Subject: Physics
Topic: Fuse and Circuit Breakers
Objectives:
1. Discuss The reasons for using parallel connections for domestic appliances
2. Explain the purpose of a fuse or circuit breaker.
3. Select a fuse or circuit breaker of suitable current rating for an appliance.
4. Explain the function of the Earth Wire.
5. Recall the international insulation color code.
6. Stare the adverse effects of fluctuating voltage supplies.
Teaching Strategy: Discussion, Multimedia Presentation, including, Music, Videos.
Content and Facts: A fuse or circuit breaker is a device use to protect circuit components from excess currents.
Wires are color coded for safety.
Introduction:
Lesson Development: The function of the fuse/circuit breaker will be discussed. labelled diagram of a fuse will be shown. Questions will be used to show how a fuse rating is calculated. The function of the fuse earth wire will be discussed. The international insulation color code will be displayed in a table and discussed. The effects of fluctuating voltages will also discussed.
Conclusion: a fuse protects the circuit from excess current. An earth wire prevents shock caused by stray currents.
NOTES
FuseThere are many different types of fuses for residential and commercial use, but the most common type is made up of a metal wire or filament that is enclosed in a glass or ceramic and metal casing. In a home, the fuse is typically plugged into a central fuse box where all the building’s wiring passes through. When the electricity is flowing normally, the fuse permits the power to pass unobstructed across its filament, between circuits. If an overload occurs, the filament melts, stopping the flow of electricity.
Fuse and Circuit Breakers Video
It generally takes very little time for the filament in the type of fuse used in a home to melt, so any power surge is quickly stopped. Once a fuse is blown, however, it must be discarded and replaced with a new one. There are many different voltage ratings available that handle different capacities of electricity, and the best fuse for a circuit is typically one that is rated for slightly higher than the normal operating current.
Circuit Breakers Video
A circuit breaker works in one of two ways, with an
electromagnet (or
solenoid) or a bi-metal strip. In either case, the basic design is the same: when turned on, the breaker allows electrical current to pass from a bottom to an upper terminal across the solenoid or strip. When the current reaches unsafe levels, the magnetic force of the solenoid becomes so strong that a metal lever within the switch mechanism is thrown, and the current is broken. Alternately, the metal strip bends, throwing the switch and breaking the connection.
Another use of the circuit breaker is a ground fault circuit interrupter (GFCI) outlet, which functions to prevent electric shock instead of overheating. It works by breaking the circuit in an outlet if the current becomes unbalanced, and can be reset by the push of a button. This technology is particularly useful in bathrooms or kitchens where electrocution is a risk due to the frequent use of electric appliances near a source of water.
Advantages and Disadvantages Of Fuse and Circuit Breakers
The fuse and circuit breaker both have advantages and disadvantages, each of which can depend on the situation in which they are used. Fuses are inexpensive and can be purchased from any hardware store. They also tend to react very quickly to overloading, which means that they can offer more protection to sensitive electronic devices. This quick reaction can be a disadvantage, however, if the circuit is prone to surges that regularly cause fuses to blow.
Fuses must always be replaced once they are blown, which can be challenging in a darkened room or if the appropriate replacement is not immediately available. Another issue is that a do-it-yourselfer can mistakenly select a fuse that has a voltage rating that is too high for his needs, which can result in an overheated circuit. In addition, there may be exposed electrical connections in a fuse box, which can pose a danger to someone who does not follow the proper safety precautions.
Circuit breakers have many advantages, not the least of which is how quickly they can be reset. It is usually clear which switch has tripped, and it can be easily reset in most cases. For the average homeowner, it is also safer because there is no question about choosing the right fuse rating and all of the electrical connections are hidden in a breaker box.
A drawback to using a circuit breaker is that it is usually more expensive to install and repair. A circuit breaker also typically does not react as quickly as a fuse to surges in power, meaning that it is possible that electronics connected to the circuit could be damaged by "let-through" energy. It also is more sensitive to vibration and movement, which can cause a switch to trip for reasons unrelated to an electricity overload.
A fuse and circuit breaker are not interchangeable for all power applications. For example, a fuse cannot be used in situations that require a GFCI. Electricians are best qualified to determine whether a fuse or circuit breaker system is better for a particular electrical installation or upgrade.
The earth Wire
The earth wire connects the metal frame of the electrical appliances to the ground. The convention is to have green colour for earth wires. The purpose of earthing is that in case of an insulation failure in some appliance, this wire connected to the metal body will provide a path for the current to flow on the ground. In absence of earth wire, this fault current would have flown through the body of the operator, causing electrical shock.
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