To trace a circuit, start at the supply, follow the live conductor through each protective and control part to the load, then follow the return path back. Write the path as numbered steps so nothing is skipped.
This lesson belongs to the wider SPM subjects and learning resources section. Everything here is paper-based: the diagram is original and described in words, and no live equipment is involved.
What is the path of current?
Current needs a complete loop. It leaves the supply on the live conductor, passes through the parts, does its work in the load, and returns on the neutral conductor.
If the loop is broken anywhere, for example by an open switch, current stops everywhere in that loop.
Worked example: a one-lamp lighting circuit
An original diagram shows a supply, a circuit breaker, a one-way switch, a lamp and the return path to the supply.
- Live conductor leaves the supply and reaches the circuit breaker.
- From the breaker it goes to the one-way switch.
- From the switch it goes to the lamp.
- After passing through the lamp, current returns on the neutral conductor to the supply.
Switch closed: the loop is complete, current flows and the lamp lights. Switch open: the loop is broken at step 3, so no current flows and the lamp is off.
| Part | Job |
|---|---|
| Circuit breaker | Opens the circuit automatically if the current becomes too high |
| One-way switch | Opens and closes the live conductor to control the lamp |
| Lamp | Converts electrical energy to light |
| Neutral conductor | Completes the return path |
The mistake that loses marks
A weak answer says: “The current goes from the breaker to the switch to the lamp.” It stops at the lamp, so the return path is missing and the loop is not complete.
A second error is putting the switch in the neutral line. The lamp still turns off, but the lamp holder stays connected to live, which is why the switch belongs in the live conductor.
Reading a diagram with two switches
A two-way switch circuit lets one lamp be controlled from two places, such as both ends of a corridor. Trace it by testing each switch position in turn and asking whether a complete loop exists.
Write a small table with the positions of switch 1 and switch 2 and the lamp state. The lamp is on when the two switch positions make a complete path, and off when they do not. This gives you an answer you can check by logic, not by guessing.
Check yourself
In the lighting circuit above, the circuit breaker has tripped. Write the path of current with the breaker open and say whether the lamp lights when the switch is closed.
Answer
With the breaker open, the live conductor is broken at the first part after the supply, so current cannot reach the switch or the lamp. The lamp stays off even when the switch is closed.
A careful answer adds that the loop is incomplete at the breaker, and that finding the reason it tripped is a job for a qualified person.
What to study next
Practise the same path-tracing habit on water systems in interpreting a plumbing-system diagram, and check your written answers with the structured-answer self-review tool.
For a teacher to trace more diagrams with you, see online one-to-one Domestic Wiring tuition. Live wiring work stays with your school.