Assessing Arc Flash Hazards

There are approximately 20,000 arc flash hospitalizations every year in this country (IEEE).  These incidences often cause debilitating burns which are extremely painful and require extended hospital stays often with profoundly negative affects on the lives of the worker’s and their families.  Many of these injuries may be avoided or greatly reduced if proper assessments and work methods are utilized.

What is an Arc Flash?

An arc flash is an uncontrolled and unplanned electrical event that develops heat up to 35,000° F.  As a comparison, the surface of the sun is about 10,000°F.  This level of heat will instantaneously melt hardened steel.  The severity of an arc flash incident depends on what the available fault current is, the clearing time from the breaker of fuse (generally .01 – .2 seconds) and the impedance of the arc.  The heat generated from the arc flash incident energy is measured in calories/cm².  The distance an arc flash reaches and dissipates to a safe level from the point of generation is called the arc flash boundary.   This boundary is the point where the arc flash dissipates to 1.2 calories/cm² which is the heat level that will cause a second degree burn.  Some arc flash incidences are unavoidable but may be predictable.  Injuries from arc flashes incidences can be avoided if proper assessment, work methods, mitigation techniques, and personal protective equipment (PPE) are used. 

Why do we need to assess arc flash hazards?

OSHA requires that employers perform a hazard assessment to determine methods to protect workers and the level of PPE required if necessary.  The OSHA regulations associated with this requirement are in the general industry regulations in Subpart I and Subpart S.

OSHA 1910.132 The employer shall assess the workplace to determine if hazards are present, or are likely to be present, which necessitate the use of personal protective equipment (PPE).

OSHA 1910.335(a)(1)(i) Employees working in areas where there are potential electrical hazards shall be provided with, and shall use, electrical protective equipment that is appropriate for the specific parts of the body to be protected and for the work to be performed.

The OSHA requirements are vague and explain what is required but not how to accomplish it.   To determine how to comply, the standards referenced are the National Electric Code (NFPA 70) and Standard for Electrical Safety in the Workplace (NFPA 70E) and for voltages greater than 600 volts, National Electric Safety Code (IEEE C2).

What are methods of assessing arc flash hazards?

There are several ways to assess the arc flash hazards including using the NFPA 70E tables, using NFPA 70E Annex D to calculate arc flash boundaries and incident energy levels at the working distance of 18” (for low voltage) or doing an engineering study.   We will examine the pros and cons of each method.

1. Using the Hazard Risk Category (HRC) and PPE NFPA 70E tables is the most inexpensive method to determine PPE for workers.  These tables may only be used if the equipment worked on has a lower available fault current than stated each category of equipment listed.  As an example to remove an individual starter bucket from a 600 V class motor control center, the available fault current must be less than 42,000 amps and the maximum fault clearing time is 0.33 seconds.  This requires some level of assessment to determine this.  Some companies with low fault current utilize this table and use the Simplified Two Category Arc Rated Clothing table in Annex H.  The drawback to this method is often the PPE requirements exceed the arc flash hazard which discourages workers from using it and in some cases, especially for extremely low fault current conditions coupled with long protective device clearing times, the arc flash level exceeds the recommended PPE listed in the HRC table.  Another downside is the arc flash boundary is unknown.

2. Using the calculations in NFPA 70E Annex D.5 is a bit more accurate than using the tables alone and provides the arc flash boundary.  This will most likely provide adequate PPE but usually requires an engineer or a high level electrician to perform the calculations.  Limitations are that it is applicable for systems 600 volts and less with short circuit current between 16,000 and 50,000 amps..

3. An engineering study is based on IEEE Guide for Performing Arc-Flash Hazard Calculations (IEEE Std 1584) in most cases and provides the most accurate assessment.  The drawback is the expense.  Every part of the electrical infrastructure must be collected on-site including transformer data, wire and conduit sizes, types and length;  breaker and fuse make, model, rating, settings; panel make, model, ratings; bus duct make, model, rating and length; and any other data related to generators, MCC’s motors, etc.  Once collected by an electrician, the data is sent to an engineer who enters it into the software to create the model of the electrical system and perform the analysis of the system.  The benefit of doing this level of assessment is the inclusion of short circuit and coordination studies.  The short circuit study will determine the available fault current at each device studied.  All panels and electrical equipment have a fault current rating and if the level is exceeded, the breakers may not operate and cause an arc flash.  The coordination study is important for the reliability of the service and often justifies the cost of the study.  As an example, this will determine if a fault in a control panel operates the breaker feeding it or another breaker upstream which may take the entire plant down rather than just at that equipment.  This coordination study can also be used to identify changes to protective devices that could reduce the existing arc flash hazard.  Other benefits include the development of a one-line diagram of the electrical system and labels stating the arc flash boundary and PPE requirements at the working distance (18” for lower than 600 volts) as well as the shock hazards, approach boundaries and associated PPE requirements.

Can we prevent arc flash hazards?

Arc flashes are caused by an unplanned event that is related to failed equipment or foreign objects causing a short circuit between a grounding system and an ungrounded energized part, or between two or three ungrounded phases.  This may include switches or breakers not operating properly, non-insulated tools placed or left across electrical parts, grounds left in place or other objects like mice, bolts, cobwebs, combustible dust, or other conductive apparatus inadvertently falling into electrical parts.  Foremost is recognizing that there may be a hazard.  This requires that a trained worker capable of understanding the construction and operation of the equipment perform an assessment before starting work on the equipment.  

Proper maintenance is also an important component to prevent these unplanned events.  Recommended Practice for Electrical Equipment Maintenance (NFPA 70B) is a good guide for maintaining the electrical infrastructure.  An arc flash analysis is based on breakers operating within the perimeters designed.  If maintenance is not done the breakers may operate in a longer duration than designed which may cause the arc flash hazard to be greater than stated on the label, or the breaker may catastrophically fail.  Even if no worker is injured this could cause an extended outage and downtime for plant operations, sometimes weeks depending on the damage and location of the flash.

How can we mitigate or reduce the arc flash hazards.

A full engineering study will state methods to reduce the arc flash hazard.  The incident energy level from an arc flash is directly proportional to clearing time of the protective device, so recommendations may include changing breaker settings, installing different or new upstream fuses or installing arc flash relays.  These changes will all lower the clearing time.  Also, replacing transformers with lower impedance may lower the arc flash level, however consideration should be given to increased fault current creating other problems with overdutied panels.

Other methods to mitigate the exposure to employees include extending the distance from the exposed or operating parts by extending operating rods, or remote switching devices.

Lastly, PPE is required to protect employees from hazardous arc flash levels.  Protective clothing systems are based on the arc flash hazard level in cal/cm² at working distance of 18” of the exposed part.    If the arc flash PPE is at a proper level, the employee should not receive a second degree burn.  For hazards assessed at greater than 40 cal/cm² equipment must be de-energized.  The arc blast at higher levels may break bones so a suit rated higher would not protect from this injury.

Summary

Arc flash events cause serious damage to equipment instantaneously and where a worker is exposed may inflict an injury that is life changing.  They are not always avoidable but may be greatly reduced by reducing clearing time, training employees to assess the hazards before starting each job, de-energizing unless infeasible, and ensuring they use the proper PPE as dictated by the arc flash analysis performed by the company.