Explain the use and operation of trip switches and fuses and choose appropriate fuse ratings and trip switch settings

4.4 Electrical Safety

Learning Objective

Explain the use and operation of trip switches and fuses, and choose appropriate fuse ratings and trip‑switch settings.

Why Electrical Safety Matters

When the current in a circuit exceeds the safe limit of the conductors or devices, the circuit can become hazardous. Over‑current can cause:

• Excessive heating → fire risk.
• Damage to appliances or wiring.
• Electric shock or burns to users.

Protective devices such as fuses and trip switches are installed to interrupt the circuit before damage occurs.

Common Electrical Hazards (AO1)

Mains Wiring Basics (AO1)

In most domestic installations the supply consists of three conductors:

• Live (L) – carries the supply voltage (e.g., 240 V AC).
• Neutral (N) – returns current to the source and is near earth potential.
• Earth (E) – provides a low‑resistance path for fault currents.

Safety rule: Switches and protective devices must be placed on the live conductor only. This ensures that when the switch is OFF the appliance’s live parts are at earth potential, reducing shock risk.

Typical UK/EU Plug & Socket (illustrative)

Wiring Colour‑Code (UK example)

Appliance Classification (AO1)

• Class I (earthed) appliances – metal outer casing is connected to the earth wire.
• Class II (double‑insulated) appliances – outer casing is non‑conducting or has two layers of insulation; no earth connection is required.

A fuse placed on the live conductor protects both Class I and Class II appliances. Class II appliances rely solely on the fuse because their casing cannot become live.

Fuses

How a Fuse Works

1. Normal operation: Current I < I_rated. The thin metal strip remains solid.
2. Over‑current: I > I_rated. Power dissipated as heat (P = I²R) raises the strip temperature.
3. Melting: When the strip reaches its melting point it breaks, opening the circuit.

Common Types of Fuses

• Cartridge (cylindrical) fuses – used in fixed domestic circuits.
• Blade (plug) fuses – used in portable appliances.
• Thermal fuses – open when a specific temperature is reached (protect against overheating, not over‑current).

Choosing a Fuse Rating (AO1)

Step‑by‑step checklist

1. Determine the normal load current: I_normal = P / V (or read the appliance rating).
2. Apply the safety factor: I_fuse ≈ 1.25 × I_normal.
3. Round up to the next standard fuse size.
4. Confirm that the fuse’s voltage rating is at least as high as the supply voltage.

Standard Fuse Sizes (UK example)

Trip Switches (Circuit Breakers)

Operation Principles

1. Thermal (bimetallic) trip: Heat from I²R losses bends a bimetal strip, opening the contacts after 0.5–5 s.
2. Magnetic (instantaneous) trip: A solenoid produces a magnetic force proportional to current; at very high currents (≈ 10 × rated) it pulls a latch open within ≤ 0.1 s.
3. Combined type: Uses both mechanisms – magnetic for short‑duration short‑circuits, thermal for moderate overloads.

Setting the Trip Current (AO1)

Guideline for overload protection:

Choose a breaker whose rated trip current is the next standard size above this value, allowing for occasional inrush currents (e.g., motor start‑up).

Standard Trip‑Switch Ratings (UK example)

Trip‑Current vs. Rated Current

• Magnetic element trips instantly for currents roughly 10 × rated (short‑circuit protection).
• Thermal element trips after a few seconds for currents about 1.2–1.5 × rated (overload protection).

Comparing Fuses and Trip Switches

Worked Example – Selecting Protective Devices

Problem: A lighting circuit must supply a maximum load of 8 A at 240 V. Choose a suitable fuse rating and a trip‑switch setting.

1. Normal operating current: I_normal = 8 A.
2. Fuse rating:

   I_fuse ≈ 1.25 × 8 A = 10 A

   Next standard size → 13 A fuse.

3. Trip‑switch setting:

   I_trip ≈ 1.5 × 8 A = 12 A

   Next standard size → 15 A circuit breaker.

Practical Activity – Demonstrating Fuse Operation (AO3)

1. Set up a low‑voltage DC supply (≤ 12 V) with a variable resistor (or a set of resistors) and a replaceable plug‑type fuse.
2. Measure the current with an ammeter while gradually increasing the load.
3. Record the current at which the fuse blows and compare it with the fuse’s rated current.
4. Explain why the fuse opened (heating of the metal strip) and how this protects the rest of the circuit.

Key Points to Remember

• Protective devices are rated slightly above the normal load current, then rounded up to the next standard size.
• Fuses are simple, cheap, and replaceable; trip switches are reusable and give faster response to very high currents.
• Consider inrush currents (e.g., motors) when selecting trip‑switch settings.
• Only the live conductor is switched; neutral and earth remain continuous.
• Class I appliances must be earthed; Class II appliances rely on double‑insulation.
• Trip switches have two protective actions: magnetic (instantaneous, ≈ 10 × rated) and thermal (overload, ≈ 1.2–1.5 × rated).
• Breakers require periodic testing; fuses do not.

Practice Questions

1. A heater draws 1500 W from a 240 V supply. Determine a suitable fuse rating.
2. Explain why a magnetic trip element is preferred for protecting motor circuits.
3. In a circuit protected by a 5 A fuse, a short circuit causes a current of 30 A. Describe what happens to the fuse and why.
4. Identify three hazards from the “Common Electrical Hazards” table and suggest a simple preventive measure for each.

Answers to Practice Questions

1. Normal current I = P/V = 1500 W / 240 V = 6.25 A.
   

   Fuse rating ≈ 1.25 × 6.25 A = 7.8 A → choose the next standard size, 10 A fuse.

2. Magnetic trips react instantly (≤ 0.1 s) to the very high currents drawn at motor start‑up, clearing the fault before the thermal element can overheat. This rapid action limits damage to the motor windings and reduces fire risk.
3. The 5 A fuse experiences I²R heating far above its design limit. The metal strip reaches its melting temperature, melts, and the circuit opens. The fuse must be replaced before the circuit can be used again.
4. Examples:
   

   • Damaged insulation – keep cables in good condition and replace any with exposed conductors.
   

   • Over‑loading sockets – never plug more appliances into a single socket than its rating; use a correctly rated extension lead.
   

   • Damp conditions – avoid using electrical equipment with wet hands or in wet areas unless it has proper IP rating or residual‑current protection.