Explain why the outer casing of an electrical appliance must be either non-conducting (double-insulated) or earthed

4.4 Electrical Safety

Objective

Explain why the outer casing of an electrical appliance must be either non‑conducting (double‑insulated) or earthed.

Why the Casing Must Not Be Live

If the outer casing becomes live and a person touches it, a current will flow through the body to the ground. The severity of the shock depends on the voltage, the resistance of the body and the duration of the fault.

Ohm’s law gives the current that can flow:

\$ I = \frac{V}{R} \$

where \(V\) is the fault voltage and \(R\) is the total resistance of the path through the person to earth. Even a few milliamps can cause muscular contraction; currents above 30 mA can be fatal.

Two Safe Design Approaches

1. Non‑conducting (double‑insulated) casing
2. Earthed (protectively earthed) casing

1. Double‑Insulated Appliances

Double insulation means that two separate layers of insulating material surround all live parts. If the first layer fails, the second layer still prevents the live part from reaching the outer casing.

• Typical for handheld tools, hair dryers, electric shavers.
• No earthing connection is required.
• Identified by a square within a square symbol.

2. Earthed Appliances

When the outer casing is made of a conductive material (e.g., metal), it is connected to earth via an earthing conductor. If a live part contacts the casing, the fault current is diverted directly to earth, causing a large current to flow through the protective earth (PE) conductor.

• The high fault current trips the circuit‑breaker or blows a fuse, disconnecting the supply.
• Earthing also limits the voltage that can appear on the casing to a safe value (typically < 50 V).
• Identified by the earth‑symbol (three lines forming a triangle).

Comparison of Double‑Insulation and Protective Earthing

How Earthing Limits Shock \cdot oltage

When a fault occurs, the earth wire provides a low‑resistance path. The voltage that appears on the casing is given by the voltage divider formed by the earth‑wire resistance (\(R{PE}\)) and the body resistance (\(R{body}\)):

\$ V{casing} = V{supply} \times \frac{R{PE}}{R{PE}+R_{body}} \$

Because \(R{PE}\) is made very small (typically < 0.1 Ω), \(V{casing}\) is kept well below hazardous levels.

Safety Devices that Work with Earthing

• Fuses – melt when fault current exceeds rating.
• Circuit‑breakers – mechanically open the circuit under over‑current.
• Residual‑Current Devices (RCDs) – detect imbalance between live and neutral currents (typically 30 mA) and disconnect supply within 30 ms.

Key Points to Remember

• The outer casing must never become a source of live voltage.
• Double insulation provides two independent barriers; earthing provides a low‑impedance path to ground.
• Both methods aim to protect users from electric shock and to ensure that any fault quickly disconnects the supply.