2-3
because all the current would be flowing through the short and none through the motor. The battery would
become discharged quickly (perhaps ruined) and there could be the danger of fire or explosion.
The battery cables in our example would be large wires capable of carrying heavy currents. Most
wires used in electrical circuits are smaller and their current carrying capacity is limited. The size of wire
used in any given circuit is determined by space considerations, cost factors, and the amount of current
the wire is expected to carry under normal operating conditions. Any current flow greatly in excess of
normal, such as there would be in the case of a direct short, would cause a rapid generation of heat in the
wire.
If the excessive current flow caused by the direct short is left unchecked, the heat in the wire will
continue to increase until some portion of the circuit burns. Perhaps a portion of the wire will melt and
open the circuit so that nothing is damaged other than the wire involved. The probability exists, however,
that much greater damage will result. The heat in the wire can char and burn the insulation of the wire and
that of other wires bundled with it, which can cause more shorts. If a fuel or oil leak is near any of the hot
wires, a disastrous fire might be started.
Excessive Current
It is possible for the circuit current to increase without a direct short. If a resistor, capacitor, or
inductor changes value, the total circuit impedance will also change in value. If a resistor decreases in
ohmic value, the total circuit resistance decreases. If a capacitor has a dielectric leakage, the capacitive
reactance decreases. If an inductor has a partial short of its winding, inductive reactance decreases. Any
of these conditions will cause an increase in circuit current. Since the circuit wiring and components are
designed to withstand normal circuit current, an increase in current would cause overheating (just as in
the case of a direct short). Therefore, excessive current without a direct short will cause the same
problems as a direct short.
Excessive Heat
As you have read, most of the problems associated with a direct short or excessive current concern
the heat generated by the higher current. The damage to circuit components, the possibility of fire, and the
possibility of hazardous fumes being given off from electrical components are consequences of excessive
heat. It is possible for excessive heat to occur without a direct short or excessive current. If the bearings
on a motor or generator were to fail, the motor or generator would overheat. If the temperature around an
electrical or electronic circuit were to rise (through failure of a cooling system for example), excessive
heat would be a problem. No matter what the cause, if excessive heat is present in a circuit, the possibility
of damage, fire, and hazardous fumes exists.
Q1. Why are circuit protection devices necessary?
Q2. What are the three conditions that require circuit protection?
Q3. What is a direct short?
Q4. What is an excessive current condition?
Q5. What is an excessive heat condition?
CIRCUIT PROTECTION DEVICES
All of the conditions mentioned are potentially dangerous and require the use of circuit protection
devices. Circuit protection devices are used to stop current flow or open the circuit. To do this, a circuit
protection device must ALWAYS be connected in series with the circuit it is protecting. If the protection
