Safety Considerations

The method of which to analyze arc flash hazards has evolved through several iterations of NFPA 70E, NPFA 70, and IEEE 1584 standards which modified the calculation methods to provide more accurate data based on testing, as well as UL 2986. Analysis of hazards has also evolved to consider both the actual arc flash hazard and the impeding risk or chance of an occurrence. Manufacturer design and engineered controls have also taken into consideration these changes. Energy reducing methods are also now required for certain applications.

Electrical arcs form when a medium that is normally an insulator, such as air, is subjected to an electric field strong enough to cause it to become ionized. This ionization causes the medium to become a conductor which can carry current. The phenomenon of electrical arcing is as old as the world itself. Lightning is a natural form of electrical arcing. Man-made electrical arcs exist in devices such as arc furnaces. However, utilization of electrical energy invariably requires equipment where unintentional arcing between conductors becomes a possibility.

Electric arcs in equipment liberate large amounts of uncontrolled energy in the form of intense heat and light. Unintentional arcing in power equipment can impose several different types of hazards:

• Heat from arc can cause severe flash burns many feet away (temperatures can reach 20,000 K, four times the temperature at the surface of the sun.).

• Byproducts from the arc, such as molten metal spatter, can cause severe injury.

• Pressure wave effects caused by the rapid expansion of air and vaporization of metal can distort enclosures and cause doors and cover panels to be ejected with severe force, injuring personnel.

• Sound levels can damage hearing.

Example of Arcing Damage to Equipment gives an indication of the amount of uncontrolled energy an arc can contain, as seen by the amount of damage to the equipment shown.

Electrical safety has traditionally been concerned only with electric shock hazards. The recognition of arc flash hazards began formally in 1981 with a paper “The Other Electrical Hazard: Arc Blast Burns” Lee, R., “The Other Electrical Hazard: Electrical Arc Blast Burns,”, vol. 1A-18, no. 3, May/June 1982.*by Ralph Lee, presented at the 1981 IEEE IAS Annual Meeting. This paper established theoretical modeling for the heat energy incident upon a surface a given distance from the arc. Subsequent developments followed over the next 20 years, including testing to develop more accurate empirical calculation methods and to evaluate protective clothing.

At the time of publication, there are two basic standards which establish requirements for arc flash hazards. The first is NFPA 70E, Standard for Electrical Safety in the Workplace NFPA 70E, The National Fire Protection Association, 2021 Edition.*, which defines the basic practices to be followed for electrical safety, including protective clothing levels which must be worn for given levels of arc flash incident energy and what steps must be taken prior to live work on electrical equipment. The second is the IEEE Guide for Performing Arc-Flash Hazard Calculations, IEEE 1584-2018 IEEE 1584-2018, September 2018.* which gives the engineer the methods for calculating the severity of arc flash incident energy levels. The NEC The National Electrical Code, NFPA 70, The National Fire Protection Association, Inc., 2020 Edition.* requires only that certain equipment (switchboards, panelboards, industrial control panels, meter socket enclosures, and motor control centers in other than dwelling occupancies and likely to require examination, adjustment, servicing, or maintenance while live) be field marked to warn qualified persons of potential electric arc flash hazards.

NFPA 70E Standard for Electrical Safety in the Workplace NFPA 70E, The National Fire Protection Association, 2021 Edition.*is divided into four chapters: Safety Related Work Practices (Chapter 1), Safety Related Maintenance Requirements (Chapter 2), Safety Requirements for Special Equipment (Chapter 3), and Installation Safety Requirements (Chapter 4). The discussion here is centered upon Chapter 1.

Several terms are of particular importance when discussing arc flash hazards (see Standard for Electrical Safety in the Workplace NFPA 70E, The National Fire Protection Association, 2021 Edition.*):

Flash Hazard: A dangerous condition associated with the release of energy caused by an electric arc.

Incident Energy: The amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. One of the units used to measure incident is calories per square centimeter (cal/cm²).

Flash Hazard Analysis: A study investigating a worker’s potential exposure to arc-flash energy, conducted for the purpose of injury prevention and the determination of safe work practices and appropriate levels of PPE.

Live Parts: Energized conductive components.

Exposed (as applied to live parts): Capable of being inadvertently touched or approached nearer than a safe distance by a person. It is applied to parts that are not suitably guarded, isolated, or insulated.

Shock Hazard: A dangerous condition associated with the possible release of energy caused by contact or approach to live parts.

Flash Protection Boundary: An approach limit at a distance from exposed live parts within which a person could receive a second degree burn if an electrical arc flash were to occur.

Limited Approach Boundary: An approach limit at a distance from an exposed live part within which a shock hazard exists.

Restricted Approach Boundary: An approach limit at a distance from an exposed live part within which there is an increased risk of shock, due to electrical arc over combined with inadvertent movement, for personnel working in close proximity to the live part.

Qualified Person: One who has skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training on the hazards involved.

Working on (live parts): Intentionally coming in contact with live parts with the hands, feet, or other body parts, with tools, probes, or with test equipment, regardless of the personal protective equipment a person is wearing. There are two categories of “working on”: Diagnostic (testing) is taking readings or measurements of electrical equipment, circuit parts with approved test equipment that does not require making any physical change to the electric equipment, conductors, or circuit parts. Repair is any physical alteration of electrical equipment, conductors, or circuit parts.

Working near (live parts): Any activity inside the Limited Approach Boundary.

Electrically Safe Work Condition: A state in which the conductor or circuit part to be worked on or near has been disconnected from energized parts, locked/tagged in accordance with established standards, tested to ensure the absence of voltage, and grounded if determined necessary.

NFPA 70E The National Fire Protection Association, 2021 Edition.* Chapter 1 covers personnel responsibilities (both the employer and the worker have specific responsibilities for safety), training requirements, the establishment of an electrical safety program, and the establishment of an electrically safe working condition. These are not discussed in detail here, but the reader is strongly encouraged to refer to the NFPA 70E Standard for Electrical Safety in the Workplace NFPA 70E, The National Fire Protection Association, 2021 Edition.* to become more familiar with them as they are important topics.

For arc flash hazard considerations, the focus is on Article 130, “Working On or Near Live Parts”. The basic requirement is that live parts over 50 V to ground to which an employee might be exposed should be put into an electrically safe work condition prior to working on or near them, unless the employer can demonstrate that de-energizing introduces additional or increased hazards or is infeasible due to equipment design or operational limitations. In this case live work requires an Energized Electrical Work Permit, for which the requirements are given in Article 130.2. Some exemptions are given to the requirement for an electrical work permit, such as testing, troubleshooting, performed by qualified persons.

The approach boundaries to live parts are defined above, and are illustrated in Approach Boundaries. These form a series of boundaries from exposed, energized electrical conductor(s) or circuit part(s). The requirements for crossing these become increasingly restrictive as the worker moves closer to the exposed live part(s). The limited, restricted, and prohibited approach boundaries are shock protection boundaries and are defined in NFPA 70E table 130.4(E)(a) Standard for Electrical Safety in the Workplace NFPA 70E, The National Fire Protection Association, 2021 Edition.*. Qualified persons can approach live parts 50 V or higher up to the restricted approach boundary, and can only cross this boundary if they are insulated or guarded and no uninsulated part of the body crosses the prohibited approach boundary, if the person is insulated from any other conductive object, or if the live part is insulated from the person and from any other conductive objects at a different potential. Unqualified persons must stay outside the limited approach boundary unless they are escorted by a qualified person. Unqualified persons cannot cross the restricted approach boundary.

An Arc Flash Risk Assessment must be performed to protect personnel from the possibility of injury due to arc flash. This can be done by identifying the arc flash hazard and using the table found in 130.5(C) to estimate the likelihood of occurrence of an arc flash incident, and to determine if any additional protective measures are required, including the use of PPE. This is called the “Arc Flash PPE Category Method”. Using this method is only permissible if equipment is in a proper state as recommended by the manufacture. The second method permissible for determine the appropriate PPE is by performing an 130.5(G) Incident Energy Analysis Method.

Arc flash boundary is covered in 130.5(E) which can be determined two ways:

• The arc flash boundary shall be the distance at which the incident energy equals 1.2 cal/cm² or

• using the Table 130.7(C)(15)(a) or Table 130.7(C)(15)(b).

IEEE 1584 IEEE Guide for Performing Arc Flash Hazard Calculations, IEEE 1584-2018, September 2018.* is the guide for determining arc flash incident energy levels and protection boundaries. It contains an empirical calculation method based upon extensive test results using a Design-of-Experiments (DOE) method, resulting in a 95% confidence level. In situations where the empirical method does not apply, the “Lee” method from Lee, R., “The Other Electrical Hazard: Electrical Arc Blast Burns,” IEEE Transactions on Industry Applications, vol. 1A-18, no. 3, May/June 1982.* is recommended, and is described in IEEE 1584. IEEE 1584 only considers the heat of an arc, and not the secondary effects such as molten metal spatter and pressure-wave effects.

IEEE 1584 Empirical Method

This method is valid for the following systems with the following characteristics:

• Voltages in the range of 208 V–15 kV, three-phase

• Frequencies of 50 Hz or 60 Hz

• Bolted fault current in the range:

  • LV: 500–106 kA

  • MV: 200–65 kA

• Grounding of all types, not a variable in 2018 calculations

• Standard box per voltage level; Max dimension “49”

• Gaps between conductors:

  • LV: 6.35–76.2 mm

  • MV: 19.05–254 mm

• Working distance >12 in.

• Electrode configurations:

  • VCB: Vertical electrodes in a cubic box enclosure (equivalent to 2002 in box)

  • VCBB: VCB with electrodes terminating in an insulating barrier

  • HCB: Horizontal electrodes in a cubic box enclosure

  • VOA: Vertical open-air (equivalent to 2002 open air)

  • HOA: Horizontal open-air

• Enclosure Size Correction Factor: enclosure correction factor de-rates incident energy for larger-than-standard box sizes.

Steps for performing calculations:

1. Determine electrode configuration.

2. Arcing current calculation.

   • I_bf is the bolted fault current for three-phase faults (symmetrical rms) (kA)

   • I_{arc 600} is the average rms arcing current at V_oc = 600 V (kA)

   • I_{arc 2700} is the average rms arcing current at V_oc = 2700 V (kA)

   • I_{arc 14300} is the average rms arcing current at V_oc = 14300 V (kA)

   • G is the gap distance between the electrodes (mm)

   • k1 to k 10 are the coefficients provided in Table 5 of IEEE1584–2018

   • lg is log₁₀

3. Determine clearing time based on the arcing current in Step 1. The use of time current curves (TCC) may be used for this step. Ensure the clearing time considers this such as the condition of equipment, alternate fault sources, or time delays in the control circuit.

4. Determine the Incident Energy.

   E≤600 = 12.552/50(Tx10){k1+ k2lgG + (k3I arc 600/k4Ibf7 + k5Ibf6 + k6Ibf5 + k7Ibf4 + k8Ibf3 + k9Ibf2 + k10Ibf) + k11lgIbf + k12lgD + k13lgIarc + lg(1/CF)]

   (8–1)

5. Determine the Arc Flash Boundary.

   AFB≤600 = {k1+ k2lgG + (k3I arc 600/k4Ibf7 + k5Ibf6 + k6Ibf5 + k7Ibf4 + k8Ibf3 + k9Ibf2 + k10Ibf) + k11lgIbf + k13lgIarc + lg(1/CF)] - lg(20/T)]/-k12

   (8–2)

Arcing Current Variation: First pass in the calculations is done with 100% arcing current, as used in Step 2. To ensure the worst case is used, the following formulas are used to provide minimum arc rating. Use the worst case incident energy.

The incident energy is proportional to the arcing time, which is set by the overcurrent protective device time-current characteristic and the arcing current level. Because overcurrent protective device tripping times are lower for larger currents due to inverse time-current characteristics, this is an important point. Larger bolted fault currents lead to larger predicted arcing fault currents, which lead to generally lower values of arc flash incident energy. Lower bolted fault currents lead smaller predicted arcing fault currents, which lead to generally higher values of incident energy.

Simplified Device Equations

Further testing was performed for circuit breakers and current-limiting fuses, and simplified equations of the form (A+B_log⁡I_bf) were developed. These are given in IEEE Guide for Performing Arc Flash Hazard Calculations IEEE 1584-2018, September 2018.*. The equations for fuses are applicable within the bolted fault current ranges given in IEEE Guide for Performing Arc Flash Hazard Calculations IEEE 1584-2018, September 2018.*. The equations for circuit breakers yield conservative results and should only be used when they are within the ranges of applicability given in IEEE Guide for Performing Arc Flash Hazard Calculations IEEE 1584-2018, September 2018.* and where nothing else about a particular circuit breaker is known.

Manufacturers also publish device-specific equations for certain devices, such as fuses and some high-performance circuit breakers. These are preferred versus the IEEE 1584 Empirical Method since they more accurately model the arc-flash performance of a given device.

Arc Flash Calculations

The following guidelines are helpful when performing arc flash calculations (see The National Electrical Code, NFPA 70, The National Fire Protection Association, Inc., 2020 Edition.*):

• When choosing a calculation method, be sure the system conditions fall into the calculation method’s range of applicability.

• Use the newest methods given in IEEE 1584-2018. Older methods given in previously published papers are superseded by this standard.

• If the manufacturer publishes device-specific equations, use them.

• Use realistic fault current values. The actual minimum available fault current, rather than the worst-case values typically used for short-circuit analysis, give more conservative (and realistic) results.

• Consider the effects of arc fault propagation to the line side of the main overcurrent device when determining which device to use to calculate the arcing time. For example, for the electrical panel in Example Electrical Panel, device A would be used rather than device B for calculating the arcing time for a fault on the panelboard bus, since the fault can propagate to the line side of device B. Make similar considerations for switchboards, MCC’s.

  Example Electrical Panel

  

• Quantify the variables. The working distance, bus gap, equipment configuration, and system grounding are all dependent upon the particular installation and must be accurately determined.

• Be aware of motor contribution. Motor contribution can both increase and decrease the arc flash incident energy, depending upon where in the system the arcing fault occurs.

• Use a computer for analysis. This is the most efficient way to accurately calculate the incident energies and flash protection boundaries where multiple sources, such as generation and motor contribution, must be taken into account. Several commercial software packages are available for arc flash hazard analysis. Be aware, though, what the user-configurable options for the software are and be sure they are set correctly for accurate results.

System Design

Arc flash hazard analysis is typically performed after the system design process, including the time-coordination study, is complete. This can result in the need for “tweaking” of overcurrent protective device settings to obtain acceptable arc flash results or, in the worst case, system re-design with additional equipment. The following guidelines, if observed during the system design phase, can serve to minimize the need for such activities:

• Use a dedicated main overcurrent device at transformer secondaries. The secondary of a transformer is one of the most difficult places to achieve acceptable arc flash hazard levels. If multiple mains are used for transformer secondaries, the arc flash hazard level downstream from the main but ahead of the feeders must be calculated using the transformer primary device timing characteristics, significantly increasing the incident energy. If the secondary main and feeders are in the same switchboard or panel, this is usually not be applicable due to arc fault propagation to the line side of the main device as described above. For ANSI low-voltage switchgear per ANSI C37.20.1, however, this can be of real benefit, as well as in cases where the secondary overcurrent device is remote from the feeders.

• Closely coordinate devices where possible. The lower the clearing time for the predicted arcing current, the lower the arc flash incident energy.

• Use high-performance devices, such as low-arc-flash circuit breakers, where possible. These significantly reduces the arc flash incident energy.

• Use bus differential protection and/or zone selective interlocking where possible. This is high-speed protection that can significantly lower the arc flash incident energy.

Another code required can be found in NFPA 70, article 240.87 The National Electrical Code, NFPA 70, The National Fire Protection Association, Inc., 2020 Edition.*. Where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted is 1200 A or higher, one of the following means shall be provided and be set to operate at less than the available arcing current:

• Zone-selective interlocking.

• Differential relaying.

• Energy-reducing maintenance switching with local status indicator.

• Energy-reducing active arc flash mitigation system.

• An instantaneous trip setting. Temporary adjustment of the instantaneous trip setting to achieve arc energy reduction shall not be permitted.

• An instantaneous override function.

• An approved equivalent means.

Arc Flash Avoidance: Help personnel avoid the hazards or add distance between hazard and operator.

NFPA 70E article 205.32 states that a single-line diagram, where provided for the electrical system, shall be maintained in a legible condition and shall be kept current. If utilizing the incident energy analysis method of determine the arc flash hazard, the analysis must be reevaluated when changes occur in the electrical system that could affect the results of the analysis and reevaluated at intervals not to exceed five years, per NFPA 70E article, 130.5(G). An effective method of accomplishing this is to have a study performed and maintained to be kept current. The concept of a digital twin can accomplish this.

The general approach of a digital twin model is a virtual representation of a distribution system that can be as simple as a single-line diagram with relevant data to complex, updated real-time data, via digital readings through networked metering and communication. Digital twins can create a foundation for the customer and clients to support reliability assessment, asset management, real-time interfaces for SCADA systems, and system studies.