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What is an Arc Flash?

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By Douglas Krantz

An Arc Flash can be started by simply dropping a screwdriver
The act of dropping a screwdriver is all it takes to start the electricity flowing in an Arc Flash.

Thunder and Lightning

Burning, blinded, deafened, an electrician is thrown across the room; having reached into a live electrical panel, all he did was to drop his screwdriver.

This caused an Arc Flash, but what happened?
An extension cord has small wires.
Electrical cords and house wiring will inherently restrict the amount of current in an electrical short condition.

Short

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.
110 Volts Times 4 Equals 440 Volts
110 volts times 4 may equal 440 volts, but current is not limited by the voltage.

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.
Voltage
X
Current
X
Time

It's Not Just the Voltage

This is electricity ... Voltage, Current, and Time are the Enemy
Warning sign from an electrical cabinet
The sign only tells you the voltage, but current and time are also involved.

Arc Flash = Explosion

That little spark at 110 volts becomes huge at higher voltage, especially above 220 volts, producing what is known as an Arc Flash, or an electrical explosion.
A turned off relay
The Federal Agency NIOSH is concerned with Arc Flashes

Frequent

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.
Flash of Lightning
An Arc Flash inside an electrical cabinet can be considered as dangerous as a storm cloud lightning bolt.

Lightning

Not just an insignificant spark, an arc flash - or electrical explosion - is a short lightning bolt. Producing one is an definite "Do-Not-Try-This-At-Home."
But... What Causes the Arc Flash?
Ionized Air
 Conducts
  Electricity

Voltage

Lower voltages, like 110 volts, usually require physical contact with the conductors to complete the circuit; but especially higher than 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.
Tool near an electrical contact as it gets into the static electricity field
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.

Ionization

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.
The letter I stands for Current
The letter I stands for Current

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 diamerter of the wire inside the house and the extension cord is small and will limit the amount of electrical current in an arc flash
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 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.

It's the Wire Size that
Limits the Current

The extension cord has small wire.

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.
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

Entry - Large Wire

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.

The large size of the entry wires allows a huge surge current. That equals...

Explosion

Like a storm cloud lightning bolt, the arc flash explodes the air into superheated plasma with a bang. But unlike a very short duration 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.
Arc Flash exploding in an electricians face
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.

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.
Personal Protective Equipment
Personal Protective Equipment (PPE) shields the electrician from the intense heat and shock wave of an arc flash.

Personal Protive Equipment -- PPE

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.
Stop Arc Flashes
Arc Flashes cannot be fully prevented, but the incidents can be reduced.

Arc Flash

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.
















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