Fig. 4.1 shows an electric drill. The symbol on the rating plate indicates that the drill is double insulated. (a) Describe what is meant by double insulati...

(a) Double insulation means the appliance has two separate layers of insulation. [1] The live internal components are surrounded by their own insulation, and the entire outer casing is also made of an insulating material such as plastic. [1]

The double-insulation symbol (a square inside a square, as shown on the rating plate) indicates this construction. Even if the inner insulation fails, the outer insulating case still prevents any conducting path to the user.

(b) Since the outer case is made of an insulating material, it can never become live, even if an internal wire comes loose. [1] There is therefore no conducting path from the live parts through the case to the user, so no earth connection is needed. [1]

An earth wire is only necessary when a metal case could become live. With a plastic case providing a second insulating barrier, the risk of the case conducting electricity is eliminated.

(c) The colour of the neutral wire is blue. [1]

The three wire colours in the UK standard are: live = brown, neutral = blue, earth = green and yellow stripes.

(d) A circuit breaker (or residual current device, RCD). [1]

A circuit breaker operates like a fuse in that it disconnects the circuit when the current is too high, but unlike a fuse it can be reset (switched back on) after the fault is fixed, rather than needing to be replaced. An RCD specifically detects a difference between the current in the live and neutral wires, which indicates current leaking to earth (possibly through a person), and disconnects extremely quickly.