Arc Flash and Shock Hazard

Electric shock is the physiological reaction or injury caused by electric current passing through the human body. It occurs upon contact of a human body part with any source of electricity that causes a sufficient current through the skin, muscles, or hair. Very small currents can hardly be felt. Larger current passing through the body may make it impossible for a shock victim to let go of an energized object. Still larger currents can cause fibrillation of the heart and damage to tissues. Death caused by an electric shock is called electrocution.

An arc is an undesired electric discharge that travels through the air between conductors or from a conductor to a ground. The resulting explosion can cause fires and serious harm to equipment and people. The temperature of an arc flash may exceed 15 000 Degrees Celsius, which is capable of vaporizing metal and sending a blast of plasma and molten metal in all directions with extreme force. Generally, an electrical system that run at more than 125 kVA to supply an arc flash; the higher the current, the higher the risk. Damage is caused both by the explosion of the arc flash and by the heat radiating from the blast.

Voltage = Shock

Current = Arc Flash



The severity of an Arc Flash?

Several groups and organizations have developed formulas to determine the incident energy available at various working distances from an Arc-Flash.

In all cases, the severity of the Arc-Flash depends on one or more of the following criteria:

  • Available short circuit current
  • System voltage
  • Arc gap
  • Distance from the arc
  • Opening time of overcurrent protective device (OCPD)

When a severe enough Arc-Flash occurs, the overcurrent protective device (fuse or circuit breaker) upstream of the fault interrupts the current.

The amount of incident energy a worker may be exposed to during an Arc-Flash is directly proportional to the total clearing ampere-squared seconds (I2t) of the overcurrent protective device during the fault.

High current and longer exposure time produces greater incident energy. The only variable that can be positively and effectively controlled is the time it takes for the overcurrent protective device to extinguish the arc. A practical and significant way to reduce the duration of an Arc-Flash and thereby the incident energy is to use the most current-limiting OCPD’s throughout the electrical system.


10 Common Causes Of Arc-Flash and Other Electrical Accidents


The most common cause of Arc Flash and other electrical accidents is carelessness. No matter how well a person may be trained, distractions, weariness, pressure to restore power, or over- confidence can cause an electrical worker to bypass safety procedures, work unprotected, drop a tool or make contact between energized conductors. Faulty electrical equipment can also produce a hazard while being operated.

Electrical safety hazards such as exposure to shock and Arc-Flash can be caused by:

  1. Carelessness
  2. Worn or brok­en conductor insulation
  3. Exposed live parts
  4. Loose wire connections
  5. Improperly maintained switches and circuit break­ers
  6. Obstructed disconnect panels
  7. Water or liquid near electrical equipment
  8. High voltage cables
  9. Static electricity
  10. Damaged tools and equipment