Test Instruments, Measurement, and Interpretation
Instrument questions combine selection, safety rating, and interpretation. Choose the instrument for the quantity and environment: DMM for voltage, resistance, and continuity; clamp meter for load current; insulation resistance tester for insulation; low-resistance ohmmeter (DLRO) for contact and winding resistance; ground tester for electrode resistance; power quality analyzer for harmonics, sags, and transients. Check the CAT rating: CAT IV is for service entrance and outdoor conductors, CAT III for distribution and fixed equipment, CAT II for cord-connected loads. A meter’s voltage rating and CAT rating must both suit the point of measurement.
Technique and traps
Use true-RMS meters on distorted waveforms; averaging meters misread non-sinusoidal current. For insulation testing, de-energize, isolate, and discharge the circuit, then remove sensitive electronics. Insulation resistance is not a go/no-go number alone: temperature, humidity, test voltage, and cable length affect readings. Polarization index (10-minute/1-minute ratio) and dielectric absorption ratio help judge moisture and contamination. For low resistance, use a four-wire Kelvin connection to eliminate lead resistance. Ground resistance is best measured by fall-of-potential or clamp-on methods, not by an ordinary ohmmeter.
Interpretation
Compare readings to nameplate, prior trends, and manufacturer limits. A very high resistance on a winding may indicate an open circuit; a very low insulation resistance may indicate moisture or damage. Continuity does not prove a conductor can carry load current.
Worked example
A 1 MΩ insulation reading on a 480 V motor may sound high, but at 1,000 V test voltage the expected minimum is often around 1 MΩ per kV plus one, so verify against the standard and trend. A sudden drop from 500 MΩ to 5 MΩ is the real warning.