CIRCUIT PROTECTION DEVICES

LEARNING OBJECTIVES

Upon completion of this chapter you will be able to:

• State the reasons circuit protection is needed and three conditions requiring circuit protection.
• Define a direct short, an excessive current condition, and an excessive heat condition.
• State the way in which circuit protection devices are connected in a circuit.
• Identify two types of circuit protection devices and label the schematic symbols for each type.
• Identify a plug-type and a cartridge-type fuse (open and not open) from illustrations.
• List the three characteristics by which fuses are rated and state the meaning of each rating. Identify a plug-type and a cartridge-type fuse (open and not open) from illustrations.
• List the three categories of time delay rating for fuses and state a use for each type of time-delay rated fuse.
• List the three categories of time delay rating for fuses and state a use for each type of time-delay rated fuse. Identify fuses as to voltage, current, and time delay ratings using fuses marked with the old military, new military, old commercial, and new commercial systems. List the three categories of time delay rating for fuses and state a use for each type of time-delay rated fuse.
• Identify a clip-type and a post-type fuseholder from illustrations and identify the connections used on a post-type fuseholder for power source and load connections.
• List the methods of checking for an open fuse, the items to check when replacing a fuse, the safety precautions to be observed when checking and replacing fuses, and the conditions to be checked for when conducting preventive maintenance on fuses.
• Select a proper replacement and substitute fuse from a listing of fuses.
• List the five main components of a circuit breaker and the three types of circuit breaker trip elements.
• Describe the way in which each type of trip element reacts to excessive current.
• Define the circuit breaker terms trip-free and nontrip-free and state one example for the use of each of these types of circuit breakers.
• List the three time delay ratings of circuit breakers.
• Define selective tripping, state why it is used, and state the way in which the time delay ratings of circuit breakers are used to design a selective tripping system.
• Identify the factors used in selecting circuit breakers.
• List the steps to follow before starting work on a circuit breaker and the items to be checked when maintaining circuit breakers.

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. Some examples of these devices are discussed in this chapter.

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.