2.1-2.2 - Electrical units and mains safety

The required electrical units

The specification expects you to use seven units accurately. A unit tells the reader what kind of quantity the number measures.

The symbols are part of the answer. For example, $3\,\mathrm{A}$ is a current, $3\,\mathrm{C}$ is a charge, and $3\,\mathrm{W}$ is a power. The number is the same, but the physics is different.

Use the capital letters carefully. $\mathrm{A}$, $\mathrm{C}$, $\mathrm{J}$, $V$ and $\mathrm{W}$ are capital letters. The symbol for second is lower-case $\mathrm{s}$. The symbol for ohm is $\Omega$.

The capitals are there because these units are named after scientists: Ampère, Coulomb, Joule, Volta and Watt. The ohm is named after Georg Ohm, but its symbol is the Greek capital letter omega, $\Omega$.

Do not mix up a unit symbol with the symbol for the quantity in an equation. Current has the symbol $I$ but is measured in $\mathrm{A}$; charge has the symbol $Q$ but is measured in $\mathrm{C}$.

Using units in answers

In calculation answers, the unit usually comes after the numerical value:

Do not guess the unit from the number size. The same number can describe different quantities: $230\,\mathrm{V}$ is a voltage (the UK mains voltage), but $230\,\mathrm{W}$ is a power. Match the unit to the quantity named in the question.

A good habit is to read your final answer as a phrase: "the current is 0.25 amperes", "the resistance is 12 ohms", or "the energy transferred is 45 joules". If the phrase sounds like the wrong kind of quantity, the unit probably needs fixing.

Insulation and double insulation

Insulation is material that does not allow charge to flow through it easily. In domestic appliances, plastic or rubber insulation is used around wires and around live parts so that a user cannot touch a conductor.

Insulation protects in two linked ways:

• it prevents a user from making direct contact with a live conductor
• it helps keep separate conductors apart, reducing the chance of a short circuit

Double insulation means the appliance has two independent insulating barriers, or an insulating outer case as well as insulated internal parts. The key point is that there should be no exposed conducting case that could become live and shock the user.

A double-insulated appliance is marked with a square inside a square (⧈). It is connected by a two-core cable, with a live wire and a neutral wire but no earth wire.

A plastic phone charger, electric drill, or hairdryer may be double insulated. A damaged cable is unsafe because the insulation no longer separates the user from the live conductor.

Earthing metal-cased appliances

A mains cable to a metal-cased appliance has three wires. The live wire (brown) carries the alternating supply, at about 230 V in the UK. The neutral wire (blue) completes the circuit and stays close to 0 V. The earth wire (green and yellow) is a safety wire.

Earthing is used when an appliance has exposed metal parts, such as the metal case of a washing machine, toaster or electric cooker. The earth wire connects the metal case to the ground through a low-resistance path. In normal use, the earth wire should carry no current.

[DIAGRAM: asset_name: Electrical units and mains safety - diagram 01; asset_slug: p13_electrical_units_and_mains_safety__diagram_01; recommended_method: image_gen; description: Deterministic NovaLearn plug-and-earth-fault diagram with neutral-filled plug and metal case, a fused clay live wire feeding a separate heating element, isolated blue neutral, sage/ochre earth and a rose spark at the inside case wall. One connected rose fault-current arrow chain runs through the fuse to the fault, through the case to earth, back through plug earth and to the earth symbol; the case is labelled case stays at 0 V.]

The diagram uses the ideal approximation that the earth connection has negligible resistance, so the case is at earth potential (0 V). In practice the important protection is a low-resistance fault path and rapid disconnection.

The important fault sequence is:

1. A live wire touches the metal case.
2. The metal case is connected to earth by the earth wire.
3. A large fault current flows from the live wire, through the case and the low-resistance earth wire, to earth. Because this path has very low resistance, the current is much larger than the appliance's normal current.
4. The large current makes the fuse melt or the circuit breaker trip.
5. The supply is disconnected, so the metal case is not left live.

Without the earth wire, the case would stay live after the fault. A person touching it would become the path to earth and could get an electric shock.

Earthing is not just "a spare wire". It only protects properly when it gives fault current a safer path and helps a fuse or circuit breaker disconnect the circuit quickly.

Fuses and circuit breakers

Fuses and circuit breakers protect by disconnecting a circuit when the current becomes too large. A high current can overheat wires, damage an appliance, or increase fire risk. If the high current is caused by a live-to-case fault in an earthed appliance, disconnection also helps protect the user.

The fuse or overcurrent circuit breaker is placed in the live conductor. Opening the live connection disconnects the appliance from the high-voltage supply; breaking only the neutral could leave its internal parts connected to live.

A plug fuse has a current rating, usually $3\,\mathrm{A}$ or $13\,\mathrm{A}$ in UK plugs, chosen to be a little above the appliance's normal current (choosing the rating uses $P = I \times V$, specification point 2.4). Circuit breakers are found in a home's consumer unit and switch off faster than a fuse melts.

In a domestic appliance, the safety features work together. Insulation reduces the chance of a fault. Earthing gives fault current a low-resistance path if a metal case becomes live. A fuse or circuit breaker then disconnects the supply.

The device or user being protected depends on the fault. A fuse in an appliance plug can protect the cable and appliance from overheating. Earthing plus a fuse or circuit breaker can protect the user by making sure a live metal case is disconnected quickly.

Quick recap

• Electrical units are A for current, C for charge, J for energy, $\Omega$ for resistance, s for time, V for voltage and W for power.
• Insulation prevents contact with live conductors; double insulation provides independent barriers and removes the need to earth a nonconducting outer case.
• In a live-to-metal-case fault, the earth wire provides a low-resistance path so a large fault current operates the protective device.
• A fuse wire melts; a circuit breaker trips open. Disconnecting the live conductor isolates the appliance from the live supply.