Optical arc flash protection and installation experience

Complying with the safe work practices dictated by NFPA 70E and implementing arc flash mitigation strategies through engineering controls will enhance workplace safety for employees and reduce the financial risk for your company. Reprinted with permission by Schneider Electric. When responding to emergencies, including structure fires in one- and two-family housing units, safety is always at the top of the list of firefighter priorities.

Let's have a live discussion about keeping things safe these holidays. What we discuss during this session transcends holidays and should be kept in mind all year round. ESFI is providing safety tips to prevent household fires and accidents caused by improper heating, cooking, and holiday decorating practices. A source of possible injury or damage to health. A combination of the likelihood of occurrence of injury or damage to health and the severity of injury or damage to health that results from a hazard.

Introduction While the threat of shock and electrocution from inadvertent contact with energized parts has long been recognized, the arc flash and arc blast hazards have only fairly recently been incorporated into the electrical safety standards.

Basic compliance to the requirements of NFPA 70E is established through a six-step process: Develop and audit electrical safe work practices policy Conduct an arc flash risk assessment to evaluate the likelihood of occurrence and severity of arc flash hazards Follow strategies to mitigate and control arc flash hazards Conduct regularly scheduled safety training and audits for all electrical workers Maintain electrical distribution system components Ensure adequate supply of personal protective equipment PPE and proper tools Companies can take additional steps to reduce the potential for an arc flash.

Figure 1. Electrical equipment should be installed, operated, serviced, and maintained only by properly trained and qualified personnel. What is Arc Flash Mitigation? Figure 2. Reduce Arc Flash Energy Levels Arc flash reduction systems do not eliminate the electric shock hazard of working on or inside energized equipment.

As a result, proper selection of overcurrent protective devices — in particular, selecting devices that will quickly clear arcing faults from the power system — is a powerful mitigation strategy Over-Current Protective Device OCPD Coordination Study An OCPD coordination study optimizes the protective device setting for reliability and arc flash protection. Specialized Relaying, Such as Optical Technology Quickly clearing faults is a key to arc flash mitigation.

Figure 3. Virtual Main Arc Flash Mitigation System Switchgear and switchboards can be subjected to dangerous levels of arc flash incident energy when fed directly from a power transformer. This mitigation solution can take one of two forms: A maintenance selector switch, which temporarily lowers the instantaneous short circuit current setting. The maintenance setting lowers the available arc flash incident energy and temporarily forfeits selective coordination.

Zone-selective interlocking with downstream branch circuit breakers in the switchgear eliminates the need for the maintenance selector switch. Arc flash energies can be permanently reduced with zone-selective interlocking. Before and After One-Line Diagrams An arc may propagate to the supply side of all devices in the same enclosure.

Figure 4. Example of a before and after single line diagram The following arc flash mitigation solutions remove a worker from the location of, or place a barrier between, the worker and exposed energized parts: Remove Workers from Harms Way Infrared Viewing Windows Having infrared IR windows permanently installed into electrical equipment enables IR scans to be performed without exposing the worker to hazardous energy.

Figure 5. Infrared IR windows allow hot spots to be registered by a thermographic camera. Figure 6. Remote Racking System A remote racking system RRS allows circuit breaker racking operations to be performed via a control panel located away from the cell, removing the operator from manual contact with the circuit breaker.

Figure 7. Remote racking systems remove the operator from manual contact with the circuit breaker. Conclusion Electrical hazards are a significant safety and financial risk for electrical workers and their employers.

Antony C. Parsons May 1, Parsons, Ph. He has more than 19 years of experience in design, analysis, and troubleshooting of commercial and industrial power systems. Antony is a global resource on arc flash analysis and electrical safety for Schneider Electric, and has delivered seminars, trainings, and consultations for clients and colleagues both in the US and abroad.

He is a licensed engineer in the state of Texas. The 5 ft. Ground Rod and its little-known use in the NEC. Find Us on Socials. Load More. To illustrate the inherent increase in safety provided by AR switchgear, consider a worker operating a circuit breaker in a 5-kV switchgear lineup.

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. 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. Therefore, certain components may need to be derated. For example, one manufacturer derates its 2,amp circuit breakers to 1,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 C 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 in. This can lead to larger lineups. 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. 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.

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.

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. 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. Duct routing. This ductwork is a unique aspect to AR designs that is not applicable to conventional switchgear. 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. 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. 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. 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.

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. 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. 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.