PhysicsICSE Class 10 · Household Circuits

Earthing and the Three-Pin Plug: Need, Socket Positions and Colour Codes (ICSE Class 10)

Updated 16 October 2026 · Fact-checked

Earthing connects the metal body of an appliance to the earth through a low-resistance earth wire. If the live wire touches the body, a large current flows to earth and blows the fuse, so the body stays at earth potential and no one gets a shock. In a three-pin plug the earth pin is longer and thicker.

Understand Earthing and the Three-Pin Plug

Three wires in a house.

Wire	Potential	New colour	Old colour	Connected to
Live (L, phase)	High (220 V AC)	Brown	Red	Fuse and switch, then the appliance
Neutral (N)	About zero (earthed at the substation)	Light blue	Black	Other end of the appliance
Earth (E)	Zero	Green or green-and-yellow	Green	Metal body of the appliance

The new (international) convention is the one asked for unless a question says otherwise. The live and neutral carry the working current; the earth wire normally carries no current.

How an earth connection is made. A thick copper wire is joined to a copper plate buried deep in the ground near the house. The plate is surrounded by a mixture of charcoal and common salt, which keeps the soil around it moist and conducting, so the earth connection has a very low resistance. The earth wire runs through the house to the top hole of every three-pin socket.

Why earthing is needed. Suppose the insulation of the live wire inside an iron wears away and the wire touches the metal body.

Without earthing: the body is now at the potential of the live wire. A person who touches it provides a path to the ground, and current flows through the body: a severe electric shock.
With earthing: the body is connected to earth by a wire of very low resistance. A very large current flows from the live wire, through the body, to earth ( $I = \frac{V}{R}$ with $R$ tiny). This blows the fuse (or trips the MCB) at once, cutting off the supply. Meanwhile the body stays at earth potential, so touching it is safe.

Appliances with metal bodies, such as an iron, a refrigerator, a washing machine, a geyser, a toaster, a microwave oven and a cooler, must be earthed. Earthing works together with the fuse: the earth wire creates the large fault current; the fuse then disconnects the live wire.

The three-pin plug. It has three pins: live (L), neutral (N) and earth (E). The earth pin is longer and thicker than the other two.

Longer: it enters the socket first and leaves it last, so the metal body is earthed before the appliance is connected to the live wire, and stays earthed until the live is disconnected.
Thicker: it cannot be pushed into the live or neutral hole by mistake.

The three-pin socket (seen from the front).

Earth hole: at the top (the larger hole).
Live hole: on the right.
Neutral hole: on the left.

When the plug is pushed in, each pin meets the hole of the same name.

Wiring an appliance correctly. The brown (live) wire goes through the fuse and the switch to the appliance; the light blue (neutral) wire goes to the other end of the appliance; the green-and-yellow (earth) wire goes to the metal body. When checking a wiring diagram, check both things: the switch must be on the live (brown) side, and the colours must go to the right pins.

Safety precautions with plugs and earthing. Use a three-pin plug for any metal-bodied appliance; never remove or bend the earth pin; check that the earth wire is firmly connected; do not use a two-pin adaptor for an appliance that needs earthing; keep plugs and sockets dry and never touch them with wet hands.

Earthing and the Three-Pin Plug: formulas and key facts

New colour code: Live brown
Neutral light blue
Earth green or green-and-yellow; International convention.
Old colour code: $L i v e : r e d ∣ N e u t r a l : b l a c k ∣ E a r t h : g r ee n$; Still found in older wiring.
Socket, front view: Earth top (larger hole)
Live right
Neutral left; Plug pins match these holes.
Fault current through the earth wire: $I = \frac{V}{R _{e a r t h p a t h}}$; Very large because R is very small; blows the fuse.

Live	brown
Neutral	light blue
Earth	green or green-and-yellow

Earth	top (larger hole)
Live	right
Neutral	left

How to solve Earthing and the Three-Pin Plug questions

Most questions are explanations; give the reason in terms of potential and current, not just the fact.

1Identify the three wires by colour, using the new code unless told otherwise.
2For 'why earthing', describe the fault (live touches the metal body), then what happens with and without earthing.
3For a fault current, use $I = \frac{V}{R}$ with the resistance of the earth path, and compare it with the fuse rating.
4For a wiring diagram, check the switch is in the live (brown) wire AND every colour goes to the right place.
5For the earth pin, give both features (longer, thicker) with a reason for each.

Earthing and the Three-Pin Plug in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

The tingling washing machine

Suppose you feel a slight tingling every time you touch the metal top of your washing machine, though it works normally.

What could be wrong, and why is it dangerous?

Try it first, then show the solutionSolution

Some current is leaking from the live wire to the metal body, and the body is not properly earthed (the earth wire may be loose or broken).
If the leak grows, the body can reach a high potential and give a severe shock.
Switch off at the main switch and have the earth connection and the insulation checked.

Answer: A leakage with a faulty earth connection; get it repaired before using the machine.

Memory hook: Tingle means trouble: check the earth.

The cut-off earth pin

Imagine someone breaks off the earth pin of a refrigerator's plug so that it fits a two-pin socket.

Explain why this is unsafe even though the refrigerator still runs.

Try it first, then show the solutionSolution

The live and neutral pins still connect, so the refrigerator works.
Its metal body is no longer earthed.
If the live wire ever touches the body, no large current flows to earth, the fuse does not blow, and anyone touching the body gets a shock.

Answer: The safety path is lost; a fault would leave the body live.

Memory hook: Works fine is not the same as safe.

Tricks to remember Earthing and the Three-Pin Plug

Short memory aids. Say each one aloud once, then use it on a question the same day.

Brown is Live, Blue is Neutral
'Brown Lives, Blue Never shocks, Green-Yellow grounds.'
Use it when: New colour code.
ELN clockwise from the top
Socket front: Earth at the top, then Live on the right, Neutral on the left. 'Left is Neutral, Right is Live.'
Use it when: Socket positions.
First in, last out
The long earth pin is the first to connect and the last to leave.
Use it when: Why the earth pin is longer.
Earth makes the current, fuse makes the cut
The earth wire carries the big fault current; the fuse melts because of it.
Use it when: Explaining how earthing protects.

Earthing and the Three-Pin Plug: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: The thicker earth pin · 1 mark · MCQBoard level

The earth pin of a three-pin plug is made thicker than the other two pins so that:
(a) it carries a larger current
(b) it cannot be inserted into the live or neutral hole
(c) it is the last to connect
(d) it has a higher resistance

Try it first, then show the solutionSolution

Being thicker, it fits only the larger earth hole. (Being longer makes it connect first, not last; it carries no current normally.)

Answer: (b) it cannot be inserted into the live or neutral hole

Example 2: What earthing is and how it is done · 3 marks · Short answerBoard level

(a) What is meant by earthing an appliance? (b) Describe how the earth connection of a house is made. (c) Why is the earth plate packed with charcoal and salt?

Try it first, then show the solutionSolution

(a) Connecting the metal body of the appliance, through the earth wire, to the earth. (1)
(b) A thick copper wire is joined to a copper plate buried deep in the ground near the house. (1)
(c) Charcoal and salt keep the surrounding soil moist and conducting, so the earth connection has a very low resistance. (1)

Answer: (a) Body joined to earth (b) Copper wire to a buried copper plate (c) Keeps the soil moist and conducting

Example 3: Reading an appliance flex · 2 marks · Short answerHarder board level

The flex of an electric kettle has three wires: brown, light blue and green-and-yellow. (a) Which wire is connected through the switch and fuse? (b) Which is connected to the metal body? (c) Which pin of the plug does the light blue wire go to? (d) Which of the three normally carries no current?

Try it first, then show the solutionSolution

(a) Brown (live). (b) Green-and-yellow (earth). (1)
(c) The neutral pin. (d) The green-and-yellow earth wire. (1)

Answer: (a) Brown (b) Green-and-yellow (c) Neutral (d) Earth wire

Example 4: Fault current through the earth wire · 3 marks · Short answerHarder board level

In a 220 V geyser, the live wire touches the metal body. The earth wire and earth plate together have a resistance of 1.1 Ω. The circuit has a 15 A fuse. (a) Find the current that flows to earth. (b) What happens to the fuse? (c) What would happen to a person touching the body if it were not earthed?

Try it first, then show the solutionSolution

(a) $I = \frac{V}{R} = \frac{220}{1.1} = 200$ A. (1)
(b) This is more than 13 times the 15 A rating, so the fuse melts at once and the supply is cut off. (1)
(c) The body would be at the live potential; current would flow through the person to the ground, giving a severe shock. (1)

Answer: (a) 200 A (b) It melts at once (c) The person would get a severe shock

Example 5: Order of the pins · 2 marks · Short answerHarder board level

A three-pin plug is pushed into a socket and later pulled out. In what order do its pins make and break contact, and why does this order matter for safety?

Try it first, then show the solutionSolution

The longer earth pin enters the socket first, so the metal body is earthed before the appliance is connected to the live wire. (1)
It leaves last, so the body stays earthed until the live wire has been disconnected. (1)

Answer: It connects first and disconnects last, so the body is always earthed while the appliance is live.

Example 6: Earth and live swapped · 3 marks · Short answerHOTS / competency

While wiring a plug for a metal-bodied iron, a student connects the brown wire to the earth pin and the green-and-yellow wire to the live pin. The neutral is correct. (a) Will the iron heat up when plugged in and switched on? (b) Why is this wiring extremely dangerous?

Try it first, then show the solutionSolution

(a) The heating element (on the brown wire) is now between the earth pin and the neutral pin, both at about zero potential, so almost no current flows and the iron does not heat. (1)
(b) The green-and-yellow wire, joined to the metal body, now goes to the live pin. The whole body is at the live potential as soon as the plug is in, even with the switch off. (1)
No fault current flows, so the fuse does not blow; anyone touching the iron gets a shock. (1)

Answer: (a) No (b) The metal body becomes live and the fuse cannot protect

Example 7: Case study: earthing a washing machine · 4 marks · Case-basedHOTS / competency

A 2 kW washing machine with a metal body works on 220 V and is protected by a 15 A MCB. Its earth path has a resistance of 2 Ω.
(a) Name the wire, and its colour under the new convention, connected to the metal body.
(b) Find the normal current of the machine.
(c) The live wire touches the body. Find the current to earth and state what the MCB does.
(d) Over time the earth wire breaks without anyone noticing. Explain why the machine still works and why it is now unsafe.

Try it first, then show the solutionSolution

(a) The earth wire, green or green-and-yellow. (1)
(b) $I = \frac{2000}{220} \approx 9.09$ A, within the 15 A rating. (1)
(c) $I = \frac{220}{2} = 110$ A, far above 15 A, so the MCB trips at once and cuts off the live supply. (1)
(d) The earth wire carries no current in normal use, so the machine works. But if the live touches the body now, no large current flows, the MCB does not trip, and the body stays live: touching it gives a shock. (1)

Answer: (a) Earth, green-and-yellow (b) ≈ 9.09 A (c) 110 A; MCB trips (d) Earth wire normally idle; a fault would leave the body live

Where students lose marks in Earthing and the Three-Pin Plug

The slips examiners see most often, and what to write instead.

Mistake: Old and new colour codes mixed
The new code (brown, light blue, green-and-yellow) is different from the old one (red, black, green).
Wrong working
'The switch is on the red wire and the body on the black wire' in a new-code question.
Right working
New code: switch on brown (live); body on green or green-and-yellow (earth).
Do this: Learn both codes side by side and read which one the question asks for.
Usually costs: Reported by CISCE examiners for 2025 Q3(ii).
Mistake: Checking only one condition in a wiring diagram
A correct diagram needs both the switch on the live side and the right colours at the mains.
Wrong working
Choosing the first option with the switch on the brown wire without checking the colours going to the mains.
Right working
Check switch position AND colour code in every option.
Do this: Tick off both conditions for each option before answering.
Usually costs: Reported by CISCE examiners for 2024 Q1(ix).
Mistake: Saying earthing 'stops the current'
Earthing creates a large fault current; it is the fuse that then breaks the circuit.
Wrong working
'Earthing stops current from reaching the body.'
Right working
'A large current flows to earth, the fuse blows, and the body stays at zero potential.'
Do this: Mention the fault, the earth current and the fuse in that order.
Usually costs: Usually costs the reasoning mark.
Mistake: Only one reason for the earth pin
Longer and thicker serve two different purposes.
Wrong working
'The earth pin is longer so that it does not go into the wrong hole.'
Right working
'Longer: connects first, disconnects last. Thicker: cannot enter the live or neutral hole.'
Do this: Pair each feature with its own reason.
Usually costs: Usually costs one mark.

Exam tips

When drawing a three-pin socket, label E at the top, L on the right and N on the left (front view).
Say 'new convention' or 'old convention' when you give colours.
In an earthing answer, use the words 'low resistance', 'large current', 'fuse blows' and 'earth potential'.
In MCQs on wiring diagrams, check every option, even if the first looks right.

Practice questions on Earthing and the Three-Pin Plug

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which pair is correctly matched under the new colour convention?
(a) live: red
(b) neutral: black
(c) earth: green-and-yellow
(d) neutral: brown
Hint
New: brown, light blue, green-and-yellow.
Answer
(c) earth: green-and-yellow
Q2 · 1 mark · Very short answerBoard level
Name two household appliances that must be earthed.
Hint
Metal bodies.
Answer
Any two with metal bodies, such as an electric iron, a refrigerator, a washing machine, a geyser, a toaster or a cooler.
Q3 · 1 mark · Very short answerBoard level
Which of the two supply wires, live or neutral, is at a high potential?
Hint
The neutral is earthed at the substation.
Answer
The live wire (the neutral is at about zero potential).
Q4 · 1 mark · Very short answerHarder board level
Why is the earth plate surrounded by a mixture of charcoal and salt?
Hint
Moisture.
Answer
To keep the surrounding soil moist and conducting, so that the earth connection has a very low resistance.
Q5 · 2 marks · Short answerHarder board level
Why must the earth wire have a very low resistance?
Hint
$I = \frac{V}{R}$ in a fault.
Answer
So that, if the live wire touches the body, a very large current flows to earth and blows the fuse at once, and the body stays at earth potential.
Q6 · 2 marks · Short answerHarder board level
The live wire touches the earthed metal body of a 230 V appliance. The earth path has a resistance of 0.5 Ω and the circuit fuse is rated 5 A. Find the fault current and say what happens.
Hint
$I = \frac{V}{R}$ .
Answer
$I = \frac{230}{0.5} = 460$ A, about 92 times the fuse rating, so the fuse melts at once.
Q7 · 2 marks · Short answerHarder board level
Why is the switch of an appliance connected to the brown wire?
Hint
Brown is live.
Answer
Brown is the live wire. With the switch in the live wire, turning it off disconnects the appliance from the high potential, so it is safe to touch or repair.
Q8 · 2 marks · Short answerHarder board level
In a three-pin socket seen from the front, an electrician finds the brown wire connected to the left-hand hole and the light blue to the right-hand hole. Is this correct? Explain.
Hint
Live right, neutral left.
Answer
No. Seen from the front, live (brown) should be on the right and neutral (light blue) on the left; the two wires have been interchanged.
Q9 · 2 marks · Short answerHOTS / competency
Many hair dryers with plastic bodies are fitted with two-pin plugs. Why is an earth connection less necessary for them?
Hint
What happens if a live wire touches plastic?
Answer
Their outer body is an insulator, so even if a live wire inside touches the casing, the outside does not become live and the user is not shocked. Earthing protects metal (conducting) bodies.
Q10 · 2 marks · Short answerHOTS / competency
An earthed refrigerator works normally for years, then a fault makes its body live and the user gets a shock. The fuse has not blown. Suggest what had gone wrong with the earthing, and explain your answer.
Hint
Why did no large current flow?
Answer
The earth connection had failed (a broken or loose earth wire, or a dry, high-resistance earth plate). Without a low-resistance path, no large current flowed to earth, so the fuse did not melt and the body stayed live.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Colour of the live wire (new code)?Show answer
Brown.
2.Colour of the earth wire (new code)?Show answer
Green or green-and-yellow.
3.Position of the earth hole in a socket?Show answer
At the top.
4.Why is the earth pin longer?Show answer
It connects first and disconnects last.
5.What surrounds the earth plate?Show answer
Charcoal and salt.

Earthing and the Three-Pin Plug: frequently asked questions

What is earthing and why is it necessary?

Earthing connects the metal body of an appliance to the earth through a low-resistance wire. If the live wire touches the body, a large current flows to earth, the fuse blows, and the body stays at earth potential, so the user is not shocked.

Why is the earth pin of a three-pin plug longer and thicker?

It is longer so that it connects first and disconnects last, keeping the body earthed whenever the appliance is live. It is thicker so that it cannot be pushed into the live or neutral hole.

What are the colour codes of household wires?

New convention: live brown, neutral light blue, earth green or green-and-yellow. Old convention: live red, neutral black, earth green.