How to maximize the benefits of arc flash hazard analysis

 

Arc flash hazard analysis insights

• Arc flash hazard analysis requires more than just calculating bolted fault current.
• Arc flash analysis calculates incident energy and arc flash boundaries.
• Proper electrical system design, construction of arc resistant equipment and requirements for safe work practices help minimize the risk of electrical arc flash.

---

Many employers are one fatality away from compliance with current standards. The question is, “which employee do you want to sacrifice?”

Treatment costs for burn victims can approach $120,000 per month, according to the National Institutes of Health. A workplace fatality can cost an estimated $1.39 million, according to the National Safety Council. Cost should never be a consideration when it comes to workplace safety — especially for the hazards that electrical workers face. Electrical workers rarely return to their previous job assignment following an arc flash catastrophe — they are either killed, maimed or their quality of life is significantly impaired.

Potential electrical hazards include shock or electrocution, arc flash and arc blast. An electrical arc occurs when insulating materials can no longer contain the applied voltage. A short circuit or insulation breakdown creates a bypass, which can be from phase-to-phase, phase-to-ground or a combination thereof, around a circuit. The heat generated by the high current flow melts or vaporizes the conducting material, creating an arc. The resulting arc flash produces a brilliant flash, intense heat and a fast-moving pressure wave that propels the arcing materials.

As electrical demand increases, transformers at utility and industrial levels are upgraded or replaced with those having higher kVA ratings and lower impedances. Transformers are operated in parallel to satisfy requirements for higher system reliability. These modifications can cause dramatic increases in available fault current, resulting in more electrical energy available.

The downside to these system changes is an increase in the electrical current that could feed a fault within existing equipment. This can wreak havoc on underrated or improperly maintained equipment.

Most employers and employees understand electrical shock hazard. Few understand electrical arc flash hazard — let alone how to perform an analysis.

An arc flash hazard analysis is an extension of a power system analysis. Companies once had electrical facilities engineers dedicated to keeping the electrical system up-to-date. But with budget cuts and downsizing, this function has virtually vanished.

Necessity of arc flash hazard analysis

An electrical arc could happen at any time — when no one is around, when someone is walking in proximity or when someone is working on the equipment. When someone is working on or near energized equipment, they face the most hazardous situation. The equipment doors may be open, placing workers close to electrical components, conductors and connections.

An electrical arc can form when an electrical worker makes contact between phases or from phase to ground with a conductive object such as a screwdriver, pliers or body parts while working inside an energized electrical panel. The temperature of the arc is intense enough to produce radiation burns, which could result in long-term internal bodily damage. The explosive energy released by this electrical arc creates a pressure wave. When this wave comes into contact with a surface, which could be a person, it is called incident energy.

Arc flash safety standards

Organizations such as the Occupational Safety and Health Administration (OSHA), National Fire Protection Association (NFPA), ASTM International (ASTM), and Institute of Electrical and Electronics Engineers (IEEE) aspire to protect electrical workers from electrical hazards through training, proper equipment maintenance, proper use of tools and protective equipment and sound engineering methods for design and analysis of electrical systems.

While NPFA 70E provides equations and a quick table to determine the level of personal protective equipment (PPE) necessary to keep a worker safe when performing a task, it is based on a specific system configuration. Every electrical distribution path in every plant is different. Each component in each of these paths is a variable that must be considered when evaluating potential arc flash hazards. An arc flash hazard analysis is more than recommended — it is urged because of the likelihood that your specific system configuration is quite different from the configuration used to develop the table, which should be used as a guideline — not a safety net.

The equations in IEEE Standard 1584 are based on significant high-current testing to determine incident energy. From this incident energy, one can accurately determine the proper PPE for the task to be performed. Refer to the standard for the equations and how to use them.

Calculation differences

For starters, one can calculate the 3-phase bolted fault current on the low side of a transformer feeding a switchgear line, for example. First, use a worst-case scenario by assuming an infinite bus, which assumes that the impedance ahead of a device is essentially zero, to provide maximum fault current. Second, assume a fault-clearing time of approximately 0.2 seconds and a working distance of 18 inches. Based on these data, one may think they are calculating the worst-case scenario for incident energy, which typically is expressed in calories per centimeter squared (cal/cm2).

Calculations using 3-phase bolted fault current values based on an infinite bus may indicate a faster time/current response from protective devices. However, the results are lower calculated incident energy and arc flash boundary values, which could give an electrical worker a false sense of protection, when actually he or she is not adequately protected.

When using current-limiting fuses, it is important to calculate a realistic fault-current value based on actual system impedance. A current-limiting fuse interrupts all available current above its threshold current and below its maximum interrupting rating, and limits the clearing time to equal to or less than half cycle at rated voltage. Proper maintenance and coordination of protective devices is imperative when doing an arc flash hazard analysis.

For example, a 1000 kVA 13.8/0.480 kV (which steps 13.8 kV down to 480 V) transformer with 6% impedance should supply approximately 20 kA bolted fault current at 480 V, assuming an infinite bus. With a clearing time of 0.11 seconds, the incident energy at 18 in. is approximately 4.4 cal/cm2.

However, after a thorough system analysis, the fault current is closer to 10 kA because of system impedance. The clearing time is actually 2.5 seconds because this lower fault current falls into the long-time pickup range of the upstream protective device. At a distance of 18 inches, the actual incident energy is 53.2 cal/cm2. A person working on this switchgear would most likely receive life-threatening external and internal burns as well as broken bones from the arc blast. A very serious burn can occur without ever making physical contact with the energized equipment.

How to calculate incident energy accurately

Arc flash hazard analysis requires more than just calculating bolted fault current based on an infinite bus using a generalized table or a calculator on the internet. The analysis starts with gathering up-to-date equipment information, then performing a detailed analysis comprised of load flow, short circuit, and protective device coordination studies as well as equipment evaluations to determine if the current-withstand rating is acceptable. For facilities with generators and large motors (100 horsepower or larger) a motor starting and fault-contribution analysis should be performed also.

Experienced in-house personnel using proper procedures and methodology can perform an arc flash hazard analysis. However, the process described below assumes an independent electrical consultant will carry out these tasks.

Site assessment and data gathering

Data gathered during the initial site visit and the site overview are critical elements in performing a safe and realistic arc flash hazard analysis. The system one-line diagram, supporting electrical system schematics and pertinent documents should be checked and updated during this visit. Data gathering consists of acquiring the nameplate data of all electrically powered equipment, protective device settings and load information. Source impedance data from the utility are required to accurately calculate the short circuit current. Upgrades planned within the next few years should be noted as they could affect the analyses.

Short-circuit analysis

The site data are used to build a system model, which enables the short-circuit analysis to be performed. Short-circuit studies determine the magnitude of current flowing through each section of the power system at various time intervals after a fault. Next, the current magnitude data are used to determine the 3-phase bolted short-circuit current, which is used to calculate the arc fault current.

Protective device coordination analysis

Protective devices, such as fuses, circuit breakers, and relays, have curves that are plotted on a log graph that shows current with respect to time. Protective device coordination requires setting the devices according to these curves. This way,  when a fault occurs, the upstream protective device closest to the fault opens as rapidly as possible to minimize risks to people and equipment, as well as to isolate the problem with minimum disruption to the rest of the plant’s electrical system.

The equipment and protective device data used to build the system model are used for protective device coordination analysis also. When making changes or upgrades to plant electrical systems, you should revisit the existing protection scheme to ensure that devices are coordinated properly. A change in load or equipment could change the timing and coordination of the protective devices.

Arc flash hazard analysis

Arc flash analysis calculates the incident energy and arc flash boundary for each location in a power system. Trip times from protective device settings and arcing fault current values from the short-circuit analyses are used in the arc flash hazard analysis. Incident energy and arc flash boundaries are calculated following the IEEE 1584 standard. Clothing or PPE requirements are specified for given tasks. You can print required arc flash hazard warning labels on adhesive labels and place them on equipment.

Equipment evaluation analysis

The equipment evaluation analysis compares equipment withstand ratings with calculated operating and short-circuit analyses. This is very important when upgrading electrical facilities — especially when increasing available power or adding/replacing transformers, motors or generators. Systems may be operated in paralleled to increase reliability. But this has a significant impact on the available short-circuit current to a downstream device. Circuit breakers and other devices that are underrated for fault-current withstand pose a serious arc and blast hazard to anyone close to the device.

If the completed arc flash analysis identifies hazards and noncompliance, your plant’s electrical system will require engineering changes to reduce potentially high incident energy levels. Only a complete electrical system analysis can identify the level of PPE required at each location in the system.

Preventing arc flash hazards

Obviously, the best way to prevent an arc flash hazard is to de-energize the equipment. However, even if you can totally de-energize the equipment, you still must open devices upstream. It is best if this can be done remotely. If remote operation is not possible, you must be trained and know the proper arc flash protection required for the given task.

Proper procedures for lockout/tagout call for testing for zero voltage and applying grounds. This testing also requires proper training and PPE. There is no substitution for training and following best practices in electrical work.

An arc flash hazard analysis provides you with the knowledge required to keep electrical workers out of harm’s way.

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.