Exercises: Domestic Electrical Safety

1. a) What is the purpose of the earth wire in a three-pin plug?
2. b) What colours are the live, neutral and earth wires in an Irish plug?
3. c) To which pin is the fuse fitted?

1. a) What does RCD stand for?
2. b) State what an RCD in a household fuse board detects.
3. c) Explain why an RCD protects a person from electric shock.

A metal-cased kettle has a live wire that comes loose and touches the casing.

1. a) Explain the danger if there is no earth wire.
2. b) Explain what happens if the casing is earthed.

A household RCD trips when the difference between live and neutral currents exceeds $30\ \text{mA}$. A faulty appliance draws $8.50\ \text{A}$ in the live wire, but only $8.45\ \text{A}$ returns in the neutral wire.

1. a) Calculate the leakage current.
2. b) Will the RCD trip? Justify.
3. c) Where does the missing current go?

A $2.3\ \text{kW}$ metal-cased electric heater is connected to the $230\ \text{V}$ mains in a house in Limerick.

1. a) Calculate the normal current drawn.
2. b) Select the most suitable fuse from $3\ \text{A}$, $5\ \text{A}$, $13\ \text{A}$ and explain your choice.
3. c) If the live wire touches the earthed casing, explain why the fuse blows and how this protects the user.

A person with body resistance $2,000\ \Omega$ touches a live conductor at $230\ \text{V}$ while standing on a wet floor.

1. a) Calculate the current through the body.
2. b) Compare it with the $30\ \text{mA}$ RCD trip threshold.
3. c) Explain why an RCD gives better protection to the person than a fuse rated at $13\ \text{A}$.

Explain with reference to a fault in a washing machine why both an earth connection and an RCD are used together in a house, and not just one of them.

1. a) Describe the fault path with an earth connection and a fuse only.
2. b) Describe the situation where a person touches a live part and the earth wire gives no protection.
3. c) Explain how the RCD operates by comparing live and neutral currents, mentioning the role of a toroidal core and a coil in the device.
4. d) Explain why RCDs have a test button.

A $3.0\ \text{kW}$ electric shower runs from a $230\ \text{V}$ supply. The live wire has a resistance of $0.050\ \Omega$ and the earth fault loop resistance in the circuit is $0.80\ \Omega$.

1. a) Calculate the normal operating current.
2. b) Calculate the fault current if the live wire touches the earthed metal casing.
3. c) A $16\ \text{A}$ MCB trips quickly when the current is about five times its rating. Will it trip? Justify.
4. d) Comment on why low earth loop resistance is important.

An electrician tests an RCD with a test current. The RCD must trip at $30\ \text{mA}$. A person of body resistance $1,500\ \Omega$ touches a live $230\ \text{V}$ conductor.

1. a) Calculate the current through the person.
2. b) The RCD trips in $30\ \text{ms}$. Calculate the charge passing through the person.
3. c) Calculate the energy dissipated in the person's body in this time.
4. d) Explain why a short time of exposure is crucial for safety.