Chapter 8 of 8

All 8 chapters
  1. 1Charge and current, readunderstood
  2. 2Potential difference, readunderstood
  3. 3Ohm's law, readunderstood
  4. 4Different components, readunderstood
  5. 5Series circuits, readunderstood
  6. 6Parallel circuits, readunderstood
  7. 7Power in circuits, readunderstood
  8. 8Mains electricity and safety, readunderstood

S4.12.8 Chapter 8

Mains electricity and safety

Hana can take a torch apart, but she must never take apart a plugged in lamp. What is different about the mains?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

  • You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain the difference between direct and alternating current
  • Explain the jobs of the live, neutral and earth wires and the fuse
  • Explain why mains electricity is never used for home experiments

A torch cell pushes charge round the loop the same way all the time. This is direct current, d.c. The p.d. of the cell stays at about 1.5 V, and the current keeps its direction. Look at a graph of p.d. against time.

A graph with the axes p.d. (V) and time (s). The time runs from 0 to 4 s in steps of 1 and the p.d. from 0 to 2 V in steps of 0.5. A flat horizontal line at 1.5 V is labelled cell. Labels: p.d. (V), time (s), cell.cell12340.511.520time (s)p.d. (V)
Direct current. The p.d. of a cell stays the same and never changes sign.

The mains is different. It gives alternating current, a.c. The p.d. swings up, falls through zero, swings the other way, and returns. The current reverses direction again and again. The electrons do not travel round the loop. They shuffle to and fro about their places, and the energy still reaches the appliance.

A graph with the axes p.d. (V) and time (ms). The time runs from 0 to 40 ms in steps of 5 and the p.d. from −400 to 400 V in steps of 100. A smooth wave crosses zero at 0 ms, rises to a peak of about 325 V at 5 ms, falls through zero at 10 ms, reaches a trough at 15 ms and rises again, with its next peak at 25 ms. An arrow across the top from the peak at 5 ms to the peak at 25 ms is labelled period 20 ms, with dashed lines down to the peaks. Labels: p.d. (V), time (ms), period 20 ms.−400−20002004000510152025303540period 20 msp.d. (V)time (ms)
Alternating current from a 230 V, 50 Hz supply. One whole swing takes 20 ms. The peaks, at 5 ms and 25 ms, are 20 ms apart.

Read the graph like a wave. The time for one whole swing is the period T. The number of swings each second is the frequency f. They are linked by f = 1 ÷ T. A supply with a frequency of 50 Hz swings 50 times each second. Each swing takes 1 ÷ 50 = 0.02 s, which is 20 ms.

The 230 V we quote is not the top of the swing. It is the root mean square, or r.m.s., value: the steady p.d. that would give the same heating. For a smooth swing like this, the peak is always about 1.41 times the r.m.s. value. That 1.41 is the square root of 2. So the peak is 230 V × 1.41, about 325 V.

Two loops, one above the other, each with an alternating supply on the left, drawn as a circle with a wave, and the label 230 V. Each has a closed switch labelled switch and a lit lamp labelled lamp on the top wire. In the top loop arrowheads on the wires point clockwise. In the bottom loop they point anticlockwise. Labels: 230 V, switch, lamp, 230 V, switch, lamp.230 Vswitchlamp230 Vswitchlamp
The current in the loop in one half swing, above, and in the next half swing, below. The direction reverses, and the lamp stays lit both ways.

Worked example

The p.d. of a supply repeats every 0.02 s. What is the frequency?

  1. Write the equation: f = 1 ÷ T.
  2. Put in the numbers: f = 1 ÷ 0.02 s.
  3. f = 50 Hz.

Answer: 50 Hz

The mains p.d. and frequency are not the same everywhere. Many places use 230 V and 50 Hz. Others use 120 V and 60 Hz. Some use other values. The label on an appliance says which supply it is made for.

A table with the columns Place, V and Hz. Row 1: many places, 230, 50. Row 2: some places, 120, 60. Row 3: a few places, 100, 50 or 60.PlaceVHzmany places23050some places12060a few places10050 or 60
Examples of mains supplies. The values differ from place to place.

A mains cable has three wires, each with insulation. The live wire carries the alternating p.d. of the supply. The neutral wire completes the circuit, so the current can return. The earth wire is only for safety. It does not carry current in normal use.

A mains cable cut across, as a circle. The outer sheath is the big circle. Inside are three round wires. Each wire has a copper core in the middle and insulation round it. The live wire has dotted insulation, the neutral wire has hatched insulation and the earth wire has cross hatched insulation. Labels: live, neutral, earth, outer sheath, insulation, copper.liveneutralearthinsulationcopperouter sheath
Inside a mains cable. The dots, hatching and cross hatching tell the three wires apart on a black and white copy.

The wires are told apart by the colour of the insulation. One common scheme is brown for live, blue for neutral and green with a yellow stripe for earth. Colours vary from place to place, for example black, white and green in North America. Always follow the rule of your own place, and leave wiring to a trained person.

A table with the columns Wire and Job. Row 1: live, carries the supply. Row 2: neutral, completes the loop. Row 3: earth, safety only.WireJoblivecarries the supplyneutralcompletes the loopearthsafety only
The jobs of the three wires. In the common scheme, live is brown, neutral is blue and earth is green with a yellow stripe.

Why a safety wire? Many appliances, such as a kettle or a toaster, have a metal case. Suppose the live wire inside comes loose and touches the case. The case is then live, and anyone who touches it gets a path to the ground through their body.

The earth wire joins the case to the ground. Now the fault has a path of very low resistance. A large current flows at once through the earth wire, and the fuse in the live wire melts. The appliance is cut off from the live supply, and the case is safe.

Two pictures of a metal cased appliance with a fault, one above the other. Each has a live wire with a fuse labelled fuse, a neutral wire, a box labelled metal case, and a fault where the live wire touches the case. In the top picture a thick wire labelled earth wire joins the case to a ground symbol labelled ground, and the fuse wire is melted apart. In the bottom picture a dashed line labelled through a person joins the case to the ground symbol labelled ground, and the fuse wire is whole. Labels: live, neutral, fuse, metal case, fault, earth wire, ground, live, neutral, fuse, metal case, fault, through a person, ground.liveneutralfusemetal casefaultearth wiregroundliveneutralfusemetal casefaultthrough a personground
Above, with an earth wire, the fault current melts the fuse. Below, with no earth wire, the case stays live and the only way to the ground is through a person.

Appliances with a plastic case, called double insulated, have no earth wire. No metal part can become live, so there is nothing to earth.

The fuse goes in the live wire. When it melts, the appliance is cut off from the live supply. A fuse in the neutral wire would leave the appliance joined to the live wire after it melted. On its own, a fuse does not stop a shock to a person who touches a live wire. 13 A is far more than can kill, and a fuse takes time to melt. With an earth wire, it cuts off a faulty metal case before anyone touches it.

Many homes also have a residual current device. It cuts off the supply very fast when a little current leaks to earth, for example through a person. A trained person fits and tests it.

Now see why the mains is never used for experiments. The current through a person is I = V ÷ R. Here R is the resistance of the body between the two touched points. R varies a lot. Dry skin can have 100 000 Ω or more. Wet skin or a cut can have 2 000 Ω or less. These are rough values.

Worked example

A person touches both ends of a 1.5 V cell. The resistance of the body between the touched points is 5 000 Ω. What is the current, in milliamps?

  1. Write the equation: I = V ÷ R.
  2. Put in the numbers: I = 1.5 V ÷ 5 000 Ω = 0.0003 A.
  3. Change to milliamps: 0.0003 A × 1 000 = 0.3 mA.

Answer: 0.3 mA

Worked example

A person with wet skin touches a live mains wire at 230 V. The resistance of the body is 2 kΩ. What is the current, and how does it compare with the 0.05 A that can kill?

  1. Change to ohms: 2 kΩ = 2 × 1 000 = 2 000 Ω.
  2. Write the equation: I = V ÷ R.
  3. Put in the numbers: I = 230 V ÷ 2 000 Ω = 0.115 A.
  4. Compare: 0.115 A ÷ 0.05 A = 2.3, more than twice the current that can kill.

Answer: 0.115 A

The cell pushes less than a thousandth of an amp through the body, too little to feel. The mains pushes a current that can stop the heart. Rough guides for the current through the body are in the table. They depend on the path through the body and on how long the current lasts.

A table with the columns Current and Effect. Row 1: 1 mA, tingle. Row 2: 10 mA, may not let go. Row 3: over 50 mA, can kill.CurrentEffect1 mAtingle10 mAmay not let goover 50 mAcan kill
Rough guides to what a current through the body does. A mains p.d. can drive far more than 50 mA through wet skin.

A phone charger turns the mains into a few volts of d.c. inside a sealed case. The sealed case is the safe part. Never open a charger or any plugged in appliance. Any lamp, motor or sensor you build for a test should run from cells, and never from the wall socket.

Try it

Alternating current by hand

You need: one torch cell of 1.5 V, a small torch bulb, two pieces of bare wire or kitchen foil strips, sticky tape.

Be safe: Use only a torch cell. Do not leave the wires on the cell for long, because they and the bulb get warm. Never try this with anything that plugs in to a socket.

  1. Tape one wire to the tip at the bottom of the bulb and the other to its metal side. Touch the free ends to the two ends of the cell. The bulb lights.
  2. Swap the wires, so that each wire touches the other end of the cell. The bulb lights again. The current now goes the other way round the loop.
  3. Swap the wires back and forth, about once each second, and say "one, two" aloud. You are making an alternating current of about 0.5 Hz.

What you will see: The bulb lights whichever way the current goes. A lamp does not care about the direction. A 50 Hz mains reverses the current 100 times each second, once every hundredth of a second.

If you cannot try it, this is what happens.
A table with the columns Cell and Bulb. Row 1: one way, lit. Row 2: turned round, lit.CellBulbone waylitturned roundlit

A wrong idea many people have

The neutral wire is at about zero p.d., so it is safe to touch.

Why it feels right: The neutral wire is labelled as the return wire and sits close to zero p.d. in a working circuit.

The test that breaks it: A fault, or a break in the neutral wire, can leave the neutral wire at a high p.d. A wire you cannot see is working may carry the live p.d. You cannot tell by looking.

What is true: Treat every wire in a mains circuit as live until a trained person has made it safe. Never touch exposed wire in any socket, plug or appliance.

A wrong idea many people have

A fuse stops a person who touches a live wire from getting a shock.

Why it feels right: A fuse is a safety part, and with an earth wire it does cut off a faulty metal case.

The test that breaks it: A 13 A fuse lets 13 A flow before it melts. That is more than 200 times the 0.05 A that can kill, and a fuse can take a moment to melt.

What is true: A fuse melts at currents far above what can kill. It protects the wires from overheating, and with the earth wire it cuts off a faulty metal case. A residual current device, and keeping away from live parts, protect a person who touches a live wire.

A wrong idea many people have

In alternating current the electrons travel from the power station to the lamp.

Why it feels right: We say that electricity is sent from the power station, and we picture the electrons going the whole way.

The test that breaks it: With a 50 Hz supply the current reverses 100 times each second. An electron that moves forward for a hundredth of a second is moved back again for the next hundredth. It ends where it started.

What is true: The electrons shuffle to and fro about their places. The energy travels along the wire in the changing p.d. and reaches the appliance.

Key words

r.m.s.
root mean square. The r.m.s. value of an alternating p.d. is the steady p.d. that would give the same heating. The quoted mains p.d. is an r.m.s. value.
direct current
current that flows one way only, such as from a cell. It is written d.c.
alternating current
current that keeps reversing direction, such as from the mains. It is written a.c.
live
the wire that carries the alternating p.d. of the mains supply.
neutral
the wire that completes the circuit so the current can return.
earth
the safety wire that joins a metal case to the ground. It carries no current in normal use.

Back to your guess

The answer is C: A torch cell gives a few volts. The mains gives about 230 V, which can push a deadly current through the body.

A torch cell gives a few volts and cannot push a harmful current through dry skin. The mains gives hundreds of volts, and the current it can drive through the body can kill. The mains is also alternating, but it is the p.d. and the current that make it dangerous.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which of these describes alternating current?

    Tap your answer.

    1. Yes, that is right. In alternating current the p.d. and the current swing to and fro. The direction reverses many times each second.

    2. Not this one. That is direct current, as from a cell. In alternating current the direction keeps reversing. Try another.

    3. Not this one. That is a changing direct current. In alternating current the direction itself reverses. Try another.

    4. Not this one. In alternating current the size changes too: it rises, falls through zero and reverses. Try another.

    See the answer

    The current keeps reversing direction

    In alternating current the p.d. and the current swing to and fro. The direction reverses many times each second.

    • If you chose B: That is direct current, as from a cell. In alternating current the direction keeps reversing.
    • If you chose C: That is a changing direct current. In alternating current the direction itself reverses.
    • If you chose D: In alternating current the size changes too: it rises, falls through zero and reverses.

  2. The graph shows the p.d. of an a.c. supply against time. What is its frequency?

    A graph with the axes p.d. (V) and time (ms). The time runs from 0 to 80 ms in steps of 10 and the p.d. from −10 to 10 V in steps of 5. A smooth wave has a first peak of 10 V at 10 ms and its next peak at 50 ms. Labels: p.d. (V), time (ms).−10−5051001020304050607080p.d. (V)time (ms)
    Tap your answer.

    1. Not this one. You read the period in ms as a frequency in Hz. The frequency is 1 divided by the period in seconds. Try another.

    2. Not this one. You changed 40 ms to 0.04 s and stopped. The frequency is 1 ÷ 0.04, not 0.04. Try another.

    3. Yes, that is right. The peaks are at 10 ms and 50 ms, so the period is 40 ms = 0.04 s. Then f = 1 ÷ T = 1 ÷ 0.04 s = 25 Hz.

    4. Not this one. You read the length of the graph, 80 ms. The period is from one peak to the next. Try another.

    See the answer

    25 Hz

    The peaks are at 10 ms and 50 ms, so the period is 40 ms = 0.04 s. Then f = 1 ÷ T = 1 ÷ 0.04 s = 25 Hz.

    • If you chose A: You read the period in ms as a frequency in Hz. The frequency is 1 divided by the period in seconds.
    • If you chose B: You changed 40 ms to 0.04 s and stopped. The frequency is 1 ÷ 0.04, not 0.04.
    • If you chose D: You read the length of the graph, 80 ms. The period is from one peak to the next.

  3. Which wire joins the metal case of an appliance to the ground, as a safety wire?

    Tap your answer.

    1. Not this one. The live wire carries the supply p.d. into the appliance. A live wire touching the case is the fault we guard against. Try another.

    2. Not this one. The neutral wire completes the circuit for the working current. It is not the safety wire. Try another.

    3. Not this one. The fuse wire is the thin wire inside the fuse. It melts to break the circuit, and it does not go to the ground. Go to S4.12.7. Try another.

    4. Yes, that is right. The earth wire joins the metal case to the ground. If the live wire touches the case, a big current flows through it and melts the fuse.

    See the answer

    The earth wire

    The earth wire joins the metal case to the ground. If the live wire touches the case, a big current flows through it and melts the fuse.

    • If you chose A: The live wire carries the supply p.d. into the appliance. A live wire touching the case is the fault we guard against.
    • If you chose B: The neutral wire completes the circuit for the working current. It is not the safety wire.
    • If you chose C: The fuse wire is the thin wire inside the fuse. It melts to break the circuit, and it does not go to the ground. Go to S4.12.7.

  4. Where is a fuse joined, and why?

    Tap your answer.

    1. Not this one. The live and neutral wires carry the same current in normal use. A fuse in the neutral would leave the appliance joined to the live wire when it melts. Try another.

    2. Yes, that is right. A fuse in the live wire cuts the appliance off from the live supply when it melts. In the neutral wire it would leave the appliance live.

    3. Not this one. The earth wire carries no current in normal use. A fuse in it would not cut off the supply. Try another.

    4. Not this one. The current is the same, but where the fuse sits matters. If it is not in the live wire, the appliance stays joined to the live supply after it melts. Go to S3.10.8. Try another.

    See the answer

    In the live wire, so that when it melts the appliance is cut off

    A fuse in the live wire cuts the appliance off from the live supply when it melts. In the neutral wire it would leave the appliance live.

    • If you chose A: The live and neutral wires carry the same current in normal use. A fuse in the neutral would leave the appliance joined to the live wire when it melts.
    • If you chose C: The earth wire carries no current in normal use. A fuse in it would not cut off the supply.
    • If you chose D: The current is the same, but where the fuse sits matters. If it is not in the live wire, the appliance stays joined to the live supply after it melts. Go to S3.10.8.

  5. Sipho touches a live wire at 230 V with wet hands. The resistance of his body is 1 150 Ω. What current goes through him?

    Tap your answer.

    1. Yes, that is right. I = V ÷ R = 230 V ÷ 1 150 Ω = 0.2 A. That is four times the 0.05 A that can kill.

    2. Not this one. You divided the resistance by the p.d. The current is the p.d. divided by the resistance, I = V ÷ R. Try another.

    3. Not this one. You multiplied the p.d. by the resistance. Turn V = IR round to I = V ÷ R. Try another.

    4. Not this one. You added the two numbers. Divide the p.d. by the resistance. Try another.

    See the answer

    0.2 A

    I = V ÷ R = 230 V ÷ 1 150 Ω = 0.2 A. That is four times the 0.05 A that can kill.

    • If you chose B: You divided the resistance by the p.d. The current is the p.d. divided by the resistance, I = V ÷ R.
    • If you chose C: You multiplied the p.d. by the resistance. Turn V = IR round to I = V ÷ R.
    • If you chose D: You added the two numbers. Divide the p.d. by the resistance.

  6. Why is the mains never used for home experiments?

    Tap your answer.

    1. Not this one. Readings are not the reason. The danger is the size of the current the mains can drive through a person. Try another.

    2. Not this one. The energy used is not the problem. The danger is the current through the body. Try another.

    3. Not this one. A working fuse is not the reason. The mains p.d. can push a deadly current through the body before any fuse helps. Go to S2.8.5. Try another.

    4. Yes, that is right. About 0.05 A through the body can kill, and the mains p.d. can push far more through wet skin. Cells of a few volts cannot, so every experiment uses cells.

    See the answer

    A small current through the body can kill, and mains drives more

    About 0.05 A through the body can kill, and the mains p.d. can push far more through wet skin. Cells of a few volts cannot, so every experiment uses cells.

    • If you chose A: Readings are not the reason. The danger is the size of the current the mains can drive through a person.
    • If you chose B: The energy used is not the problem. The danger is the current through the body.
    • If you chose C: A working fuse is not the reason. The mains p.d. can push a deadly current through the body before any fuse helps. Go to S2.8.5.

  7. From an earlier chapter

    A heater takes a current of 9 A. The fuses available are 3 A, 5 A, 10 A and 13 A. Which fuse should it have?

    Tap your answer.

    1. Not this one. That is far below 9 A. It would melt every time the heater is used. Go to S4.12.7. Try another.

    2. Yes, that is right. The fuse must be just above the normal current of 9 A. The smallest rating above 9 A is 10 A.

    3. Not this one. That is well above 9 A. A fault could send much more than the normal current through the wires without melting it. Go to S4.12.7. Try another.

    4. Not this one. That is below 9 A. It would melt in normal use. Go to S4.12.7. Try another.

    See the answer

    The 10 A fuse

    The fuse must be just above the normal current of 9 A. The smallest rating above 9 A is 10 A.

    • If you chose A: That is far below 9 A. It would melt every time the heater is used. Go to S4.12.7.
    • If you chose C: That is well above 9 A. A fault could send much more than the normal current through the wires without melting it. Go to S4.12.7.
    • If you chose D: That is below 9 A. It would melt in normal use. Go to S4.12.7.

  8. The p.d. of a supply repeats every 2.5 ms. Find its frequency.

    See the answer

    400 Hz

    2.5 ms = 0.0025 s. f = 1 ÷ T = 1 ÷ 0.0025 s = 400 Hz.

  9. A p.d. of 3 V is across a body of resistance 6 000 Ω. Find the current, in milliamps.

    See the answer

    0.5 mA

    I = V ÷ R = 3 V ÷ 6 000 Ω = 0.0005 A = 0.5 mA.

Keep it

Copy this into your exercise book.

d.c. one way, a.c. keeps reversing. f = 1 ÷ T. Live: supply. Neutral: return. Earth: safety. Fuse in the live wire. Mains can kill: use cells only.

Draw: A metal cased kettle with a live, a neutral and an earth wire and a fuse in the live wire. Mark where a fault would put the live wire on the case.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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