Mitigating arc flash hazards

Designers must account for several special constraints when mitigating arc flash hazards.

By Joseph Thornam, PE, Stanley Consultants September 14, 2017

Learning objectives

  • Know the codes and standards as they apply to arc flash.
  • Learn about the various products and systems that can help mitigate arc flash.
  • Understand how arc-resistant switchgear can redirect energy away from personnel.

We are all responsible for safety. Engineers, contractors, and facility operators must do their part to minimize the risk electricity poses to personnel. Good installation, training, and maintenance practices are indispensable, but engineers also can do a great deal to design for safety.

Arc flash is one of the major personnel hazards associated with electrical equipment. NFPA 70E: Standard for Electrical Safety in the Workplace defines the arc flash hazard as, “A dangerous condition associated with the possible release of energy caused by an electric arc.” This article aims to highlight some of the little-known design peculiars that can plague the specification/application engineer during a design project.

Arc-resistant (AR) switchgear

Redirecting energy produced by an arc flash away from personnel can be an important safety strategy. AR switchgear can accomplish this. Manufacturers design this specialized equipment to contain and exhaust arc blasts. This requires heavy-duty construction and other design features, which can lead to unique limitations not present in conventional switchgear. Engineers who specify and apply this equipment must be aware of these limitations.

NFPA 70E defines AR switchgear as, “Equipment designed to withstand the effects of an internal arcing fault and that directs the internally released energy away from the employee. Arc-resistant switchgear provides protection from internal arcing faults when the equipment is closed and operating normally.” AR equipment redirects energy away from personnel through flaps in the roof (see Figure 1). The energy may either exhaust directly into the room where the gear resides or into a plenum and ducting system to convey the blast out of the room. See Figure 2 for an example of a switchgear, plenum, and ducting system.

To illustrate the inherent increase in safety provided by AR switchgear, consider a worker operating a circuit breaker in a 5-kV switchgear lineup. If conventional, non-AR switchgear is used, NFPA 70E-2015 Table 130.7(C)(15)(A)(b) assigns arc flash PPE Category 4 to the task. According to NFPA 70E Table 130.7(C)(16), this category requires 40 cal/cm2 arc-rated personal protective equipment (colloquially known as a “moon suit”). Conversely, if the switchgear is AR, the task would not have an applicable arc flash PPE category. In fact, NFPA 70E-2009 assigned a Hazard Risk Category 0 to this task, which required only limited PPE. Note, however, NFPA 70E-2015 has removed references to Category 0.

IEEE Standard C37.20.7-2007: Guide for Testing Metal-Enclosed Switchgear Rated Up to 38 kV for Internal Arcing Faults establishes testing requirements for AR switchgear. This standard assigns the “arc-resistant” rating to specific switchgear designs. AR gear comes in two principal accessibility types: 1 and 2. Type 1 only protects a worker if they are standing in front of the gear while Type 2 offers protection on the front, sides, and rear. Type 2 has two further classifications: 2B and 2C. Type 2B offers protection when low-voltage compartments are open and Type 2C offers protection when adjacent breaker compartments are open. Type 2B is the most common type of AR gear; some manufacturers do not offer Type 1. Currently, there is no accepted testing procedure for arc-resistant low-voltage motor control centers, switchboards, or panelboards.

Rating limitations. To meet the testing requirements in IEEE C37.20.7, manufacturers often are forced to make compromises. When specifying AR switchgear, it is important to understand these compromises and the resulting design restrictions. Specifically, AR gear may have reduced thermal capability, less short-circuit capability, or layout restrictions relative to conventional counterparts.

First, consider thermal capability. Manufacturers design AR gear to contain gases produced by an arcing fault. The heavy-duty construction intended to withstand an arc blast can impede the equipment’s ability to reject heat under normal operating conditions. Therefore, certain components may need to be derated. For example, one manufacturer derates its 2,000-amp circuit breakers to 1,750-amp in some medium-voltage switchgear layouts. In other cases, manufacturers may reduce heat losses by using larger bus ratings than otherwise required.

Next, there are instances where manufacturers cannot achieve the same short-circuit ratings with AR gear as they can with conventional gear while continuing to satisfy the requirements of IEEE C37.20.7. For example, another manufacturer’s low-voltage switchgear has an arcing short-circuit current rating of 85 kA at 635 Vac in its AR line, but bus bracing ratings up to 200 kA in conventional switchgear.

Physical limitations. Electrical ratings are not the only restrictive aspects. Physical layout limitations also exist in AR designs. For example, yet another manufacturer requires one 57-in. instrument compartment for every two 2,000-amp circuit breakers. This can lead to larger lineups. Further, one manufacturer’s narrow and front-accessible designs are not available with AR ratings. This can also cause AR switchgear to have a larger footprint than can be achieved with non-AR switchgear.

Additionally, AR designs typically provide far less room to locate relaying and metering equipment. Type 2B designs have isolated instrument compartments that are segregated from the circuit breaker compartments. These compartments provide much less real estate relative to conventional designs, which can accommodate relaying on the entire circuit breaker cell door (see Figure 2). This lack of space may necessitate additional sections or separate, externally mounted relay racks. See Figure 3 for an example of how limited space for relaying can be in AR gear; conventional gear is on the left while AR gear is on the right.

Now, consider where the switchgear will be installed. It is important to know where the gear provides protection, and where it doesn’t. For example, IEEE C37.20.7 requires Type 2 switchgear to provide protection to personnel in the front, back, and sides, but does not provide protection above or below the switchgear. This fact may be important if there are walkways above the switchgear or cable vaults below. Designers should consider this when they are arranging the equipment.

Arc exhaust chamber (plenum) and duct. The most noticeable characteristic of AR gear is the arc blast venting system. This system can include an arc exhaust chamber (also called a plenum) and an arc exhaust duct. The plenum sits atop the gear while the duct channels the arc blast from the plenum to a safe location (typically outside the room or building). Figure 2 shows an example of the duct. Where the plenum and duct aren’t provided, the arc blast can be exhausted directly into the building. However, if exhausted directly into the building, IEEE C37.20.7 requires that testing simulate the actual room conditions. This fact can push users to favor the duct method.

The plenum and the duct come with a number of constraints that designers must consider when laying out their equipment. First, one must consider the manufacturer-required clearance above the plenum. Manufacturers recommend anywhere from 18 to 40 in. of installation clearance. Without this clearance, the contractor will not be able to install the plenum and associated ductwork. This can result in as much as 13 ft 2 in. of ceiling height in the electrical room—a much greater ceiling height than what is required for conventional switchgear, which can be as little as 9 ft 6 in. Even more restrictive, up to 10 ft of clearance may be required above low-voltage switchgear designs without plenums. A manufacturer may require that this area is to remain free of any obstruction including conduit, lighting, smoke alarms, cable trays, and HVAC ductwork. This requirement can be quite onerous.

While AR gear is intended to contain the arc byproducts during an arc flash, heat from normal operation of the switchgear must be vented. As stated above, this can be done with plenum vents that remain open during normal conditions but are forced closed during an event. The application engineer must consider the manufacturer’s required clearance around these devices to allow for adequate ventilation under normal circumstances.

Duct routing. There is a great deal of detail in manufacturers’ application guides regarding the routing of the arc venting ductwork. This ductwork is a unique aspect to AR designs that is not applicable to conventional switchgear. The duct typically can exit from the front, sides, back, or top of the gear’s plenum (see Figure 4). Ducts often need to make elevation changes, which necessitates vertical portions, but it is typically recommended that the duct terminate horizontally to prevent weather ingress. If the switchgear room resides in a larger building, the designer may want to route the duct vertically through the roof of the switchgear room. Bends are permissible, and there usually isn’t a length limit. Sometimes, it is advantageous to combine ducts from multiple switchgear lineups. Although, when this method is used, a blast from either could take both lineups out of service. This should be considered during the design. Lastly, it is prudent to slope the duct down and away from switchgear to avoid moisture ingress. Manufacturers may want to approve the duct routing. In some cases, consultants provide general room-layout details and let the manufacturer design and furnish the duct.

After the routing of the ductwork has been determined, the engineer must design its supports. Threaded rod and framing channel can be used to support the ducts, but the manufacturer will provide specific guidelines for the support system. Often, the ductwork is made from 11-ga steel and a single duct doesn’t usually weigh more than 75 lb/ft. Note that in some cases, two parallel runs of arc duct are required depending on the switchgear lineup section quantity. This will be dictated by the manufacturer.

Exhaust assembly. As mentioned above, it is preferred to route the duct horizontally through an exterior wall so that arc byproducts are vented outside. This not only keeps the exhaust point away from personnel and sensitive equipment, but it also vents the toxic arc byproduct outside the enclosed space. Because the routing will penetrate an exterior wall, weatherproofing is important. A manufacturer’s standard system may provide a wall-penetration kit, but be aware that these kits can have a maximum wall thickness. The engineer must coordinate this with the building design.

It is surprising to some that the internal pressures during an arc fault can be reduced to less than 2 psi at the exhaust duct cover or hinged flap. Because the cover must open in response to these low pressures, the cover must be quite sensitive. To maintain this sensitivity, it is important to prevent the accumulation of ice. Often, space heaters are employed to accomplish this. Further, manufacturers require clear zones around the exhaust point ranging from 8 to 15 ft of horizontal distance. The application engineer may want to consider chain-link fencing to keep this area clear of personnel. Lastly, avoid arranging the duct so an arc blast exhausts into a hazardous/classified area.

Cabling and conduit. As discussed, many AR applications use an arc plenum on the top of the switchgear lineup. This can dramatically reduce the area available for conduit penetrations and cable-tray routing. Some manufacturers completely disallow top-entry power-cable penetration in AR designs. In these cases, the designer must accommodate bottom entry. Even when bottom entry is used, it may be important to use steel seal plates and/or cable glands to ensure no arc byproduct escapes through cable penetrations. Additionally, when cabling passes between low-voltage and medium-voltage compartments, silicone sealant or specialized boots may be required at the penetration locations. It also is worth noting that cable-termination compartment doors in AR gear may slow down work due to the heavy-duty construction and the many bolts.

Cost impacts. One may be wondering what these safety features cost. It is estimated that AR switchgear can carry a 10% to 15% premium over conventional metal-clad switchgear. Note that these figures do not include the cost associated with any additional building height or floor that may be required. Those costs would depend heavily on the specific application. Additionally, installation costs are not included above.

Reducing arc energy

While redirecting blast energy with AR switchgear can be an important safety strategy, there also is a number of features the engineer can use to reduce the energy of the arc flash in the first place. NFPA 70-2017: National Electrical Code (NEC) contains important requirements related to reducing the energy. Specifically, NEC Article 240.87 requires arc energy to be reduced by one of the following methods: differential protection, zone-selective interlocking (ZSI), maintenance-mode switches, active arc flash mitigation systems, instantaneous protection, instantaneous overrides, and approved equivalent means.

Differential protection. This method works by comparing the current entering and exiting a switchgear bus. If these two do not match, the scheme believes there is a fault and it trips circuit breakers. There are many different forms of differential protection, but high-impedance differential protection often is used in switchgear because of the large quantity of terminals (or breakers). These schemes work by paralleling each circuit breaker’s current transformers (CTs) with a known impedance (see Figure 5). Current does not travel through this impedance until there is a fault. When a fault occurs, the relay detects a voltage across the impedance and operates. The relay manufacturer typically recommends a minimum voltage threshold that should establish a trip condition, at least instantaneously. The CTs must be able to provide enough voltage to achieve this threshold. Because the amount of voltage a CT can produce is roughly proportional to its size, achieving the voltage threshold can require large CTs. This can be difficult in switchgear because of the physical space limitations. The application engineer must consider these aspects.

ZSI. This technology provides the benefits of selective coordination while reducing the time delay associated with conventional schemes. Traditionally, selectively coordinated systems delay upstream or main circuit breakers to give downstream breakers time to operate. This delay gives rise to a larger arc flash hazard. ZSI overcomes this problem with communication connections between upstream and downstream circuit breakers. To illustrate this, consider an example where a fault occurs at Location A in . Both the main breaker and Feeder Breaker 1 will see this fault. When ZSI is used, the feeder breaker will send a restraint signal to the main breaker, instructing it to wait to allow the feeder breaker to operate. The feeder breaker will trip while the bus remains operational. Conversely, if the upstream breaker sees the fault and the downstream breaker does not (fault at Location B), the feeder breaker does not send a restraint signal and the main breaker trips quickly. This allows for faster tripping during faults inside the switchgear (relative to conventional coordination), which are especially hazardous to personnel. In this way, ZSI mimics the behavior of differential protection. However, it is cheaper because the differential CTs are not required. This scheme is used in low-voltage systems more than medium-voltage. When ZSI is applied to low-voltage switchgear, ZSI-capable trip units are used. These units are interconnected with communication links allowing signals to be passed from downstream to upstream breakers. If the engineer wants to apply this scheme, it must be considered when the trip units are specified.

Maintenance mode. An operator can enable maintenance mode by toggling a switch (typically on the circuit breaker), which temporarily enables a more sensitive instantaneous overcurrent element. This allows the system to achieve greater coordination under normal circumstances, but have reduced fault-clearing time during maintenance. A problem with having the maintenance mode switch on the breaker itself is that the worker must enter the limited-approach area to toggle the switch. This exposes the worker to the “nonmaintenance mode” hazard during the approach. A possible solution for this problem is to locate the switch to a location near the door of the electrical room.

An additional problem arises when the operator forgets to toggle the breaker out of maintenance mode after the work is complete. This can dramatically impact the facility’s overcurrent coordination, which can turn minor faults into a widespread outage. Potential solutions for this scenario could be audible alarms or flashing lights triggered by timers. Alternatively, signals can be sent to the plant’s control system indicating active maintenance mode.

Optical arc flash detection. These schemes detect the light from an arc flash and often are supervised by an overcurrent element to avoid tripping from sunlight or camera flashes. The problem with this approach is that the protected area is limited to the conductor downstream of CTs. If an arc flash occurs upstream of the CTs, current will not be detected and the breakers will not trip. A possible solution to this problem is to use a current-supervision signal from an upstream breaker. For example, if you had optical detection on a switchgear’s feeder breaker, the main breaker could provide the overcurrent supervision. This would require interconnection-control cabling. This problem becomes even more difficult on the main breaker. When a fault occurs upstream, the main breaker cannot do anything to limit the incident energy. Devices even further upstream must address the problem. Very high incident-energy values can exist at these locations.

Design for safety

Applying AR switchgear and energy-reduction measures comes with a fair amount of complications. A lot of these aspects didn’t exist in the past, when conventional switchgear was the norm. These additional items can sound intimidating at times, but specifying engineers shouldn’t let their trepidations prevent them from using the safety features available to them. It is important that the engineer leverage the tools available to design for safety.

Joseph Thornam is a senior electrical engineer at Stanley Consultants