An Arc Flash is the result of a short circuit that occurs when the electrical current flows through the air (hence "electrical arc"). The amount of energy associated with the arc is related to the available (fault) current in the circuit. An Arc Flash could occur because of something as simple as a screwdriver dropped at the bottom of an electrical switchboard/cabinet/cubicle or as the result of a "general" system failure. In any case, the arcing will continue until the protective circuits (e.g., fuses) kick in. The amount of available energy in an Arc Flash situation is tremendous and as a result personnel can expect to be exposed to molten metal, metal vapour, shrapnel, extreme temperatures and also the blast wave from the "incident" (called an Arc Blast). Certain protective measures can be taken against Arc Flash situations - but there is little protection afforded to the Arc Blast.

The first real push beyond the IEEE 1584 standard (1982) was the NFPA 70E "Standard for Electrical Safety Requirements for Employee Workplaces" (1995, 2000, 2003/2004, 2009). This document outlined various aspects of what an Arc Flash is (basically an unintended release of energy involving a bolted short condition with the available fault current) and what can be done about it. This includes:

• Protective clothing such as fire retardant shirts;
• Improved lock-out/tag-out and test programmes;
• Safe approach boundaries with electrical equipment.

In order to protect personnel, the NFPA use a system of "boundaries of approach" when examining a potential Arc Flash event - these boundaries combined with training, barrier systems and protective personal equipment (PPE) dictate how to manage potential Arc Flash events in any facility. Via NFPA 70E (2009 Edition):

• "Prohibited Approach Boundary" is defined as "An approach limit at distance from an exposed live part within which work is considered the same as making contact with the live part". Entry is forbidden;
• "Restricted Approach Boundary" is defined as "An approach limit at a distance from an exposed live part within which there is an increased risk of shock, due to electrical arc over combined with inadvertent movement, for personnel working in close proximity to the live part". Licensing/training and Arc Flash PPE required - work order programme and risk-based analysis also normally employed;
• "Limited Approach Boundary" is defined as "An approach limit at a distance from an exposed live part within which a shock hazard exists". Appropriate training and Arc Flash PPE required;
• "Flash Protection Boundary" is defined as "An approach limit at a distance from exposed live parts within which a person could receive a second-degree burn if an electrical Arc Flash were to occur". Arc Flash PPE is required.

The boundaries of approach are given in order of proximity to the "energised electrical part". The first three boundaries use voltage-based distance measurements - provided in the NFPA 70E guide. The "Flash Protection Boundary" requires a single-line diagram (or equivalent) in order to perform a mathematical calculation. Once within the "Flash Protection Boundary", all personnel must be wearing Arc Flash PPE. "Flash Protection Boundaries" range from approximately 15 metres (230 kV AC) down to one metre or less (600 V AC).

Arc Flash events are categorised according to hazard and risk. The Arc Flash category is clearly labelled on each piece of equipment. The categories defined by NFPA 70E are as follows (summarised in simplified form only, refer to standard for full list of requirements). The incident energy associated with an Arc Flash (within the "Flash Protection Boundary" are calculated for each system - refer to NFPA 70E 2009 for more details - all clothing must meet ASTM F1506):

• Category 0 - incident Arc Flash energy "not applicable" - non-flammable material (wools, cottons, rayon, silk or blends thereof). Fabrics must be at least 153 grams per square metre and all garments must have long sleeves / long legs. Safety glasses required. Hearing protection recommended (for Arc Blast) along with electrically-rated safety shoes/boots;
• Category 1 - incident Arc Flash energy of 16.74 Joules per Square Centimetre. Minimum arc-rating of "4". PPE includes flame-resistant shirt and trousers or overalls. Safety glasses and safety hat (electrically rated) required. Hearing protection, safety shoes/boots (electrically rated), hearing protection and leather (voltage-rated) gloves as required;
• Category 2 - Incident Arc Flash energy of 33.47 Joules per Square Centimetre. Minimum arc-rating of "8". PPE includes flame-resistant shirt and trousers or overalls. Face shield (arc rated), safety glasses, safety hat, hearing protection, safety shoes/boots, hearing protection and leather (voltage-rated) gloves required;
• Category 3 - Incident Arc Flash energy of 104.6 Joules per Square Centimetre. Minimum arc-rating of "25". PPE includes includes flame-resistant shirt and trousers or overalls and typically an overall on top. Face shield (arc rated), flash suit hood, safety glasses, safety hat (with fire retardant liner), hearing protection, safety shoes/boots and leather (voltage-rated) gloves required;
• Category 4 - Incident Arc Flash energy of 167.36 Joules per Square Centimetre. Minimum arc-rating of "40". PPE includes includes flame-resistant shirt and trousers or overalls and an overall on top. Additional flash suit (jacket and trousers) are required. Face shield (arc rated), flash suit hood, safety hat (with fire retardant liner), safety glasses, hearing protection, safety shoes/boots and leather (voltage-rated) gloves required.

   

The images above show a Category 4 Personal Protective Equipment kit and a simulated Arc Flash incident.

Approximately five to ten Arc Flash explosions occur every day in the USA.

Over 80% of electrically related injuries involve some type of Arc Flash. It is the Arc Flash and not a fire that is the leading cause of burns in electrical accidents. When an Arc Flash occurs, the explosion is as hot as 20,000ºC (if the protective equipment does not trip in a timely manner or fails to trip). Even small incidents have temperatures hot enough to melt and even vapourise copper. There is also a significant Arc Flash Blast which, beyond limited hearing protection, is extremely difficult to protect against (this involves shrapnel, the blast wave, molten copper particles and other hazards).

As well as NFPA 70E / IEEE 1584, (and ISO 61010) your safety programme will include all the relevant AS/NZS standards including AS/NZS 1337 (eye protection), AS 2225 (insulating gloves), AS/NZS 1270 (hearing protection), AS/NZS 1801 (head impact protection) and AS/NZS 2210 (footwear). We will assist you in converting a generic safety programme into a true electrical safety programme with the correctly rated multimeters, the correct workwear and correct personal protective equipment (PPE).

Your electrical safety programme may require check lists or programme steps for the following:

• Site hazard checks - safety barriers, arc flash procedures, exits, confined spaces, noise levels;
• Climate hazard checks (weather checks for outdoor work, heating/cooling checks for indoor work);
• Fall safety checks (working at heights or other fall hazards);
• Mechanical hazards (exposure to moving equipment, steam lines etc.);
• Biochemical, biological and chemical hazards;
• Manual lifting hazards;
• Risk to customers' staff, other workers;
• Risk to the general public;
• Test equipment procedures (e.g., residual current device leads/cords, multimeter rating, inspection and calibration programme);
• Procedures for work on energised circuits;
• Procedures for work on de-energised circuits.