FUNDAMENTAL Physics – Mains and Magnetism: 6.2 The fuse and Circuit Breaker

Electrical Safety

Learning outcomes

I know some uses of electrical heating in the home

I can describe how fuses, earthing, circuit breakers, and double insulation can protect me

I can use a formula to calculate the best fuse for a given appliance

Electrical Heaters

Whenever a ………current……… flows in a wire, the wire will ………heat… up.

This is called Ohmic Heating.

Think of five devices in our homes that use electrical heating:

Kettle, Heater, Radiator, Oven, Computer

The Fuse

The fuse is a thin wire, which is deliberately the ……weakest… part of a circuit.

If the current ever goes over the safe limit, the fuse will get so hot that it will “blow” …or melt…. (fuses) and hence the circuit is broken and the dangerously high electrical current is stopped.

‘test circuit’.

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FUNDAMENTAL Physics – Mains and Magnetism: 6.2 The fuse and Circuit Breaker

Question:

What is the maximum current that the fuse wire can take before melting?

This is the “rating” of the fuse wire.

I found that the rating of my fuse wire was ……5… A

We are using wire wool, which is fun because the steel wire burns.

Manufacturers will use copper wires engineered to an exact diameter for real fuse wire.

Stupid Mistake:

Here a fuse has blown and a stupid person has replaced the fuse with some copper pipe.

What can happen here?

The fuse won’t work properly

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FUNDAMENTAL Physics – Mains and Magnetism: 6.2 The fuse and Circuit Breaker

Causes of a Blown Fuse:

1. Short Circuit

A wire become frayed or loose and it touches another wire – This is a …short… circuit. A Short circuit will lower the ……resistance…… and the current becomes …higher…… . The causes the wires to over-…heat…. and an electrical fire to start. To protect us, before an electrical fire occurs the fuse will …blow…! The fuse has …open… the circuit and a fire is prevented.

2. Loose Live Wire

The live wire becomes loose and touches the metal box of the device. The metal box is LIVE and will electrocute anyone who touches it (the person completes a circuit to ‘earth’)

Solution 1: All metal cases on electrical devices must be earthed this means that the earth wire is connected to the metal box.

If the box becomes live, a large ……current… runs from the live wire, through the metal box and then the …earth…. Wire and then to …earth… This large current causes the fuse to ……melt… This is the purpose of the earth wire! The circuit is now broken and this prevents electrocution.

Solution 2: Double Insulation – devices have plastic cases, which provide a second layer of insulation and make it impossible for the user to be electrocuted. No earth wire is required.

Blowing a fuse:

An appliance will rarely blow a fuse during normal operation.

Instead a fault is normally responsible for a blown fuse. A qualified technician should repair the device before being used again.

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FUNDAMENTAL Physics – Mains and Magnetism: 6.2 The fuse and Circuit Breaker

Choosing a fuse:

Way back in history when your Physics teacher was young and had hair, there was a choice of 4 standard fuses: 1A, 3A, 5A and 13A

Now we only have 2 standard fuses:

3A and 13A, but it is essential to choose the correct one (will be specified on the manufacturers safety label)

Eg 1. For example an electric iron has an electrical power of 3000W.

Power = p.d. x Current

P = V x I

3000 = 240 x I

I = 12.5A (use a 13A fuse)

Eg 2. For example a desk-top computer has an electrical power of 600W.

Power = p.d. x Current

P = V x I

600 = 240 x I

I = 2.5A (use a 3A fuse)

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FUNDAMENTAL Physics – Mains and Magnetism: 6.2 The fuse and Circuit Breaker

Practice Calculations

Appliance Power / W Current / A Choice of fuse

Iron 2800 11.6 13A

Kettle 1872 7.8 13A

Electric blanket 280 1.167 3A

TV 204 0.85 3A

Microwave oven 750 3.125 13A

The Circuit Breaker.

Modern homes use RCCBs (residual current circuit breakers) which are a more sensitive, resettable replacement for a fuse. We can see rows of these in the “fuse box” in our homes. (RCCBs are not on our syllabus)

The good old fashioned circuit breaker (on our syllabus) is an electromagnetic device:

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Iron contact

Iron core

FUNDAMENTAL Physics – Mains and Magnetism: 6.2 The fuse and Circuit Breaker

We will describe our electromagnetic devices using ‘Five easy pieces’

Normal Operation

1. A normal safe current flows through the device and through the coils of wires.

2. The iron core becomes ……magnetic…… creating an electromagnet

3. The electromagnet ……attracts…. the iron contact.

4. However, the iron contact is held in place by the ……spring…

5. The circuit remains ……closed…….. .

Short Circuit

1. A short circuit causes a dangerously …high… current to flow

2. The electromagnet becomes more strongly …attracted….

3. The contact is attracted by a greater force.

4. The iron contact is pulled past the spring and ……breaks…. the contacts

5. The circuit is …open…. and the dangerous current is stopped.

Good old fashioned circuit breakers are a good idea for powerful devices that require a high currents or devices where there is a risk of cutting the wire (lawn mower, hedge trimmers, electric saws etc…)

Pop quiz – what is the most expensive form of transport in Hong Kong?

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FUNDAMENTAL Physics – Mains and Magnetism: 6.2 The fuse and Circuit Breaker

Homework; P193, Q 1-5

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