Four areas that affect electrical safety in electrical, power systems
Here are four facets that impact electrical safety for facility staff and occupants: arc flash, electrical room working space, testing and commissioning and egress lighting
- Understand the key issues associated with arc flash and electrical safety in general.
- Appreciate the importance of electrical room design and safe working space.
- Learn the basics of safe lighting design for both maintenance and egress.
Electrical safety insights
- Electrical safety requires several areas of expertise: System design, arc flash mitigation, room design, testing and commissioning and egress lighting.
- Mitigating arc flash incidents often becomes the No. 1 challenge and adherence to codes and standards ensures proper design.
The topic of electrical safety is an extensive one. Various codes including NFPA 70: National Electrical Code (NEC), NFPA 101: Life Safety Code, the National Electrical Safety Code NESC IEEE/ANSI Standard C2, NFPA 70E: Standard for Electrical Safety in the Workplace and other codes cover a wide variety of electrical safety topics.
Arc flash and electrical safety
One of the fastest growing concerns over the past decade in electrical safety has been arc flash hazards. While the requirement for arc flash hazard warning labels on electrical equipment has been in the NEC as early as 2002, the 2017 version added in a requirement to include the actual available fault and clearing time indicating the possible energy levels at the equipment.
An arc flash is an electrical explosion made of heat and light that occurs when electrical current escapes the normally current carrying components and “arcs” to another voltage reference or ground. Arc flashes are traditionally caused when a fault or a breakdown of electrical insulators occurs in the electrical system in a line to ground fault. The causes for arc flash can include damage to the conductors during installation, age, overvoltages, transients and insufficient testing.
This burst of energy can be tremendous with heat up to 35,000°F, vaporizing metal, causing damage to nearby equipment, fires and even death to nearby occupants. Due to the large hazard that arc flash presents, it is required by NFPA 70E to provide arc flash hazard warning labels for any piece of electrical equipment that may need maintenance or service while energized.
The labels provide guidance for electrical service personnel to use levels of personal protective equipment (PPE) based on calculated incident energy measured in cal/cm2. The standards PPE category includes from the least protective clothing at 0 to the most protective at 4. See Table 1 for classification of PPE levels based on incident energy.
Table 1: Arc flash categories
|Arc flash range cal/cm2
Table 1: Arc flash energy categories are shown. Courtesy: SmithGroup
By understanding the components that make up an arc flash, an electrical engineer can use various methods to lessen or mitigate an arc flash upon a fault condition. Reference the IEEE 1584-2018: Guide for Performing Arc-Flash Hazard Calculations for a full purview of the full methods that are used when calculating arc flash.
Simply put, the incident energy of an arc flash is a calculation of fault current and clearing time at the overcurrent protective device. It should be noted, however, that a higher fault current may not elicit a higher incident energy due to the clearing time of a fault. A lower fault value may not trip an upstream protective device whereas a higher fault value may instantaneously trip the upstream protective device.
For this reason, a coordination study should be performed to determine how each individual circuit breaker trips under each individual fault condition (single line-to-ground, line-to-line, 3-pole, etc.).
An engineer should exhaust all possibilities to lessen the potential incident energy of an arc flash by modifying circuit breaker settings, limiting fault current when possible or adding additional protective devices within the electrical system. When the aforementioned techniques have been performed, there are various equipment components and accessories that can be procured to mitigate the potential hazard.
NEC article 240.67 and 240.87 provide guidance for arc energy reduction. Specifically, 240.87 provides methods to reduce the clearing time (breaker trip) to minimizing arc flash. These include:
- Zone-selective interlocking.
- Differential relaying.
- Energy-reducing maintenance switching.
- Energy-reducing active arc flash mitigation system.
- An instantaneous trip setting.
- An instantaneous override.
- Approved equivalent means.
In addition, the consulting engineer should consider the location of the electrical equipment as placing equipment within an electrical room normally restricts access to qualified personnel whereas equipment located within a hallway may elicit safety concerns with unqualified occupants.
With the performed arc flash analysis, labels should be created and attached to each respective piece of equipment to identify the incident energy, arc flash boundary, nominal system voltage, device name and PPE required (see Figure 1).
Safety in electrical rooms
The purpose of an electrical room is to house electrical equipment, providing a space that is both safe and secure for the operations and maintenance of the electrical equipment and only accessible to authorized personnel. Electrical equipment ratings and types can significantly affect the room requirements. Switchboards, switchgear, transformers, generators, uninterruptible power supplies (UPS) and low- and medium-voltage ratings all impact the requirements for an electrical room.
NEC Article 110, Part II (1,000 V and below) and Part III (more than 1,000 V) is the primary source of these requirements.
Coordination with architectural requirements centers on electrical room space needs including working space around and above the equipment and access to and from the electrical room. Working space is based on NEC Table 110.26(A)(1) and Table 110.31(A) from the 2020 NEC. Table 2 combines this information along with typical system voltage nomenclatures.
Table 110-16(A)(1) has three conditions. These conditions consider the distance from the accessible side of the electrical equipment enclosure to various wall construction types or other electrical equipment on the opposite side of the working space. Note that the voltages referenced in the table are from any single “hot”/“leg” as referenced to ground. So, a 208 V, 3-phase panel board is 120 V from any single leg referenced to the ground conductor (see the three conditions illustrated in Table 2).
Note that the working space discussed is this table is the depth in front of the equipment. The working space required by this article also includes width and height. The width must be 30 inches or the width of the equipment, whichever is greater. The vertical space (height) required by NEC 110.26 is 6.5 feet or the height of the equipment, whichever is greater. This height requirement means that you cannot install the associated gear in question in a sublevel space (crawl space) or even allow pipes or other accessory equipment to be installed in this imaginary “box” created by the requirement(s).
The electrical engineer also should carefully review the type of electrical equipment to be placed in an electrical room. Nonelectrical engineers commonly use the terms “switchboard” and “switchgear” interchangeably, but these two types of equipment types have very different requirements for access and clearance. Because switchboards are built to the UL 891 standard and typically come with fixed breakers, they may require only front access for cable terminations.
Larger or more specialized switchboards may require rear access for cable terminations and have optional draw-out insulated case devices. Switchgear is built to the ANSI C37 standard and has draw-out front power breakers and requires rear access. Draw-out breakers will extend into the pathway in front of the gear and require space from either overhead hoists or portable lifts to remove the breakers. A good recommendation is to show these additional space needs on the electrical drawings with dashed lines extending from the equipment footprint.
Another important requirement in NEC Article 110, Part II addresses the means of entrance and egress for an electrical room. In electrical rooms with equipment more than 6 feet wide with equipment-rated and rated 1,200 amps and above, there must be one means of egress at each end of the room. The doors must be a minimum of 2 feet wide and 6.5 feet high. The doors should swing out of the room and have panic hardware. Also note that these door requirements also apply to electrical rooms with equipment rated 800 amps or more under NEC Article 110.26 (C)(3).
Other codes and good practice may point to larger doors for egress. Beyond the code requirements for doors, the engineer and architect should consider real working space needs. For instance, the primary intent of the NEC minimum door size noted above is for egress purposes, not equipment entry or removal. Some types of electrical equipment can be 7.5 feet high and 30 inches wide or larger. Code sized doors at 2 x 6.5 feet would not permit move-in or replacement of that equipment. A good suggestion, therefore, might be 9-foot-high double doors that would accommodate these types of equipment.
Most basic switchboards require only front access and have fixed group-mounted breakers. However, some sophisticated switchboards or standard switchgear require additional front access space for draw-out breakers, and rear access to pull and terminate cables on the bus. The NEC-mandated 3 or 4 feet may not suffice to support these needs.
Further, the height of the electrical equipment and the clearance to the ceiling above should be evaluated where top entry conduit bending space may be needed; 3 feet or more may be needed above the top of the equipment. A suggested minimum height for some electrical rooms therefore might be 12 feet or more.
Lighting is another area that engineers should coordinate with the architect. The NEC does not mandate lighting levels, but a good recommendation for lighting is 30-50 footcandles (fc). It is good standard practice to consider putting a portion of the lighting in a main electrical room on battery or generator power. Note that the newer codes also state that “Control by automatic means shall not be permitted to control all illumination within the working space.”
Testing and commissioning for electrical safety
Electrical testing has become a key component in all modern electrical equipment and installations. It has expanded beyond the key foundations for equipment safety to include performance, operations and energy requirements for electrical facilities. However, its basis remains on electrical safety. Some of the key electrical testing options include:
Manufacturer factory tests: Electrical equipment manufacturers test their equipment in the factory according to established standards. However, consulting engineers may wish to specify additional “factory witness testing.” This additional witness testing permits the consulting engineer to perform preinstallation review of the equipment and request simulated functional testing performed by the manufacturer.
Witness testing is only required for large or complicated electrical equipment and for mission critical projects. Witness inspections can include confirmation of equipment dimensions and weights, proper nameplates and labels, locations and sizes of conduit openings, communication wiring points and cable lug configurations. Functional witness testing may include simulated sequences of operations such as startups and load transfers, fault conditions, load loss and alarms and display information
Manufacturer field tests: More commonly specified than factory witness tests are requirements for the electrical equipment manufacturer to require factory-trained technicians to perform additional testing and adjustments in the field during or after installation. Often larger complex electrical equipment is shipped in parts and having a factory authorized technician on-site to test equipment after the contractor has installed it can be helpful. The most common tests are functional tests and demonstration for owner staff. Additional work may include relay and protective device settings.
Contractor field tests: Field testing by the installing electrical contractor is a common specification requirement. Some of the more common contractor field tests include medium-voltage cable testing, load balancing, phase rotation and infrared scanning of terminations and connections
Third-party field testing
For some types of critical facilities such as health care, data centers, laboratories and government facilities, it is important to specify additional electrical testing by independent third-party contractors. The primary recommended standard for independent testing of the installation of electrical systems is ANSI/NETA ATS-2021 Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems.
As part of these field tests there are NEC requirement for the primary injection testing in sections 240.67 and 87 (C):
“The arc energy reduction protection system shall be performance tested by primary current injection testing or another approved method when first installed on-site. This testing shall be conducted by a qualified person(s) in accordance with the manufacturer’s instructions. A written record of this testing shall be made and shall be available to the authority having jurisdiction.”
While it is possible that some or all, of these tests could be performed by the installing contractor, there is a benefit to the consulting engineer and the owner to use a third-party testing agency who can independently assess that electrical equipment comply with the engineer’s design and specification documents and have been installed to meet all codes.
Commissioning and testing for electrical safety
Beyond testing individual electrical equipment components, there is a need to verify installed electrical systems match design documents, construction submittals, owner’s program requirements and to document functional performance testing. Electrical commissioning may be optional or required depending on the code or certifications required by the project. The commissioning authority is a critical part of the design and construction team and should be engaged early in the design and construction process.
For a facility to operate initially effectively and safely and over its life span, it is important to consider post-occupancy commissioning and testing.
There is a companion standard for post-occupancy testing called ANSI/NETA MTS 2015: Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems. According to the International Electrical Testing Association (NETA), this document “was developed for use by those responsible for the continued operation of existing electrical systems and equipment to guide them in specifying and performing the necessary tests to ensure that these systems and apparatus perform satisfactorily, minimizing downtime and maximizing life expectancy.” NETA MTS is an excellent resource for ongoing testing as part of a preventive maintenance program in a facility.
One of the more code-driven aspects of electrical design is the performance of egress lighting during both normal and emergency power situations. A consulting engineer should reference NEC Article 700; International Building Code (IBC) Chapter 10, Section 1008; and NFPA 101 Chapter 7.8 to ensure full compliance of the egress lighting system with the various mandated codes.
NEC Article 700 Part IV indicates that emergency illumination — which includes egress lighting, illuminated exit signs and all other needed luminaires that are required to provide the required illuminance along the egress pathway — must be connected to the emergency system of the electrical distribution. The source to supply power to the egress lighting must be compliant with the sources indicated in NEC Article 700.12.
Traditionally, the most common source of power supplying egress lighting is through a generator set, a lighting inverter system or batteries integral to the egress light fixtures. Other items of note as mentioned in NEC Article 700 Part IV include the requirement:
- Emergency lighting supplies automatically transfer from normal to emergency power during a normal power failure or outage.
- Emergency lighting must be designed so that a failure of a single device does not leave a room needing egress illumination in complete darkness.
NEC Article 700 Part IV details the requirements of controlling emergency lighting circuits. When using a dimmer or relay system, there must be an automatic override to the controls to select fixtures to provide the minimum emergency illumination required. This requirement is typically met with the use of a UL 924. Emergency light fixtures supplied normally by a normal branch circuit shall be allowed to switch over to an emergency branch circuit provided that the branch circuit does not exceed 20 amperes. This requirement is typically met with the use of a UL 1008 transfer switch as a branch circuit emergency lighting transfer switch.
The IBC Section 1008 details the locations where emergency illumination is required as well as the illumination levels required under both normal and emergency power. With some exceptions, egress illumination should be provided to the following spaces: Aisles, corridors, exit stairways and ramps, exit passageways, vestibules, electrical equipment rooms, fire command centers, fire pump rooms, generator rooms and public restrooms greater than 300 square feet.
It should be noted that illumination shall be provided along the path of travel from the exit discharge from each exit to the public way and the duration of the emergency power for illumination shall be not less than 90 minutes.
For illumination under normal power, the egress illumination shall not be less than 1 footcandle (fc) at the walking surface. For illumination under emergency power, the egress illumination shall not be less than an average of 1 fc with no points below 0.1 fc measured along the walking path. The emergency illumination levels shall be permitted to decline down to an average of 0.6 fc with a minimum of 0.06 fc at the end of the emergency lighting time duration of 90 minutes. In all conditions, a maximum-to-minimum illumination ratio of 40:1 shall not be exceeded.
NFPA 101 Section 7.8 also elaborates on the location requirements of egress lighting as well the required illumination levels. Most notably, NFPA 101 identifies the egress stairwells illuminance levels of a minimum illumination of at least 10 fc measured at the walking surface. The minimum illumination for floors and other walking surfaces shall be to values of at least 1 fc measured at the floor.
Walking surface requirements between the IBC and NFPA 101 differ in their requirements. It is up to the engineer of record to confirm with the authority having jurisdiction which code is applicable to the project. If the engineer is unable to receive an answer, they should refer to the more stringent of the two codes.
The electrical safety topics of arc flash mitigation, electrical rooms, testing and egress lighting are just a few key topics for consulting engineering to consider for electrical safety. Many more topics can be found in relevant codes and standards.