Reducing The Potential for an Arc Flash

By jim schuster, ceo Martin technical

Abstract

“How can a crystal or polymer optic stand up to the enormous blast-pressure, heat and molten shrapnel produced by an arc flash?” This is a question that many engineers ask when they begin researching infrared windows. Part of the reason for the question is a misconception that there is an infrared viewport standard that qualifies some infrared (IR) windows as “arc resistant IR windows.”

The fact is, there is no standard for arc resistant infrared windows, and no infrared window is capable of protecting a worker in the event of an during inspection.

This paper will explore the dangers of arc flash and the forces that the resulting arc blast produces. The reader will also gain an understanding of the considerable safety benefits, and arc flash risk control, that infrared windows provide, as well as the realistic limitations of the devices and their role in arc resistant switchgear and MCCs.

What is an arc flash?

NIOSH (National Institute for Occupational Safety and Health) definition of an arc flash:

“An arc flash is the sudden release of electrical energy through the air when a high–voltage gap exists and there is a breakdown between conductors.”2

The causes of arc flash are many, ranging from rodents, to insulation breakdown, to dust and contaminants. However the predomination of causes are human initiated and occur when the panel covers are not in place, or during panel removal or reapplication or when opening or closing equipment doors.

In less than 1/1000th of a second, the center of an arc flash can reach temperatures of 35,000℉ / 19427℃ 3 -- nearly four times the temperature of the surface of the sun (roughly 9,000℉ / 4982℃). This rapid heating causes copper bus bar to turn from solid to plasma state in a fraction of a second, expanding 67,000 times. At that rate, a pea sized piece of copper will expand to the size of a rail car.

This instantaneous expansion of machine parts and the surrounding air creates an “arc blast” carrying a pressure wave of thousands of pounds of force, super-heated gases and molten shrapnel.4 The bomb-like blast can be as powerful as three sticks of dynamite blowing up just an arm’s length from the worker. It’s not surprising that victims of arc blast trauma report horrific burns, shrapnel wounds, damaged internal organs, hearing loss, blindness and lung damage.

Arc Resistance Versus Arc Avoidance:

Every industrialized country has instituted electrical safety standards to ensure workplace safety. Most of these standards are similar to the US standard: NFPA 70E Electrical Safety in the Workplace. In fact, many, like Canada’s CSA Z462 are based in part or in whole on the NFPA 70E standard. As such, many / most of these international standards will have a large degree of focus on protecting workers from the effects of arc flash by seriously limiting the worker’s exposure to “energized electrical conductors or circuit parts” over 50 volts. Eliminating the exposure, and therefore the risk, is at the heart of the ANSI Z10 Risk Control Hierarchy (sometimes referred to as the “Hierarchy of Risk”).

Unfortunately, there is no way to completely prevent an arc flash happening in electrical distribution systems. The best one can do is to mitigate or reduce the risk.

The Risk Control Hierarchy systematically reduces risk to its lowest practicable level by prioritizing ways to mitigate a given  risk. Higher priority and weight are given to methods that seek to control risk by proactive means as close as possible to the root cause. Meanwhile lower priority is placed on reactive methods of controlling damage after an incident has occurred. Specifically, Risk Control Hierarchy ranks the most effective to least effective ways to reduce risk as follows:

1. Elimination -- remove the hazard
   1. Only work on energized equipment, when absolutely necessary. De-energizing equipment removes the arc flash hazard, although there is some risk of arc flash hazard when testing to make sure that the equipment is de-energized.
2. Substitution -- replace higher risks with lower risks. This can be done with two methods:
   1. Arc Flash Analysis with short circuit study and protective device coordination study can identify how to reduce an arc flash hazard category for some equipment.
   2. Technologies can be implemented to reduce risks. This includes equipment such as arc limiting fuses and remote racking technologies.
      
3. Engineering Controls -- reinvent ways to limit/prevent the risk
   1. This can be done by replacing it with equipment that will lower the incident engery, including but not limited to: adjusting breaker settings and redesigning electrical distribution systems. While most of these mitigations require electrical engineering analysis, steps that don'r require engineering can be taken as well, such as implementing electrical covers with ifrared inspection ports.
      
4. Awareness -- raise knowledge of risks and consequences thereof
   1. Train all workers on the hazards of arc flash Although this is #4 on the Risk Control Hierarchy, it should be the first thing done and is mandated by OSHA. It is only through training that workers will understand how to eliminate, substitute or otherwise lower the risk.
   2. Conduct an arc flash analysis to properly identify the hazards, boundaries and required PPE
5. Administrative Controls -- create regulations, work processes, etc.
   1. Make sure that you have a written electrical safety program AND that it is understood by everyone AND that it is enforced.
   2. Allow only Qualified Persons wearing the proper PPE and using the correct tools to work on or around energized equipment.
   3. Implement a preventive maintenance plan for electrical systems based on NFPA 70B. A proper preventive maintenance program will help identify or fix electrical hazards before they become big problems.
      
6. PPE -- use Personal Protective Equipment as last defense.

An effective electrical safety program will include components of multiple levels of risk control, including PPE; but the most prized level of control is risk elimination. With this in mind, it is not surprising that OSHA specifically states “...with respect to arc-flash burn hazard prevention, the general provisions for the selection and use of work practices... generally require de-energization of live parts before an employee works on or near them.” 8  

Arc Flash Protection:

By removing high-risk, hazard-inducing activities, IR windows help to eliminate risks and thereby proactively protect workers by reducing risk in the most efficient manner. However, the word “protect” must be used with caution since there is not a window on the market that has been proven to actually offer “protection” to workers in the exceedingly unlikely event that an arc flash were to occur during inspection.

Arc resistant switchgear and similar systems utilize engineering controls, such as barriers, compartmentalization, and pressure relief mechanisms to redirect arc flash / arc blast gasses and forces away from panels where personnel are most likely to be interacting with equipment. In so doing, these engineering controls (in Risk Control Hierarchy terms) offer reactive protection to personnel from the effects of the arc flash / arc blast.

References:

1 CapSchell Group

2 NFPA 70E; Electrical Safety in the Workplace; Annex K.3; 2009

3 NIOSH; Arc Flash Awareness; DHHS (NIOSH) Publication No. 2007-116D; 2007

4 NFPA 70E; Electrical Safety in the Workplace; Annex K.4; 2009

5 IEEE.37.7 Standard; Guide for Testing Metal-Enclosed Switchgear Rated  Up to 38kV for Internal arcing Faults; Section 1.2.4; 2007

6 K. Heid, R. Widup; Field Measured Total Clearing Time of Protection Devices & its Effect on Electrical Maintenance; from the Proceedings of the 2009 IEEE IAS Electrical Safety Workshop; St. Louis, MO, 2009

7 ANSI / AIHA Z10 Standard; American National Standard for Occupational Health & Safety Management Systems; 2005

8 OSHA 1910.303, Linhardt interpretation

9 NFPA 70E; Electrical Safety in the Workplace; Section 130.7(C)(9); 2009

10 NFPA 70E; Electrical Safety in the Workplace; Article 100 FPN No. 1; 2009

11 IEEE C37.20.2; IEEE Standard For Metal Clad Switchgear; Section A.3.6 ; 1999