Explore ways to go beyond requirements for electrical safety

Electrical safety insights

• Engineers should prioritize exceeding minimum safety requirements by specifying advanced safety features.
• The design process should include comprehensive short circuit, coordination, and arc flash studies to specify appropriate overcurrent protection devices.

To meet baseline safety criteria and design requirements for electrical power systems, engineers review a wide range of codes, standards and product literature throughout the design process. The consulting engineer must understand that codes present minimum safety requirements, and that there are many opportunities to enhance system safety for facility personnel and third-party electricians by going above and beyond code requirements. When selecting equipment, the consulting engineer should seek opportunities to leverage product features and improve end-user safety. They must prioritize limiting operational hazards that electricians face when working directly on energized equipment or maintaining machinery and fuel-burning equipment.

Facilities may have policies in place regarding live or energized work that can influence design decisions related to phasing, data collection, temporary conditions and points of interconnection. While, from a safety standpoint, it’s desirable to work on de-energized equipment, sometimes live work is advantageous to avoid an electrical service shutdown and preserve operations. Additionally, equipment surveys or metering performed pre-construction may require live equipment cabinets to be opened, exposing personnel to energized buses.

For buildings where a lengthy shutdown could be especially disruptive, like hospitals, it is important to discuss shutdown protocols with the project stakeholders to understand if energized or de-energized work should be assumed for construction. If the client requests live work to be done by the contractor, it should be clearly noted in the construction documents so associated costs are captured in the bid, and so contractors who are unable to perform live work are filtered out of the awarding process. If included in their scope of services, the engineer should monitor bid forms and bid qualifications to confirm bidders have priced live work.

Arc flash considerations

During design, the engineer may be required by code to specify products that reduce the arc flash incident energy levels at the equipment to make live work safer for electricians. Arc flash incident energy can be defined as how much energy is released during an electric arc per unit area at a certain location. An arc flash occurs when an electrical current jumps between two conductors (busbars, feeders, etc.) through the air, effectively creating a short circuit. In commercial power systems, the energy released can be significant and extremely dangerous. NFPA 70: National Electrical Code (NEC), Article 240.67: Arc Energy Reduction and Article 240.87: Arc Energy Reduction for Switchboards and Panelboards require fuses and circuit breakers rated 1,200A or higher to be equipped with means to reduce arc flash incident energy levels by reducing the time it takes for the overcurrent protection device to clear a fault. The lengthier the fault, the more energy can be released during the fault.

Figure 1: The single-line diagram for a partial short circuit study shows available short circuit, or fault, currents at equipment buses. Courtesy: Kohler Ronan Consulting Engineers, LLC

Examples of technologies that satisfy NEC requirements include zone-selective interlocking, differential relaying, energy-reduction maintenance switching and instantaneous trip settings. Certain technologies are more sophisticated than others and can be more effective in reducing hazards to equipment and personnel. Intelligent differential relaying and arc quenching relays are more advanced than simple, fast-acting fuses that have fixed trip characteristics determined by their physical properties. Furthermore, the robustness of equipment construction can influence arc flash hazard potential. Products such as arc-quenching or metal-insulated switchgear reduce the risk of arc-related hazards compared to basic switchboard construction.

Short circuit, coordination and arc flash study

The design engineer can use short circuit, coordination and arc flash analysis software to determine key parameters. Parameters include maximum available fault currents and arc flash incident energy levels at equipment throughout their project’s power system. By producing a comprehensive short circuit, coordination and arc flash study, the design engineer can specify the appropriate overcurrent protection devices that can limit short circuit current and clear faults quickly to limit arc flash hazards.

It should be specified in the construction documents for the contractor to engage a third-party engineer to produce their version of a short circuit, coordination and arc flash study. This study accounts for the exact field routing of feeders along with the characteristics of submitted equipment and devices to verify that the design intent is met. The facility should keep an up-to-date record copy of the study on-site for easy reference when planning equipment maintenance and energized work.

Figure 2: The partial arc flash study single-line diagram shows arc flash boundaries and incident energy levels at equipment. Courtesy: Kohler Ronan Consulting Engineers, LLC

The contract specifications should require the contractor to provide system-specific arc flash hazard labeling for each piece of equipment that informs those operating the equipment of safety parameters, such as arc flash incident energy levels, arc flash boundary and the required level of personal protective equipment (PPE) required for energized work. These are outlined in NFPA 70E: Standard for Electrical Safety in the Workplace. Note that the arc flash labeling standards indicated in NFPA 70E are referenced in NFPA 70, Article 110.16: Arc-Flash Hazard Warning via an informational note regarding labeling requirements for service equipment. Adding arc flash labeling with the detailed information noted above to distribution equipment throughout the facility is a recommended best practice that goes beyond the requirements of NFPA 70, Article 110.16, which requires more basic warning labels.

This study will also enable the design engineer to specify equipment with adequate short-circuit current, interrupting and withstand ratings. It also allows them to specify fault current limiting overcurrent protection devices to reduce available equipment short-circuit currents to the lowest possible levels. Short-circuit and withstand ratings each define a piece of equipment’s ability to maintain its physical, electrical and functional integrity after a fault event. The interrupting rating of a device defines its ability to clear a fault up to a certain fault current level.

Electrical faults

A fault may occur when operating a switch mechanism, and if a piece of equipment is under-rated, it may catastrophically fail during a fault event and cause harm to personnel. NFPA 70, Article 110.9: Interruption Rating and Article 110.10: Circuit Impedance, Short-Circuit Current Ratings and Other Characteristics require adequate short-circuit and interrupting ratings. Withstand ratings of equipment are parameters listed by the manufacturer. For switches upstream of transformers with potentially high inrush load currents, selecting switches with adequate close-on ratings is important. A switch’s close-on rating is its ability to close during an inrush up to a certain current, usually expressed as a multiple of the equipment’s nameplate current. Specifying high close-on rated switches can reduce the risk of a switch arcing between its contacts if closed during an inrush.

Figure 3: This arc flash warning label includes information per NFPA 70E, including arc flash boundary, incident energy levels and certain personal protective equipment requirements. Courtesy: Kohler Ronan Consulting Engineers, LLC

At other high-power switching equipment, such as switchgear, remote switching options should be considered in addition to standard manual switching. This allows personnel to operate large switch mechanisms with the push of a button, leaving them out of potential harm’s way. Whenever possible, it is also recommended to specify mechanically held switches as opposed to electrically held switches, which rely on power to hold their position. Infrared scanning of distribution level feeder terminations and splices should be specified to evaluate the integrity of the termination at the time of initial energization. A loose termination could leave the feeder vulnerable to electrical stresses that could ultimately result in an arc event.

Follow up scanning should happen months after the initial scan to ensure that there are no noticeable changes in heat mapping, which could signal a failing connection. Adding surge and switch operation counters to key distribution switches can help facility personnel track the condition of their infrastructure and schedule necessary maintenance before equipment becomes less safe to operate.

There may be a large range of short circuit currents and arc flash incident energy levels experienced in buildings’ electrical systems, so the design engineer should evaluate each piece of equipment and their application against the project’s budget and space constraints before specifying a product. For example, switchgear with electronic fault-clearing relays and low-voltage power circuit breakers takes up significantly more space and costs significantly more than a less substantial switchboard with molded case circuit breakers. However, the switchgear provides greater resilience to faults and improved arc mitigation.

The engineer should also familiarize themselves with the technical knowledge of the facility personnel and equipment operations protocols to specify a product appropriate to their level of involvement with the systems. In the construction documents, the engineer should include training sessions for the personnel on how to operate and maintain specified arc energy reduction and fault mitigation equipment.

Maintenance or isolation bypasses

Maintenance or isolation bypasses are other features that can limit hazards due to energized work and are available on a wide variety of products. Bypass switches, or sections in automatic transfer switches, variable frequency drives and uninterruptible power supplies can allow maintenance personnel to work on energized critical equipment while reducing, or eliminating, their exposure to live parts. Bypasses are available in different configurations, with the safest being a separate section that is electrically isolated from the primary section by a vertical barrier within the equipment cabinet.

Figure 4: This arc flash warning label is generic. It meets the requirements of NFPA 70, but lacks information per NFPA 70E. Courtesy: Kohler Ronan Consulting Engineers, LLC

Even safer than an integral bypass is a separate, redundant piece of equipment. Bypasses and redundant equipment often add to project space needs and costs, so it is worth discussing with the client early in the project to understand their tolerances for shutdowns and energized work compared to the spatial and financial costs. Common applications for bypasses and redundant equipment include hospitals, data centers and other occupancies where shutdowns to work on de-energized equipment would be costly to business operations and occupant safety or would otherwise compromise the building’s internal environmental conditions.

Electrical rooms and mechanical equipment

During design, the engineer should meet with the owner’s electricians and facility personnel to review their procedures for maintenance and shutdowns and ensure specifications align with their operations. As a best practice, all distribution-level switches and switches feeding mechanical equipment should be lockable. Lockable switches allow electricians to enact their lock-out tag-out procedures during regular maintenance and shutdowns, reducing the risk of inadvertent energization of equipment. It is also recommended to provide disconnecting means within the line of sight of mechanical equipment and other equipment that requires regular maintenance for an additional layer of safety.

Note that it is not always required to have disconnecting means within the line of sight of equipment served. The NEC often permits it to be omitted, so long as the upstream disconnecting means is lockable in accordance with Article 110.25: Lockable Disconnecting Means. Emergency power off, emergency pull stations and shunt trip modules are useful products an engineer can employ in design to add means for the safe shutdown of remote equipment. Specific applications include emergency shutdowns of machinery and fuel supplies to mitigate hazards such as kitchen grease fires, entanglement in rotating shop machines and fires at classroom science labs or fuel-burning equipment.

Certain items pertaining to safety in power systems design may be of particular interest to architects for coordination, space-proofing and aesthetic purposes. Detailed signage, labeling requirements and record documentation should be included in the construction documents as diagrammatic placards at conspicuous locations. This ensures that system configurations are easily identifiable by facilities personnel and first responders in the event they need to act quickly to remedy a situation.

Electrical rooms should always be clearly labeled and should have access control or keyed locks accessible only to qualified personnel. In terms of electrical room design, it is recommended that doors swing outward so that someone working on a panelboard within the room does not get inadvertently bumped by a door swing into harm’s way. Note that this is in excess of NFPA 70, Article 110.26: Spaces about electrical equipment, which only requires doors to swing outwards and be equipped with panic door hardware when equipment within the room is rated 800A or greater. It is recommended that all electrical rooms be provided with emergency lighting for safe egress in the event of an outage, but this is only required by certain building codes.

Figure 5: This diagram shows a basic application of an emergency power-off switch and a contactor relay panel to control receptacles at science lab tables. Courtesy: Kohler Ronan Consulting Engineers, LLC

At key equipment, such as service and generator rooms, it is often required to provide emergency backup battery lighting packs, even if generator backup is present, to allow maintenance personnel to safely leave equipment working clearance during a power outage. If an electrical panel is recessed in a wall within a space that is required by energy code or building operations, then controls must be provided to bypass or override the automatic shut-off so that an electrician can safely work on the panelboard without lights potentially shutting off. Where electrical equipment is floor-mounted, concrete housekeeping pads are also recommended to keep equipment off the ground and away from potential pooling water, particularly if located in a room with piping. It is recommended to extend concrete housekeeping pads a few inches outside the electrical equipment’s footprint to protect it from physical impacts, such as accidental kicking or rolling equipment.

In conclusion, design engineers have many opportunities to go beyond the baseline requirements of construction codes and standards. Engineers should work with equipment manufacturers and clients to pursue such opportunities whenever possible.

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