Electric Arc Flash

Arc Flash, defined by the National Fire Protection Association (NFPA), is “a dangerous condition associated with the release of energy caused by an electric arc.” It occurs when electrical insulation or isolation between conductors is broken or can no longer withstand the applied voltage. More specifically, arc flash defined by NFPA 70E Annex K.3 is - "When an electric current passes through air between ungrounded conductors and grounded conductors, the temperatures can reach 35000° F. Exposure to these extreme temperatures both burns the skin directly and causes ignition of clothing, which adds to the burn injury. The majority of hospital admissions due to electrical accidents are from the arc flash burns, not from shock. Each year more than 2,000 people are admitted to burn centers with severe arc flash burns. Arc flash can and will kill at distances of 10 ft."

The massive energy released in the fault rapidly vaporises the metal conductors involved, blasting molten metal and expanding plasma outward with extreme force. A typical arc flash incident can be inconsequential but could conceivably easily produce a catastrophic explosion. The result of the violent event can cause destruction of equipment involved, fire and injury or death, not only to immediate workers but also to nearby people. In addition to the explosive blast of such a fault, destruction also arises from the intense radiant heat produced by the arc. The metal plasma from an arc produces tremendous amounts of light energy from far infrared to ultraviolet. Surfaces of nearby people and objects absorb this energy and are instantly heated to vaporising temperatures. The effects of this can be seen on adjacent walls and equipment - they are often ablated and eroded from the radiant effects.

The rapidly expanding superheated vapour produced by an electric arc flash can cause serious injury or death and the intense UV, visible and IR light produced by an arc can temporarily or sometimes permanently blind or cause eye damage. The extremely loud noise which accompanies an electric arc flash also has the potential to permanently damage hearing.

There are four different arc type events to be assessed when designing PPC safety programs. Only the first two are usually considered in ATPV calculations and standards. In workplace 480-volt or higher environments, the open air arc can quickly turn into an ejected or tracking arc. The tracking arc is most common at very high voltages or during an electrical contact.

The four types of electric arcs are:

• Open Air Arc - the primary arc used in arc testing.

• Arc-in-a-box - used in one form for arc testing in the EU.

• Ejected Arc - when arc plasma hits the worker.

• A Tracking Arc - most common at higher voltages; arc plasma conducts on the skin or through the clothing.  

Westex Arc Flash Testing Overview

This Westex arc flash testing video was conducted at an independent laboratory with the assistance of 70E Solutions. The goal was to create ‘real-life’ arc flashes using common 480-volt equipment to help companies understand the magnitude of the arc flash hazards that exist in nearly every facility in the world and highlight the importance of complying with the NFPA 70E standard. This video clearly demonstrates that if you work on or near energised parts and equipment, wearing market-proven flame resistant clothing and other PPE can and does dramatically reduce injury and saves lives.

An electric arc flash incident at an Exxon Mobil Corporation refinery.

Arc Flash Blast

Arc Flash Blast is the result of a rapid release of energy due to an arcing fault between a phase bus bar and another phase bus bar, neutral or a ground. The cause of the fault normally burns away during the initial flash and the arc fault is then sustained by the establishment of highly-conductive plasma. The plasma will conduct as much energy as is available and is only limited by the impedance of the arc. This massive energy discharge burns the bus bars, vaporising the copper and thus causing an explosive volumetric increase, the arc blast, conservatively estimated as an expansion of 40,000 to 1. This fiery explosion devastates everything in its path, creating deadly shrapnel as it dissipates.

WARNING – NFPA 70E does not have a PPE Category above 40 cal/cm². Working in environments above 40 cal/cm² should be avoided because of the blast hazards caused by arc flash. Arc flash levels above 40 cal/cm² can be fatal and usually result in a massive pressurised blast with sound pressure waves and projectiles. PPE is available for 100 cal/cm² however the force from the pressurised arc blast can be fatal regardless of the PPE. 

Catastrophic Arc Flash Incident Costs

Arc flash incidents are extremely prevalent in the electrical industry with the most severe burns and fatalities being a consequence of ignition of non-FR work wear, not the arc flash itself. Burn injury is the second most costly hospitalisation in the US today, trailing only neo-natal ICU care. Burn units are isolation wards; burn treatment generally requires weeks or months of hospitalisation and often multiple surgeries sometimes spanning years. This type and duration of care is extremely expensive and hospitalisation is only one major cost of an arc flash burn injury. When a worker’s garments ignite during an arc, resultant burn injury costs to the employer will be enormous. Conversely, if a worker in the same arc is wearing FR clothing, which will not ignite and will also insulate against the heat hazard, there will be little or no burn injury and therefore little or no cost. Many corporations have concluded that any cost of an FR clothing program is more than paid back by avoidance of injury costs over time.

OSHA’s current NFPA 70E and Electrical Safe Work Practices presentation states that the cost of arc flash burn injuries to the employer is now between USD $1m and $3m per incident, with a few exceeding USD $6m. A corporation who avoids these costs even once in a ten year period may be able to pay for any FR “premium” many times over. Initial and life cycle garment costs are most important but are only a part of the true uniform cost equation when workers are potentially exposed to electric arc flash.

Electrical Hazards

Each year in the US, several hundred workers are injured or killed due to inadvertent contact with energised conductors: surprisingly, over half those killed are not employed in the traditional electrical field but are from related fields such as painters, labourers and drivers.

Studies have revealed the three major hazards of electricity as:

Electric Shock - it takes a very low value of current flowing through the human body to cause death or serious injury.

Electric Arc Flash - there are two different issues with this hazard:

• The arc temperature - the main concern being the flash flame and ignition of clothing.

• The incident energy - a radiant energy, which can pass through the clothing, igniting undergarments and/or burn the skin.

Electric Arc Blast - the main concern is that the pressures developed by an electric arc can be extremely high. Heavy duty PPE will protect workers against the flash flame and incident energy hazards of the arc flash but may not protect against the pressures of the arc blast.

The issues to be considered with electric arc flash and electric arc blast, noting that an electric arc is a multi-hazard event, are:

• Electric Arc Flash:

• Flash/flame temperature

• Incident energy

• Electric Arc Blast:

• Fragmented metal 

• Molten metal

• Vaporised metal

• Pressure