Purpose

Apply a repeatable fault-finding process that begins with safety, evidence, and circuit boundaries before measurements or replacement.

Learning objectives

• Define the reported symptom
• Establish safe boundaries
• Choose discriminating tests
• Record and verify the repair

Good fault finding reduces uncertainty in controlled steps. It does not begin by changing the part that seems most likely.

Core theory

Clarify what happened, when, under which loads, and whether the symptom is reproducible; inspect records, labels, recent work, environmental changes, and obvious damage.

Risk-assess, identify every source including alternative/backfeed sources, isolate and secure where required, prove the voltage indicator, prove dead at relevant conductors, and re-prove the indicator.

Form hypotheses from the circuit topology, choose the safest test that separates them, record results, repair only after identifying cause, then inspect, test, functionally verify, document, and explain limitations.

Assumptions: No immediate damage or shock report.

1. Confirm: Check the symptom and circuit identity without exposed live work.
2. Localise: An operating upstream/downstream pattern suggests a local connection, lamp/driver, or branch issue rather than total supply loss.
3. Test safely: Isolate and use continuity/inspection in a planned order before energised diagnostics are considered.

Localise the fault from topology, then use isolated inspection and continuity tests.

Reasonableness check: Unaffected points provide evidence about which shared portions remain intact.

Knowledge check

Answer: The precise symptom, conditions, boundaries, and available evidence. A symptom is not itself a diagnosis.

Practical exercise

Build a decision tree for a dead lighting point that uses inspection and dead tests before any live test.

Summary

• Define before diagnosing
• Test hypotheses safely
• Verify and document the completed repair