Arc Flash Studies

Arc Flash Studies

An arc flash is an unexpected release of electrical energy, with the potential for serious or possibly fatal injury to workers exposed to the arc.
An arc flash event occurs when there is an insulation breakdown between two conductive surfaces of different potential. This would typically be between phases or phase/s and earth of an HV/LV electrical system. The breakdown results in a ‘short circuit’ through the air separating phases or phase/s and earth.

Some possible causes of an arc flash are:

• Momentary shorting due to, for example, a dropped tool or bolt, or accidental contact with live parts
• Ionisation of air due to over-heating (hot spots) or transient over-voltage (e.g.lightning)
• Solid insulation deterioration due to partial discharge or ageing
• Pollution by water, dust, or foreign matter
• Corrosion
• Conductive dust particles
• Misalignment of moving contacts
• Entry of foreign bodies (e.g. insects, rodents, snakes)

Once initiated, the arc flash can rapidly develop into a plasma cloud. High temperatures within the initial arc path, breaks the air molecules into a plasma of positive and negative ions, which are electrically conductive. The electrical fault current flows through this conductive path in the air, dissipating a large amount of energy. The energy is released as an instantaneous explosion of light, heat, hot gases and molten metal, with temperatures of up to 20,000ºC.

The resulting rapid and destructive expansion of air and vaporised metal often leads to switchgear structural failure and the explosive propulsion of molten metal, dislodged panels/doors, equipment parts and other debris at speeds of up to 300 m/s.

Consequences of an Arc Flash

The consequences for personnel exposed to an arc flash incident are potentially very serious and can result in death. Typical exposure consequences are:

• Severe burns – arc plasma, radiation and the secondary burns from the ignition of flammable clothing
• Burns – metal spray and material combustion
• Electric shock/electrocution from the projected arc plasma
• Pressure wave – lung, eye and ear trauma
• Pressure wave – secondary injuries from projectiles and shrapnel
• Poisoning – toxic gasses

Financial losses and equipment damage may also result from arc flash incidents and typically present as:

• Initial explosive damage
• High temperature melting
• Fire damage
• Carbon deposits reducing insulation quality
• Consequential damage
• Adjacent panel damage
• Auxiliary plant damage

How Is An Arc Flash Study Conducted?

The purpose of the Arc Flash Study is to identify incident energy on the busses and define the arc flash hazard level. Based on this level, the class of clothing for workers will be determined. Additionally, the Arc Flash Study shows the arc-flash protection boundaries for the new 11 kV and 3.3 kV switchgears.