An Arc Flash is a very large, hot, and loud momentary short circuit. For all practical purposes, it is a lightning bolt; Just being near one can give third degree burns over large sections of the body, create permanent deafness, and cause blindness. Don't try this at home.
What is an Arc Flash?
By Douglas Krantz
Thunder and Lightning
Burning, blinded, deafened, an electrician is thrown across the room; having reached into a live electrical panel, all the electrician did was to drop a screwdriver.
This caused an Arc Flash, but what happened?
The act of dropping a screwdriver is all it takes to start the electricity flowing in an Arc Flash.
Many of us have experienced sparking at the end of an extension cord from an accidental short circuit. There's a spark, a poof of smoke, and the circuit breaker trips.
That's a very small electrical explosion.
Electrical cords and house wiring will inherently restrict the amount of current in an electrical short condition
Simple Math - Good for Arithmetic, Bad for the Electrician
Intuitively, one would think that shorting a wire in a 440 volt electrical switch cabinet would produce a spark 4 times larger than 110 volts, and actual contact with the wires or terminals is necessary to cause an arc.
110 volts times 4 may equal 440 volts, but current is not limited by the voltage
It's Not Just the Voltage
This is electricity ... Voltage, Current, and Time are the Enemy
Arc Flash = Explosion
That little spark at 110 volts becomes huge at higher voltages, especially above 220 volts, an Arc Flash, or an electrical explosion.
The sign only tells you the voltage, but current and time are also involved
In the United States, Arc Flashes are common. According to the National Institute for Occupational Safety and Health (NIOSH), Arc Flash explosions in electrical equipment occur 5 to 10 times a day.
The Federal Agency NIOSH is concerned with Arc Flashes
Not just an insignificant spark, an arc flash - or electrical explosion - is a short lightning bolt. Producing one is a definite "Do-Not-Try-This-At-Home."
An Arc Flash inside an electrical cabinet can be considered as dangerous as a storm cloud lightning bolt
Lower voltages, like 110 volts, usually require physical contact with the conductors to complete the circuit; but especially above 220 volts, the air surrounding the conductors becomes ionized: the higher the voltage, the greater the ionization - and the greater distance the arc flash can jump out.
By itself, the ionized air surrounding the wires and terminals will conduct; the air inside an electrical cabinet is ready to conduct - even without actual contact.
Getting near the wires and terminals, a tool can complete the electrical path, starting the Arc Flash.
Electrical current jumps the gap between exposed copper to the screwdriver through ionized air, and in a "flash", that current will further ionize the air to conduct even more current.
Intuitively, after looking at the 400 amp main breaker, one would think the current inside the electrical cabinet is limited to 400 amps.
That's not really the case.
The letter I stands for Current
It's the Surge of Current
The house wiring and the size of the wire inside the extension cord limit the momentary current surge in that little spark to well under 100 amps.
The diameter of the wire inside the house and the extension cord is small and will limit the amount of electrical current in an arc flash
It's the Wire Size that Limits the Current
Extension Cord - Small Wire
The amount of current that can flow in the extension cord can be considered to be minuscule compared to the current that can flow through the entry wiring.
Entry - Large Wire
The large size of the entry wires allows a huge surge current. That equals...
The diameter of the electrical entry wires and the diameter of the wires in the outside transformer are the only limits to the amount of current that can flow during an arc flash.
Like a storm cloud lightning bolt, the arc flash explodes the air into superheated plasma with a bang.
But unlike a lightning bolt, until the up-stream fuse or circuit breaker trips and gets around to shutting off the current, the arc flash lightning bolt will continue flaring: flash-melting tools and copper, and producing intense heat, burning whatever is nearby, leaving heat shadows etched into the wall.
What Can Be Done About The Arc Flash?
There's no way to fully prevent an arc flash.
There are things, though, that can be done to protect the worker.
An arc flashes happen. Arc flashes can be caused by dirt in the cabinet conducting electricity, a hand too near the contacts, moisture in the air, tools too near the contacts, old insulation, etc. An electrician needs some protection to be safe when an arc flash occurs.
The dangers from flying molten metal, extreme heat and light, and deafening shock during the arc flash incident can be reduced for the electrician by the use of the correct Personal Protective Equipment (PPE): face shield, protective clothing, gloves, etc.
Personal Protective Equipment (PPE) shields the electrician from the intense heat and shock wave of an arc flash
Personal Protective Equipment (PPE), common sense, and a great deal of respect for the hazards of working inside a live electrical cabinet will reduce the chances injury due to an arc flash electrical explosion.
Arc Flashes cannot be fully prevented, but the incidents can be reduced.