arc-quenching systems can reduce incident energy, they can decrease or completely eliminate damage to the equipment from arc flash events. Consequently, arc-quenching systems can reduce the downtime that results from an arc flash event since damage to the equipment is minimal to none.
This may make these systems ideally suited for use in critical processes—processes where extended downtime can be costly or lead to other hazards.
Nevertheless, arc-quenching systems have some drawbacks. Since they are electronic systems that contain components such as relays, optical sensors, quenching device(s) and all of the associated interconnect wiring, they must be periodically tested and maintained to ensure proper functionality. The inclusion of these additional devices can also increase the footprint of the equipment.
Finally, arc-quenching systems may be more expensive than passive arc-resistant equipment if total installed costs are not considered.
The emphasis on improving safety, protecting expensive electrical equipment from arc flash damage and reducing the downtime of critical processes continues to increase in nearly every industry.Capital budgets are strained and insurance premiums continue to rise. In response to these pressures, electrical manufacturers are developing innovative solutions.
Current-limiting arc-quenching systems are one of the promising new technologies that address these concerns and changing priorities with a new approach.