What you are expected to learn

At the end of this module, you should be able to:

• enumerate the type of protection employed against short circuits
• enumerate the types of short circuits that may occur in a circuit
• Explain how fast-acting and slow-blow fuses differ.

How to learn from this module

Going through this module can be both a fun and a meaningful learning experience. All you need to do is make use of your time and resources efficiently. To do this, here are some

tips for you:

1. Take time in reading and understanding each lesson. It is better to be slow but sure than to hurry finishing the module only to find out that you missed the concepts you are supposed to learn.

2. Do not jump from one chapter to another. Usually, the lessons are arranged such that one is built upon another, hence an understanding of the first is essential in comprehending the succeeding lessons.

3. Be honest. When answering the test items, do not turn to the key to correction page unless you are done. Likewise, when performing experiments, record only what you have really observed.

4. Safety first. Perform the experiments with extra precaution. Wear safety gears whenever necessary.

5. Don’t hesitate to ask. If you need to clarify something, approach your teacher or any knowledgeable person.

What to do before (Pretest)

Lesson 9: Fuse

Fuses are the most common and least expensive way to protect a circuit from excessive currents.

A fuse is a conductive device which opens when the current through it exceeds its current rating for a certain period of time. For example a fuse rated at 30-A can only conduct when the current through it is below 30 A. When the current goes above 30 A, the fuse blows and becomes an open circuit thus opening up the circuit and preventing damage to the load, the wiring or the power source.

Fuses are rated according to the following,

1. current rating
2. voltage rating
3. blowing characteristics

The current ratings indicate the maximum current that the fuse can conduct. Above these rating the fuse will become an open circuit.

The voltage rating is important as this indicates the voltage that the fuse can block when it has opened up. Using a fuse in a circuit with a higher voltage than it is rated for could be dangerous. These is because the fuse could continue conducting even if the fuse has already blown because of the high voltage across its open terminals when arcing occurs through the fuse.

The blowing characteristics of the fuse indicates how rapidly the fuse blows or opens up. There are three categories of flowing characteristics of fuses. These are

1. slow blow fuses

2. medium blow fuses

3. fast blow fuses

In dead shorts, all these types of fuses blow at about the same time which is about 1 millisecond. In small overloads, for example at 1.35 times the rated current, they also blow up at about the same time which takes about a minute or two. However in between the two extremes, dead short and small overloads, the blow time differs dramatically.

In an overload of 5 times the rated current, for example they will have the following blow times,

1. slow blow - greater than 1 second
2. fast blow - less than 1 millisecond
3. medium blow - about 10 milliseconds

Medium blow refers to the ordinary types of fuses. Fast blow fuses are used to protect sensitive devices such as electric meters. Medium and fast blow fuses are the same in appearance.

Slow blow fuses are used whenever short-duration current surges are expected. An example of an application of a slow blow fuse is in devices which use electric motor. Electric motors have a high initial current at start-up sometimes reaching five to ten times the normal (running) current.

When replacing fuses, always use replacements that have the same voltage, current and blow characteristics to avoid damaging the equipment with which it is used. For example replacing a fast blow fuse with a medium blow fuse may cause damage to the equipment before the fuse can respond to excess current.

Circuit Breakers

Circuit breakers also prevent a device from being damaged by excess current that could result from a short circuit. As such it functions much like a fuse.

Unlike the fuse, however, the circuit breaker can be reset to conduct current again without needing any parts replacement. All that has to be done for the circuit to conduct current again is to press the reset button and then the circuit breaker can conduct current again.

A circuit breaker becomes an open circuit when the current passing through it exceeds its current rating for a specified period of time.

There are two types of breakers namely,

1. thermal breakers
2. magnetic breakers

Thermal circuit breakers have a bimetallic strip that flexes when the current passing through it exceeds a certain limit. After flexing, the bimetallic strip breaks the circuit.

Thermal circuit breakers are more commonly used for motor circuits, household circuits and battery chargers. Thermal breakers may have automatic or manual reset. Automatic thermal breakers reset themselves after tripping. Manual breakers need to be manually reset after tripping to conduct again.

Magnetic circuit breakers contain an electromagnetic coil that pulls in a lever whenever the current through the coil exceeds a certain level. The lever in turn breaks the switch contact and opens the circuit.

Important Terms

Circuit breaker a device used to protect the components and wiring in a circuit in the event of a short circuit. It is constructed of a thin bimetallic strip that expands with heat and in turn trips open the circuit. The circuit breaker can be reset after the bimetallic strip cools down and the short circuit is removed.

Fuse a device used to protect the components and wiring in a circuit in the event of a short circuit. The fuse element is made of either aluminum, tin-coated copper, or nickel. Excessive current melts the fuse element which blows the fuse.

Slow-blow fuse a type of fuse that can handle a temporary surge current that exceeds the current rating of the fuse. This type of fuse has an element with a coiled construction and is designed to open only on a continued overload such as a short circuit.

Self Check

1. An open fuse has a resistance that approaches

a. infinity b. zero ohms c. 1 to 2 d. none of the above

2. Enumerate the type of protection employed against short circuits

3. Enumerate the three categories of flowing characteristics of fuses.