Electric Arc Flash
An electric arc, often referred to as electrical flashover, is a continuous electric discharge of high current, which flows through an air gap between conductors. This generates a very bright ultra-violet light as well as intensive heat. An arc flash is typically caused by a short circuit, which is sometimes due to technical failure of electrical equipment, for example improper installation, dust, corrosion, surface impurities or sometimes simply due to normal wear and tear. However, in the majority of cases, short circuits are the result of human error. The consequences for employees working on or close to energised electrical equipment primarily depends on the amount of Incident Energy received at the body surface, which depends on its distance from the arc. The primary concern for the exposed person is skin burns.
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 in the US more than 2,000 people are admitted to burn centres with severe arc flash burns. Arc flash can and will kill at distances of 10 feet’.
THE IMPORTANCE OF ARC RATED PPE
The massive energy released in an arc fault rapidly vapourises 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 vapourising 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 and the intense UV, visible and IR light produced by an electric arc flash 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.
On layered clothing systems: can the individual component arc ratings simply be added to obtain the system rating? You cannot add the arc ratings of two individual fabrics together to receive the layered rating. In order to obtain a layered system arc rating, the fabrics must be arc tested as such.
ARC FLASH BLAST AND ENVIRONMENTS ABOVE 40 CAL/CM2
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, vapourising 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.
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 hazard caused by an electric arc. 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.
TESTING FOR ATPV AND Ebt - DUPONT™ ARC-MAN®
The open arc test determines the limit of the incident energy to which a material or material assembly will provide protection against the thermal effects of an electric arc. The electric arc testing system developed by DuPont™ measures the Arc Thermal Performance Value (ATPV) and Energy Break-open Threshold (Ebt) values of protective fabrics and garments in simulated open electric arc incidents according to IEC/EN 61482-1-1.
WESTEX ARC FLASH TESTING OVERVIEW
This Westex arc flash testing video (3:35) 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.
ELECTRICAL HAZARDS
Electricity is often referred to as a 'silent killer' however electric arc and flash fire incidents can also be fatal. PPE programs backed by proven performance must be installed to provide the correct level of workplace protection. 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 and: 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: the flash/flame temperature and incident energy.
- Electric Arc Blast: fragmented metal, molten metal, vapourised metal and pressure.