Arc flash incidents can result in significant damage to electrical distribution systems and injuries or fatalities to personnel involved in their operation. An arc flash is a rapid release of electrical energy through the air between two conductors. It can occur when an uncharacteristic flow of electricity – called an arcing fault – forces an exceptionally high current through copper or another conductive material. The resulting short circuit creates an explosion and releases a plasma fireball that can reach temperatures of more than 19,000°C – hotter than the surface of the sun.

It is a common misconception that arc flash incidents are mainly associated with high voltage installations. In fact, high arc energy levels can often be found in low voltage switchgear of the types used in many manufacturing environments. In an analysis of more than 6000 distribution and switchboards at low voltage at a variety of industrial sites, for example, GSE has found that almost a quarter presented incident energy hazards of NFPA 70E Category 3 or higher, and 6.5% were in the 'Dangerous' category where personal protective equipment (PPE) alone cannot provide adequate protection to personnel.

An arc can be initiated by a failure within electrical switchgear such as a breakdown of insulation or by the introduction of a conducting object that accidentally introduces a short circuit. Moreover, when operating and maintaining low voltage systems, protection procedures and training are usually less stringent than those applied in high voltage applications, operations are generally more frequent and are undertaken by a wider number of operators. These characteristics increase the potential risks. In many industrial environments, workers face a higher risk of injury from an arc fault than from an electric shock.

Adequate consideration of arc flash hazards forms part facility owners’ responsibilities under health and safety legislation. While the details of arc flash hazard management strategy will vary from organisation to organisation, a best practice approach will follow three basic steps: assessment of the system, selection and implementation of appropriate mitigation approaches, and management of residual risk. 

Assessment

The hazard associated with arc flash depends on the likelihood of an arc flash incident and the severity of the event. Calculating the potential severity of an arc flash requires knowledge of system fault currents and protection clearance times. Typically, specialist software is used to analyse fault levels, and detailed arc incident level calculations in accordance with IEEE 1584 are performed. Arc flash analysis and risk assessment are time consuming and require specialist knowledge to implement effectively. For this reason, many organisations choose external specialists for this work.

Mitigation

Since arc flash current is always less then short circuit current, an efficient protection assessment should be performed to select the proper settings for overcurrent protection devices, such as fuses and circuit breakers. The challenge here is to reduce potential arc flash energy without sacrificing system integrity, so the portion of a power system that is disconnected in response to an event is minimised. 

Newer technologies offer alternative forms of protection. Arc fault detection relays, for example, are triggered by light from the flash, current and sometimes sound. Other approaches involve remote operation of equipment, or the use of arc resistant switchgear designed to contain and channel the forces of an arc in a safe direction.

There are also measures which can be taken to reduce the impact of fire within electrical panels. These include suppression systems which are able to detect high temperature and locally extinguish using non-conductive liquids or gases. Such systems help to minimise equipment damage although hazard prevention and personnel protection is the ultimate aim of arc flash analysis and mitigation.

Managing residual risk

Once appropriate mitigation strategies are in place, an equipment operator must understand the residual risk, and take appropriate steps to manage them. These include clear labelling of equipment showing the incident energy at the equipment and its boundaries; appropriate training; and use of PPE appropriate to the level of residual risk.