Flash Over - Effects, Causes and Prevention

Every year, many people die or are severely burned in Flash Over incidents.

Arc Flash  or Flash Overs occur during a low impedance fault between two conductors or one conductors and the ground.  The high current causes an arc flash which results in a high energy plasma.  This high energy arc carries very high energy as it is limited only by the impedance of the arc.

The temperature reaches 35000 deg. C (three times hotter than the surface of the sun).   Metal conductors vaporise at such high temperatures.    The immense thermal energy released causes all combustible materials to burn.

Arc Blast

The high temperature causes the air to expand causing pressure waves.  This is known as arc blast.  The impact of the arc blast can be felt within a radius of several metres.  These pressure waves can cause barotrauma (injuries caused due to the pressure waves on the brain, lungs, etc).  Molten metals, and broken parts of equipments such as insulators can be projected at speeds of 700 miles/h.

The high temperature causes severe burns and the vaporised metal can damage the lungs and other internal organs.  The bright lights can cause blindness.   The noise from the blast can cause hearing loss.

Flash Overs can occur at all voltages over 120V AC.

Preventing Flash Over incidents
  • Identify all possible sources of supply.  Electric equipment such as breakers, capacitors, and bus bars can be fed from more than one supply.
  • All switching equipment should be opened and visually verified if possible
  • Lock out/Tag out devices should be applied.
  • Voltage testers with hot rods should be used to check that the devices are dead.  All devices should be considered live if not earthed.
  • Grounding rods should be used to discharge energy stored device (High voltage devices will retain voltage even after the supply has been switched off due to capacitance).  The equipment on which work is conducted should be kept grounded throughout the duration of the maintenance activity(as a precaution against induced voltage or accidentally coming in contact with live parts)
A proper Flash over Hazard Analysis should be carried out at all device locations.  A Flash over Hazard Analysis calculates the maximum current, the maximum energy which will be dissipated in the event of a Flash over and the Personnel Protection Equipment which is to be used. 

Refer link for more information on Arc Flash Hazard Analysis

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