Basic Electrical Wiring and Installation
Senior 3 · Physics
Basic Electrical Wiring and Installation
Lesson 5 of 5
25 min
A student fills an old metal kettle, plugs it in and feels a tingle as she touches its side. In one house this ends with a fright and a repair; in another it ends in hospital, or worse. The difference is not luck. It is whether the house has proper earthing, the right circuit breakers and a residual-current device, and whether people follow a few simple rules. In S1 you met fuses, circuit breakers and the earth wire. This lesson explains how they work together, what each one can and cannot do, and what to do if someone receives a shock.
Important
Two rules come first. (1) Electricians never work on a live circuit: they switch off and isolate it, lock the switch off, and prove it dead with an approved tester before they touch anything. (2) Pupils must not open mains fittings, sockets, switches, plugs, lamp holders, the consumer unit or the meter, or try to repair them. Report faults to an adult, who must call a qualified, licensed electrician.
It is the current through the body that does harm, and the current depends on the voltage and on the body's resistance. Dry skin has a high resistance; wet or sweaty skin, or standing on a wet floor, lowers it greatly, which is why water and electricity are so dangerous together. Typical effects of a 50 Hz current passing through the body:
| Current | Typical effect |
|---|---|
| about 1 mA | Just felt |
| about 5 mA | Largest current that is normally harmless |
| 10 mA to 20 mA | Muscles contract: the person may be unable to let go |
| about 50 mA | Pain; breathing may be affected |
| 100 mA to 300 mA | The heart may go into fibrillation (an uncontrolled quivering): often fatal |
Worked example 1: wet hands
Estimate the current through a person who touches 230 V (a) with dry skin, total resistance about 100 kΩ; (b) with wet hands and bare feet on a wet floor, about 1000 Ω (typical values).
The same 230 V gives a tingle in (a) and a current in the range that can stop the heart in (b).
A fuse is a short, thin wire that melts if the current exceeds its rating. A miniature circuit breaker (MCB) does the same job with an electromagnet or a bimetallic strip (unit 4) that opens a switch. Both are placed in the live conductor and are chosen with a rating just above the normal current of the circuit.
| Fuse | Miniature circuit breaker (MCB) | |
|---|---|---|
| How it works | Thin wire melts | Electromagnet or bimetal strip opens a switch |
| After a fault | Must be replaced with one of the same rating | Reset by pushing the switch back on, once the fault is found and repaired |
| Speed and accuracy | Slower; rating less exact | Faster and more precise |
| Protects | the cables against overloads and short circuits, which could overheat and start a fire | |
A current of 230 mA through a person is far below the 16 A of a socket circuit's breaker, so a fuse or MCB alone does not protect a person from a shock.
The metal case of an appliance such as a kettle, an iron or a fridge is connected to the green-and-yellow earth conductor, which leads to the earth electrode. If a loose live wire touches the case, current has an easy, low-resistance path to earth:
The fault current is so large that the fuse melts or the MCB trips at once, disconnecting the live supply. Meanwhile the case is held close to earth potential, so a person touching it is safe. Without the earth wire, the case would sit at 230 V, the breaker would not trip, and the next person to touch the kettle would complete the circuit to earth through their body.
Worked example 2: an earth fault
In the fault above, the resistance of the live conductor, the fault and the earth path together is 0.80 Ω. Find the fault current and explain what happens on a circuit protected by a 16 A MCB.
This is about 18 times the MCB's rating, so it trips almost instantly, cutting off the supply before anyone touches the case.
An RCD protects people. In a healthy circuit, every ampere that goes out along the live conductor comes back along the neutral. The RCD passes both conductors through an iron ring with a sensing coil:
When the currents are equal, their magnetic effects in the ring cancel. If some current leaks to earth, for example through a person touching a live part, the returning current is smaller, the ring is magnetised, the sensing coil drives the trip mechanism, and the RCD opens both contacts in a fraction of a second. RCDs for protecting people trip at a difference of 30 mA or less, below the range that is likely to stop the heart. Press the RCD's test button regularly, as the maker advises, to check that it still trips.
Definition
Residual-current device (RCD, also called an RCCB or, in North America, a GFCI): a switch that disconnects a circuit when the current in the live conductor and the current in the neutral differ by more than a set value, typically 30 mA, showing that current is leaking to earth.
Earthing: connecting exposed metal parts to the earth through a low-resistance conductor, so that a fault makes a large current flow and the protective device disconnects the supply.
Double insulation: an appliance with a plastic case and two layers of insulation, marked with a square inside a square; it needs no earth wire.
Worked example 3: does the RCD trip?
An RCD rated at 30 mA measures 4.000 A in the live conductor and 3.970 A in the neutral. What is happening, and does it trip?
30 mA is leaking to earth, perhaps through a damaged cable or a person. This reaches the RCD's rating, so it trips. Note that the 16 A MCB would not notice anything: 4 A is well within its rating.
Warning
If someone has an electric shock
Tip
Three protectors, three jobs. The fuse or MCB protects the cable from too much current. The earth wire makes a fault trip the breaker and keeps metal cases safe. The RCD protects people from small leakage currents. A safe house needs all three.
What to do if your baby has had an electric shock (St John Ambulance). Watch the order of the steps: make the scene safe and break the contact first, then check the casualty and call for help.
Practice
Key points
Further reading
Sources and credits
Written by Ganzaa: the lesson text, the two diagrams, the tables, worked examples and practice exercises.
