Testing, Troubleshooting, and Maintenance
Core concepts
Know the instruments: multimeters, clamp meters, insulation resistance testers (megohmmeters), continuity testers, ground resistance testers, and thermal imagers. Understand insulation resistance testing (typically 500 V or 1,000 V for low-voltage systems), polarization index, and the importance of de-energizing and discharging before testing. Troubleshooting follows a logical sequence: verify the complaint, gather information, isolate, test, repair, and confirm. Common faults include opens, shorts, grounds, high-resistance connections, and intermittent faults. Preventive maintenance includes torque checks, thermographic surveys, cleaning, and exercising breakers.
Traps
Questions confuse a megger with an ohmmeter-insulation testers apply high voltage and must never be used on energized circuits or sensitive electronics. Remember that a clamp meter measures one conductor at a time; clamping both conductors of a circuit reads zero. A reading of infinite resistance on a continuity test indicates an open, not a short. Watch for false readings from parallel paths.
Worked example
A motor trips its overload after a few minutes. Check supply voltage under load, compare running current to nameplate FLC, test insulation resistance to ground, and inspect connections for heat damage. If current is balanced but high, suspect mechanical load or wrong voltage; if unbalanced, suspect a supply or winding problem.
Rule of thumb
Divide and conquer: test from the source toward the load, and always verify your meter on a known live source before trusting a zero reading.