14-07-2012, 01:21 PM
Need for Circuit Protection
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Current and Temperature
Current flow in a conductor always generates heat. The greater
the current flow, the hotter the conductor. Excess heat is
damaging to electrical components. For that reason, conductors
have a rated continuous current carrying capacity or ampacity.
Overcurrent protection devices, such as circuit breakers, are
used to protect conductors from excessive current flow. These
protective devices are designed to keep the flow of current in
a circuit at a safe level to prevent the circuit conductors from
overheating.
Overloads
An overload occurs when too many devices are operated on
a single circuit, or a piece of electrical equipment is made to
work harder than it is designed for. For example, a motor rated
for 10 amps may draw 20, 30, or more amps in an overload
condition. In the following illustration, a package has become
jammed on a conveyor, causing the motor to work harder and
draw more current. Because the motor is drawing more current,
it heats up. Damage will occur to the motor in a short time if
the problem is not corrected or the circuit is shut down by the
overcurrent protector.
Conductor Insulation
Motors, of course, are not the only devices that require circuit
protection for an overload condition. Every circuit requires some
form of protection against overcurrent. Heat is one of the major
causes of insulation failure of any electrical component. High
levels of heat can cause the insulation to breakdown and flake
off, exposing conductors.
Short Circuits
When two bare conductors touch, a short circuit occurs. When
a short circuit occurs, resistance drops to almost zero. Short
circuit current can be thousands of times higher than normal
operating current.