CIRCUIT PROTECTION DEVICES

 CIRCUIT PROTECTION DEVICES

Electricity, like fire, can be either helpful or harmful to those who use it. 

A fire can keep people warm and comfortable when it is confined in a campfire or a furnace. 

It can be dangerous and destructive if it is on the loose and uncontrolled in the woods or in a building. 

Electricity can provide people with the light to read by or, in a blinding flash, destroy their eyesight. 

It can help save people’s lives, or it can kill

them. 

While we take advantage of the tremendous benefits electricity can provide, we must be careful to protect the people and systems that use it.

It is necessary then, that the mighty force of electricity be kept under control at all times. 

If for some reason it should get out of control, there must be a method of protecting people and equipment. 

Devices have been developed to protect people and electrical circuits from currents and voltages outside their normal operating ranges. 

While you study this chapter, it should be kept in mind that a circuit protection device is used to keep an undesirably large current, voltage, or power surge out of a given part of an electrical circuit.

The conditions that require circuit protection are direct shorts, excessive current, and excessive heat.

Direct Short

One of the most serious troubles that can occur in a circuit is a DIRECT SHORT. 

Another term used to describe this condition is a SHORT CIRCUIT. 

This term is used to describe a situation in which some point in the circuit, where full system voltage is present, comes in direct contact with the ground or return side of the

circuit.

This establishes a path for current flow that contains only the very small resistance present in the wires carrying the current.

According to Ohm’s law, if the resistance in a circuit is extremely small, the current will be

extremely large. 

Therefore, when a direct short occurs, there will be a very large current through the

wires. 

Suppose, for instance, that the two leads from a battery to a motor came in contact with each other.

If the leads were bare at the point of contact, there would be a direct short. The motor would stop running.

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.