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

Electrical circuits can become dangerous when the current exceeds the safe limit of the conductors or devices. Over‑current can cause:

• Excessive heating and possible fire.
• Damage to appliances.
• Electric shock or burns to users.

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

Fuses

A fuse is a sacrificial device that contains a thin metal strip. When the current exceeds the fuse’s rated current (\$I_{\text{rated}}\$) for a short time, the strip melts, opening the circuit.

How a Fuse Works

1. Normal operation: current \$I < I_{\text{rated}}\$, the metal strip remains intact.
2. Over‑current: \$I > I_{\text{rated}}\$ → heating \$P = I^{2}R\$ causes the strip to reach its melting temperature.
3. Melting: the strip breaks, the circuit opens, and current stops flowing.

Types of Fuses

• Cartridge fuses – cylindrical, used in domestic circuits.
• Blade (plug) fuses – used in portable appliances.
• Thermal fuses – protect against overheating, not over‑current.

Choosing a Fuse Rating

The fuse rating must be high enough to allow the normal operating current but low enough to protect the circuit. The general rule is:

\$ I{\text{fuse}} \approx 1.25 \times I{\text{normal}} \$

where \$I_{\text{normal}}\$ is the maximum expected load current.

Trip Switches (Circuit Breakers)

A trip switch is a reusable protective device that automatically opens a circuit when the current exceeds a preset value. It can be reset after the fault is cleared.

Operation Principles

1. Thermal (magnetic) trip – a bimetallic strip bends with heat generated by \$I^{2}R\$ losses, opening contacts.
2. Magnetic trip – a solenoid creates a magnetic force proportional to \$I\$; at high currents it pulls a latch open.
3. Many modern breakers combine both mechanisms for fast response to very high currents and slower response to moderate overloads.

Setting the Trip Current

The trip setting should be selected using the same principle as fuses, but with an additional safety margin for occasional surges (e.g., motor start‑up). A typical guideline is:

\$ I{\text{trip}} = 1.5 \times I{\text{normal}} \$

where \$I_{\text{normal}}\$ is the steady‑state load current.

Comparing Fuses and Trip Switches

Choosing Appropriate Ratings – Worked Example

Problem: A lighting circuit is designed to supply a maximum load of 8 A at 240 V. Select a suitable fuse rating and a trip‑switch setting.

1. Calculate the normal operating current (already given as 8 A).
2. Fuse rating:

   \$ I_{\text{fuse}} = 1.25 \times 8\ \text{A} = 10\ \text{A} \$

   Choose the next standard rating, e.g., 13 A fuse.

3. Trip‑switch setting:

   \$ I_{\text{trip}} = 1.5 \times 8\ \text{A} = 12\ \text{A} \$

   Select a 15 A circuit breaker (standard size).

Key Points to Remember

• Protective devices must be rated slightly above the normal load current.
• Fuses are simple, cheap, and replaceable; trip switches are more convenient for frequent faults.
• Always consider inrush currents (e.g., motors) when selecting trip‑switch settings.
• Use the correct type and rating for the specific circuit to avoid nuisance tripping or insufficient protection.

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.

Answers to Practice Questions

1. Normal current \$I = \dfrac{1500\ \text{W}}{240\ \text{V}} = 6.25\ \text{A}\$.

   Fuse rating \$=1.25 \times 6.25\ \text{A}=7.8\ \text{A}\$ → choose a standard 10 A fuse.

2. Magnetic trips react instantly to very high currents, clearing faults before the thermal element can overheat. This rapid response limits damage to motor windings and reduces fire risk.
3. The fuse’s metal strip heats rapidly due to \$I^{2}R\$ losses, reaches its melting point, and the strip melts, opening the circuit. The fuse must then be replaced before the circuit can be used again.