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
severity of an Arc Flash?
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.
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:
- Worn or broken conductor
- Exposed live parts
- Loose wire connections
- Improperly maintained switches and
- Obstructed disconnect panels
- Water or liquid near electrical
- High voltage cables
- Static electricity
- Damaged tools and equipment