Mitigating arc flash hazards

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

09/14/2017


This article has been peer-reviewed.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

Figure 1: These photos show arc flaps located on top of arc-resistant switchgear. The photo on the left shows flaps in their normal, closed state; the photo on the right shows how the flaps open during an arc flash. All images courtesy: Stanley Consultants

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.

Figure 2: This diagram shows components of typical arc-resistant switchgear including the arc plenum, arc duct, and various compartments.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.

Figure 3: These photos illustrate the limited relaying space in arc-resistant switchgear (right) as opposed to conventional gear (left).

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.

Figure 4: An arc-resistant switchgear design where arc ducts exit directly out the top of the plenum.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.


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