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General Electronics

An Arc Flash is a short but full power lightning bolt.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer






What is an Arc Flash?

By Douglas Krantz

Thunder and Lightning

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.

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?
A realy coil is an electromagnet
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 voltages,

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 a

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

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.
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 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

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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Douglas Krantz

Describing How It Works
writer@douglaskrantz.com
Text
612/986-4210

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Electrical Flow


On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at www.douglaskrantz.com/
ElecElectricalFlow.html
.