Introduction

Arc flash is an extremely dangerous phenomenon where a flashover of electric current leaves its intended path and travels through the air from one conductor to another, or to ground. The results are often violent and when a human is in close proximity to the arc flash, serious injury and even death can occur.

Legislative law and regulations have been adopted only recently in Countries such as Ireland, UK and USA.  Regulating bodies such as the NFPA, IEC and IEEE have developed certain criteria to improve calculations and predictions around the phenomenon.  It must be taken into consideration that an arc flash is sporadic, and mathematics developed around this phenomenon still requires refinement.

The two most common types of arc flash covered by standards and calculations are:

• Confined Space Flash (Arc-in-a-box)This occurs when an arc flash occurs in a distribution box or a motor control centre. The flash is contained on all sides except the front opening of the box. All of the focused energy escapes through the only opening onto the hands and face of the person working on the box.  Motor control center adopt standards such as IEC TR 61641 and IEC 61439 to decide on the type of confined flash mitigation. Molten metal, high temperature and poisonous gases are released during the incident.
• Open Air Flash
  Takes place when an Arc Flash occurs in an open space with the energy escaping 360° in all directions. The molten metal can ignite the PC and PPE of the worker.

Causes

Arc flash can be caused by many things including:

• Dust
• Dropping tools
• Accidental touching
• Condensation
• Material failure
• Corrosion
• Faulty Installation

ARC FLASH INTENSITY

The intensity of the Arc Flash is influenced not only by the voltage. An Arc Flash will contain the release of thermal energy, acoustical energy, pressure wave and debris.  The heat created by an arc flash is 6 times hotter than the surface of the sun, ranging at approximately 19000 degrees Celsius.  This size and energy of this heated blast is equivalent to a plethora of variables such as:

• Voltage (safety issue where greater than 220 Volts)
• Amperage
• Arc Gap
• Closure Time
• Distance away from the arc
• 3 Phase v Single Phase
• Confined Space

ARC FLASH INJURIES

The consequences of an Arc Flash can be devastating depending on the intensity of the Arc Flash, the proximity of the individual to the source of the arc and the PPE used, if any.

Injuries can include:

• 3rd degree (irreversible) burns caused by extreme heat and molten splatter.
• Loss of sight due to electromagnetic radiation
• Internal burns and intoxication due to inhalation of hot toxic gases
• Permanent hearing damage due to explosion bang
• Other physical trauma caused by the explosion (blast pressure and flying debris)
• Death

Standards and Regulations

Over the years studies and mathematics have been developed and documented into two known standards.  These standards are widely used to protect equipment and personnel against the deathly Arc Flash phenomenon. These standards provide a clear guideline to safety and hazardous work procedures:

• IEEE 1584
  The Guide for Performing Arc Flash Hazard Calculations has been widely accepted as the best mathematical expression for calculating incident energy. The incident energy is expressed as calories per square centimeter.  The acceptable level of calories per square centimeter would be 1.2 to 2 calories per square centimeter.  This would be equivalent to holding your finger directly over a candle fire for two seconds, causing second degree burns.

• NFPA 70E
  Standard for Electrical Safety in the Workplace, outlines a six-step process for the proper design and installation of electrical systems:

1. Develop and audit electrical safe work practices policy.
2. Conduct an arc flash risk assessment to evaluate the likelihood of occurrence and severity of arc flash hazards. 
3. Follow strategies to mitigate and control arc flash hazards.
4. Conduct regularly scheduled safety training and audits for all electrical workers.
5. Maintain electrical distribution system components.
6. Ensure adequate supply of personal protective equipment (PPE) and proper tools that act as the “last line of defense” for exposed workers.

Arc Thermal Performance Value (ATPV)

The ATPV is measured in cal/cm2, and it’s defined as the maximum incident heat energy that a fabric can absorb and lessen the injury to a 2nd degree burn. If the worker is exposed to a potential incident heat energy level of less than 4.0 cal/cm2, the proper ATPV clothing system is 4 cal/cm2.