State that a fuse without an earth wire protects the circuit and the cabling for a double-insulated appliance

4.4 Electrical Safety

Objective

State the hazards associated with damaged insulation, overheating cables, damp conditions and excess current, and explain why a fuse (without an earth wire) protects the circuit and the flexible cord of a double‑insulated appliance.

Glossary (quick‑reference box)

Key Statement

A fuse fitted in the plug of a double‑insulated appliance opens the circuit before the current can raise the temperature of the appliance’s flexible cord or the fixed wiring upstream to a hazardous level, even though the appliance has no earth connection.

1. Hazards in a Mains Circuit

2. The Mains Supply and Switching

• Live (L) – carries the full supply voltage.
• Neutral (N) – returns current; essentially at earth potential.
• Earth (E) – provides a low‑resistance path for fault currents, protecting the user.
• The domestic switch is placed on the live conductor so that, when OFF, the live part of the appliance is isolated from the supply, eliminating accidental contact.

3. Double‑Insulated (Class II) Appliances

• Two independent layers of protective insulation (or reinforced insulation) plus a non‑conducting outer case.
• If the basic insulation fails, the second layer prevents live parts from becoming exposed.
• Marked with the double‑square symbol (⧈); no earth pin is required in the plug.
• Safety is achieved without relying on an earth connection.

4. Over‑Current Protection

4.1 How a Fuse Works

• Contains a thin metal strip that melts when the current exceeds its rated value \(I_f\).
• Melting opens the circuit, stopping the flow of excessive current.
• Once blown, the fuse must be replaced.

4.2 How a Trip‑Switch (Circuit‑Breaker) Works

• Uses an electromagnet or bimetallic strip that trips (opens) when the current exceeds a preset value.
• Can be reset manually after the fault is cleared.
• Provides the same over‑current protection as a fuse but is reusable.

4.3 Coordination Between Fuse and Trip‑Switch

In a domestic installation the consumer‑unit breaker (typically 32 A or 63 A) is set higher than the plug fuse (3 A, 5 A or 13 A). This ensures that the fuse in the plug will open first for a fault inside the appliance, protecting the flexible cord, while the circuit‑breaker protects the fixed wiring if a fault occurs upstream of the plug.

4.4 Choosing the Correct Fuse Rating

Rule of thumb for a single appliance:

1. Calculate the normal operating current: \[I_{op}= \frac{P}{V}\] where \(P\) is the appliance power (W) and \(V\) is the mains voltage (V).
2. Select a fuse rating of about 1.25 × \(I_{op}\), but never exceed the rating of the flexible cord or the socket circuit.

Worked Example 1 – High‑power appliance

Hair dryer: \(P = 1200\; \text{W}\), \(V = 230\; \text{V}\)

\[

I_{op}= \frac{1200}{230}\approx 5.2\; \text{A}

\]

A standard 13 A plug fuse is suitable (13 A > 1.25 × 5.2 ≈ 6.5 A). If the heating element shorts, the current could rise to ≈30 A; the 13 A fuse will melt, protecting the cord and upstream wiring.

Worked Example 2 – Low‑power appliance

Electric shaver: \(P = 30\; \text{W}\), \(V = 230\; \text{V}\)

\[

I_{op}= \frac{30}{230}\approx 0.13\; \text{A}

\]

A 3 A fuse (the smallest UK plug fuse) is appropriate because 3 A > 1.25 × 0.13 ≈ 0.16 A.

What‑If Question

If the same hair dryer is used on a 240 V supply (common in some EU countries), what is the new operating current and is the 13 A fuse still adequate?

\[

I_{op}= \frac{1200}{240}=5.0\; \text{A}

\]

1.25 × 5.0 = 6.25 A < 13 A, so the 13 A fuse remains suitable.

5. Practical Investigation – “When Does a Fuse Blow?”

Aim: To observe the current at which a low‑voltage fuse opens and compare it with the fuse’s rated current.

1. Set up a variable DC power supply (≤ 12 V) with a current‑meter in series.
2. Connect a removable 5 A fuse in series with the supply and a resistive load (e.g., a power resistor).
3. Increase the supply voltage gradually while watching the current reading.
4. Record the voltage and current at the instant the fuse blows.
5. Repeat with a 3 A fuse and discuss any differences.

Safety Checklist (complete before the experiment)

• Ensure the power supply is switched OFF before making connections.
• Use only low‑voltage (≤ 12 V) equipment – no mains voltage.
• Wear insulated gloves and safety glasses.
• Verify that the resistive load’s power rating exceeds the expected dissipation.
• Keep a fire‑extinguishing blanket nearby.

6. Why a Fuse Without an Earth Wire Is Sufficient for a Double‑Insulated Appliance

• The appliance’s safety does not depend on an earth connection because live parts are fully enclosed by double insulation or a non‑conducting case.
• If a fault occurs (e.g., internal short or damaged cord), the current rises sharply.
• The fuse in the plug melts, opening the circuit before the flexible cord or the fixed wiring upstream can heat to a dangerous temperature.
• Thus the fuse protects both the appliance’s cord and the building’s wiring, even though the appliance has no earth pin.

7. Over‑Loading in a Domestic Ring Circuit (Illustrative Paragraph)

A typical UK domestic ring circuit is rated at 32 A and supplies a series of wall sockets. Each socket can accept a plug with a 13 A fuse. If several high‑wattage appliances (e.g., a kettle 3 kW, a toaster 1.2 kW, and a hair dryer 1.2 kW) are used simultaneously, the total current drawn is:

\[

I_{total}= \frac{3000+1200+1200}{230}\approx 23.5\; \text{A}

\]

Adding a further 2 kW iron would raise the total to ≈33 A, exceeding the ring‑circuit rating. The consumer‑unit circuit‑breaker (32 A) would then trip, protecting the fixed wiring. Individual plug fuses still protect each appliance’s cord.

Suggested Diagram (insert in teaching material)

Show a ring circuit loop with multiple sockets, a 32 A breaker, and a few plugged‑in appliances. Highlight the current flow and the point where the breaker would trip if the total exceeds 32 A.

8. Brief Note on Residual‑Current Devices (RCDs)

Although not required for the specific sub‑topic, most domestic installations also include an RCD (or Earth‑Leakage Circuit‑Breaker). The RCD monitors the difference between live and neutral currents and trips within 30 ms if an imbalance (typically > 30 mA) is detected, providing extra protection against electric shock, especially in damp locations.

9. Summary Diagram (suggested)

10. Check Your Understanding

1. Why does a double‑insulated appliance not need an earth connection?
2. List the four hazards mentioned in the syllabus and give a brief description of each.
3. What would happen to the circuit if the fuse were omitted from a double‑insulated appliance?
4. Explain how a trip‑switch differs from a fuse and when you might choose one over the other.
5. How do you decide the correct fuse rating for a single appliance? Include the calculation steps.
6. In a 32 A ring circuit, three appliances draw 3 kW, 1.2 kW and 1.2 kW respectively. Is the circuit safe? Show your calculation.
7. What additional protection does an RCD provide that a fuse does not?

Answers (for teacher use)

1. Because it has two independent layers of protective insulation (or a non‑conducting case), which prevent live parts from being exposed, eliminating the need for a protective earth.

2. • Damaged insulation – exposed live conductor can cause shock or fire.
   • Over‑heating of cables – excess current raises temperature, risking melt and fire.
   • Damp conditions – moisture reduces resistance, increasing leakage current and shock risk.
   • Excess current (over‑loading) – too many devices on a circuit draw more current than the wiring can safely carry, leading to overheating.

3. Without a fuse, an over‑current fault could allow the flexible cord and the fixed wiring to overheat, potentially melting insulation and causing fire or damaging the appliance.
4. A trip‑switch (circuit‑breaker) trips open when current exceeds a set value but can be reset, whereas a fuse melts and must be replaced. Trip‑switches are preferred for whole‑circuit protection in the consumer unit; fuses are used in plugs and for simple, low‑cost protection of individual appliances.
5. Calculate the appliance’s normal operating current \(I_{op}=P/V\). Multiply by 1.25 to give a safe fuse rating, then ensure the chosen rating does not exceed the rating of the flexible cord or the socket circuit.
6. Total current \(= (3000+1200+1200)/230 \approx 23.5\; \text{A}\). This is below the 32 A rating, so the ring circuit remains safe. Adding another 2 kW device would give \((3000+1200+1200+2000)/230 \approx 33\; \text{A}\), which exceeds the rating and would cause the 32 A breaker to trip.
7. An RCD detects an imbalance between live and neutral currents (typically > 30 mA) and trips within 30 ms, protecting users from electric shock caused by earth‑leakage faults—something a fuse does not detect.