Diagnostics, Troubleshooting and Fault Analysis
What the exam tests
Diagnostics questions test a logical fault-finding sequence: gather symptoms, verify the fault, isolate the system, test from supply to load, and confirm the root cause. The exam favours evidence over guesswork and safe measurement over repeated breaker resets.
Core rules
- Define the symptom precisely: when it occurs, under what load, whether it is intermittent, and what changed recently.
- Use divide-and-conquer. Confirm supply, protection, control signal, actuator, and mechanical load in a deliberate order.
- Compare three-phase currents and voltages. A significant current imbalance can indicate supply voltage imbalance, a loose connection, insulation weakness, or a winding fault.
- Use 5 Whys or a fishbone diagram to move from the failed part to the contributing cause: alignment, lubrication, environment, operation, or maintenance practice.
- Resetting a protective device without finding the cause is not diagnosis. Test insulation, windings, and mechanical drag before returning to service.
Worked example
A 400 V motor trips its overload. Phase currents are 14.0 A, 14.2 A, and 18.0 A. Average = 15.4 A; largest deviation = 2.6 A; imbalance = 2.6/15.4 = 16.9%. That is high enough to investigate supply voltage balance, terminal tightness, insulation, and winding resistance before adjusting the overload. Increasing the setting would hide the symptom and risk damage.
Traps
Changing settings before measuring; replacing a part without testing the circuit; ignoring mechanical load; treating an intermittent fault as repaired after one good run; and working without confirming isolation.