Who wins the battle between electrical safety and reliability?

Safety insights

• Minimum requirements must be met when designing a building’s electrical system. This includes meeting NFPA 70: National Electrical Code
• Arc flash mitigation and other electrical safety systems come in many forms and are often specified as part of the entire electrical system.

Tenured electrical engineers and designers who are actively involved in designing various facility types have undoubtedly noticed the increasing pressure to cut costs and deliver designs that meet established dollar per square foot costs — costs that were likely determined in a client’s boardroom years before the project entered planning, design and construction phases.

With current inflation rates outpacing construction schedules, this is even more challenging and puts a growing focus on integrated design methods involving the general contractor and its subcontractors early in the design process. These delivery methods are seen as a means for the owner to understand project budget and schedule sooner while still ensuring safety and reliability.

This not be new to most engineers, but having more entities involved in the design decision process requires more oversight by the owner’s facility maintenance staff to ensure it still has a voice when it comes to how the building will operate and be safely maintained. The dwindling number of facility managers is demonstrated through lack of owner representation in new facility design discussions and is a sign that clients are focused on project first-cost and less on cost of facility operations. The project delivery method may also become a factor.

The owner will expect the engineer to provide a design that meets the minimum codes and that its facility maintenance staff or third-party electrician will follow the Occupational Health and Safety Administration (OSHA) standards and internal safety protocols. Without a maintenance voice during design, will the resulting facility be as safe to maintain?

The engineer can add design elements that increase safety while not affecting code-required power system reliability and occupant safety.

Figure 1: A typical arc flash warning label required by the NEC Section 110. Courtesy: Affiliated Engineers Inc.

Electrical safety codes and standards

Electrical maintenance and building occupant safety typically travel on parallel paths within the codes. These paths converge occasionally, but for the most part, they are not closely related. The 2023 edition of NFPA 70: National Electrical Code (NEC) is focused mostly on occupant safety. Its eight sections dictate how an electrical system is to be designed and installed to make a facility safe for its occupants.

Over the life of this code, material has been added to increase power system reliability for certain occupancy types but is still very limited with regard to maintenance safety. Maintenance safety, as required by the NEC, can be found in primarily in Sections 110 and 240. Section 110 includes working space requirements around electrical equipment and more recently, arc flash label requirements. Section 240 most recently addresses arc energy reduction.

Other sections of the NEC can be seen as both occupant safety as well as maintenance safety, and this article will focus strictly upon those sections that address maintenance safety.

Most of the code-required language for maintenance safety is in Section 110 of the NEC. Within this section, articles such as working space requirements dictate the space an electrician will have to work in, where equipment is to be located and how an electrician safely exits away from energized equipment when there is a problem.

Guarding of live electrical parts against accidental contact, signage and proper labeling of equipment enclosures and disconnecting means are all items that should never affect building occupant if the NEC is followed with a focus on maintenance safety.

Figure 2: Melting curves for different fuses used within the same distribution system. Courtesy: Affiliated Engineers Inc.

Electrical safety

According to the NEC , this labeling is to be used in conjunction with arc flash identification required by NFPA 70E: Standard for Electrical Safety in the Workplace. This standard is used to teach and enforce electrical safety to electricians and part of that training is to look for these labels (see Figure 1).

NFPA 70E was created in 1979 to provide facility owners and electricians a path to increase their safety practices and create a common platform for institutions like OSHA to review current maintenance practices and protocols within the workforce. In the 1995 edition of NFPA 70E, arc flash was identified as an electrical hazard.

For the most part, only maintenance electricians are exposed to arc flash when working on energized equipment, not affecting building occupant safety. The current version of NFPA 70E requires facility managers to label their electrical equipment with arc flash warning labels that indicate the level of energy calculated to be found within the enclosure of the equipment. NFPA 70E also informs the electrician of what levels of protection are required (typically clothing and other personal protective equipment, known as PPE) if the enclosure is to be opened.

Facility managers will have to perform these calculations or hire an engineer who performs these calculations as a service.

Engineers who perform these calculations regularly will tell you the factors that contribute to arc flash energy are:

• Short circuit current.

• System impedance.

• Fault clearing time.

Reducing one or all of these items for the sake of safety will have a detrimental impact on system reliability. However, one of these items can be reduced temporarily to the period of maintenance when the equipment enclosure is open. In 2012, the NEC added provisions to Section 240 to allow for requirements of circuit breakers (and fuses starting in the 2017 edition) rated larger than 1,200 amps to reduce its clearing time (time to remove power from the circuit) in the case of a fault. Manufacturers have adopted different approaches to meet this requirement.

The goal of this requirement is to reduce the pickup current setting in the circuit breaker’s instantaneous region of its trip curve to its minimum setting to allow a fault to be identified earlier and cleared. The circuit breaker clearing time from fault identification to fully open is not affected, however and the arc flash event will still occur. It will just have reduced time to develop the energy that could be present if the circuit breaker did not have this function.

This circuit breaker mode is activated by a switch near the circuit breaker that is typically identified as energy reduction maintenance switch, “maintenance mode” or “RELT,” meaning reduced-energy let through.

For fused applications, the system uses electronic overcurrent or differential relays to detect the fault as it begins and opens the bolted-pressure switch using its shunt trip operator.

Other common methods the code allows to accomplish this requirement include:

• Zone-selective interlocking.

• Differential relaying.

• Arc flash mitigation system.

These three methods can have less impact to system reliability but are more complex to implement and maintain as the facility grows or is modified in the future. These methods are implemented differently depending on manufacturer. Some manufacturers provide zone-selective interlocking in the instantaneous region where they would reduce arc energy while maintaining selective coordination, while other manufacturers provide zone-selective interlocking in the short-time region.

Care must be taken to make sure the correct system is implemented. Differential relaying is a common and long-trusted method to respond to system faults in a timely manner. It works by comparing current across two or more regions where that current should be proportionately equal. During a fault that current will not be equal across, both regions and will be detected as a problem. The relay is programmed to ignore rapid changes in current that have a specific harmonic content that is accompanied by transformer inrush, for example. The fault harmonics that do not appear as transformer inrush can be perceived as a fault and a trip signal produced for upstream devices.

Arc flash safety

Arc flash mitigation systems can use an arc sensor consisting of fiber optic receivers connected to a relay that can determine the difference between ambient light and arc flash light. This relay can send a signal to an upstream device to trip a circuit breaker or close a shunt contact within an arc vault to “relocate” the fault to within the vault.

All of these arc mitigation techniques require input from the owner and facility maintenance staff for the engineer to understand how the building is to be used. Not all solutions make sense for all facility types and some have cost or maintenance implications that need to be understood.

With all these considerations in mind, it might seem that the easiest approach to safety would be to just maintain the equipment only when it is de-energized. Most facility safety procedures require all maintenance to be done while the power is off.

Article 240 of the NEC was modified in 2012 with the addition of reduced energy requirements because work was being done on energized equipment and electricians were being exposed to unsafe arc energy levels. NFPA 70E arc flash labels indicate the energy available when the cover is open.

The pressure on facility managers to maintain uptime in mission critical, health care and high revenue-generating facilities may require electrical maintenance to be done while it is energized. Different levels of PPE were created specifically for doing live work, but still some arc energy levels prohibit it from being done safely.

There will always be a need to work on energized equipment. Utility medium- and high-voltage line repair electricians have worked on live equipment since their creation and have policies and procedures in place along with specialized PPE to do this work safely. The reliability requirements of the customers they serve require a high level of uptime for safety and loss prevention.

Selective coordination

We have briefly discussed reliability of power systems and how they are impacted by maintenance safety. The NEC addresses system reliability as it pertains to selective coordination of overcurrent devices in Sections 240, 517, 620, 700, 701 and 708. The number of sections affected by this topic confirms that system reliability impacts occupant safety.

Selective coordination describes how the circuit breakers or fuses used in the power system respond to an overload or fault in a system. A system that is considered “fully coordinated” isolates the overload or faulted condition to the overcurrent device located closest to the overload or fault. Engineers accomplish selective coordination by specifying overcurrent devices that open the flow of current upon detecting a fault or overload faster than the device that feeds it or larger rated devices feeding successively smaller or faster devices.

Every circuit breaker and fuse have an engineered curve that describes its overcurrent and fault response time. This is shown in a logarithmic scale with current on the x-axis and time on the y-axis (see Figure 2).

Different devices have varied responses under different circumstances to the same conditions so engineers specify certain device types to meet reliability requirements of the project.

The facility types and electrical systems impacted by the NEC’s requirements for selective coordination are:

• Section 517: Health care facilities.

• Section 620: Elevators.

• Section 700 and 701: Emergency and legally required standby systems.

• Section 708: Critical operations power systems.

A building that has none of these systems are not code-required to be selectively coordinated. The owner’s project requirements or requirements stated by the facility engineer will indicate which systems, if any, are required to meet a certain level of reliability beyond what the code requires.

A complete study of a building’s power distribution system is required to determine if the overcurrent devices are selectively coordinated. If they are not found to be coordinated and are required to be, device settings will require adjustment or replacement with other devices having the trip characteristics needed to coordinate with other devices in the same system (see Figure 3).

In many cases, the larger upstream devices are adjusted to allow higher instantaneous currents to pass through to downstream devices and so on. The overlap of instantaneous current trip curves from upstream devices to downstream devices has to be avoided to consider the system to be “fully coordinated.”

It is this coordination of trip settings in the instantaneous current region of a devices trip curve that causes high energy levels to be found in electrical equipment and thus, increased arc flash energy. Remember, the reduced energy requirement in Section 240 of the NEC was aimed at reducing the clearing time in the instantaneous region. So by reducing the clearing time by use of the maintenance setting required in Section 240, the coordination of the system has been eliminated for the period of time the switch is left on.

Most manufacturers have an alarm tied to the switch to remind you to return it to its normal setting once maintenance is complete. The coordination of the system returns once the maintenance switch is returned to normal.

Selective coordination is typically met using a combination of device types to include electronic-trip circuit breakers, fuses and fixed-trip molded-case circuit breakers. These devices are used together in series to achieve selective coordination proven by using a computer analysis tool like SKM Systems Analysis power systems software or ETAP electrical software that models the system performance under faulted conditions.

Understanding the different circuit breaker trip or fuse curves and where those devices are used in a system is the key in making a system fully coordinated. Using matched circuit breaker sets is also an option if you can choose manufacturers and stick with that manufacturer throughout the design. Matched circuit breaker sets have been specifically designed and tested to coordinate with each other. Not every manufacturer has this option, however.

Figure 3: The time-current curve on the left is of a system that lacks selective coordination shown as overlapping trip curves in instantaneous region. The curve on the right is of a system that is selectively coordinated shown by having trip curves that do not overlap. Courtesy: Affiliated Engineers Inc.

How safety and reliability affect electrical systems

Now that safety and reliability have both been described, let’s explore how one affects the other.

Generally speaking, increased reliability can result in decreased maintenance safety and increased maintenance safety can result in decreased reliability. This holds true if the engineer follows the minimum code requirements most owners impose.

Let’s return to the three elements that control arc flash energy: short-circuit current, system impedance and fault clearing time. A reduction in short-circuit current takes energy out of a system but can result in an overcurrent device’s slower response to a fault. The trip curves for fuses and circuit breakers show the device clears the fault quicker when the current is higher. It is not uncommon to see systems with lower fault currents have more arc energy due to the slower clearing times of circuit breakers and fuses in that system. Reducing short-circuit current by itself is not the answer to making a safer system.

Meanwhile, an increase in system impedance has the same effect resulting in lower fault current. The faster clearing times in the instantaneous region result in reduced arc flash energy levels but can result in a lack of coordination and thus, reduced reliability.

The answer to these issues can come from integrating some of the lesser-used protective systems such as zone-selective interlocking, matched circuit breaker sets, differential protection relays and arc mitigation systems. The key to a safe and reliable system is selective coordination with the ability to differentiate a fault at the location of maintenance.

Safety improvements

Let’s now look back at NEC Section 110 and determine what improvements can be made in a typical design to make a facility safer to maintain. The working space around electrical equipment has not changed in the NEC even though maintenance safety requirements have within NFPA 70E. For equipment that will be maintained while energized, consider the following design elements:

• Increase working space where energized equipment is maintained to 24 inches beyond what is code required. This will allow for more space for the electrician while wearing arc flash suits and handling test equipment and tools.
• Provide additional protection of pathways away from equipment that has energy levels that require arc flash suits to maintain while energized. A clear pathway should be provided that doesn’t require the electrician to navigate past additional energized equipment on their way to exit the space.
• Provide remote breaker racking and remote breaker closing apparatus to allow the electrician to be outside the arc boundary when conducting these operations.
• Provide infrared viewing windows on the enclosure at all cable and bus terminations. This will allow for most inspections to be conducted with the enclosures intact.
• Provide two paths to critical loads. Take a play from the mission critical design book and provide a second means to supply a load, even if it just allows for a temporary connection. Adding a second (unterminated) main breaker in critical panelboards gives the electrician to back-feed a panel to de-energize its source and work more safely on upstream equipment while keeping downstream loads energized.
• Use bypass isolation transfer switches. Allow for the switch to be maintained without losing the load.
• Incorporate UL 1558 switchgear or Level III UL 891 switchboards using isolated buswork and individual branch feeder termination compartments. This allows maintenance on feeders without exposing the electrician to the horizontal bus and exposed terminations on adjacent feeder compartments. Circuit breakers in this equipment would be capable of being withdrawn for maintenance.
• Specify arc-resistant switchgear where appropriate. This will most likely be in systems that are continuously being maintained while energized and high reliability is required.

Engineers and designers working to develop the facilities of tomorrow have many options to meet code minimum requirements and are under constant pressure to design only to that minimum requirement. The owners of these facilities are working with reduced capital and shorter schedules to get these facilities opened and producing revenue. Electricians are under more restrictions than ever to work safely in these facilities where codes are more focused on occupant safety and reliability than the needs of the system maintainer.

The engineer can provide a safe and reliable facility when the goals of all parties are in alignment when it comes to how the facility is to operate after Day One.

Do you have experience and expertise with the topics mentioned in this content? You should consider contributing to our WTWH Media editorial team and getting the recognition you and your company deserve. Click here to start this process.