Test Equipment and Measurement Fundamentals
Test Equipment and Measurement Fundamentals
This domain tests accuracy, resolution, burden, calibration, and safe connection of meters and test sets.
Core rules
- Accuracy is stated as % of reading or % of full scale; a 1% full-scale meter on a 500 V range has +/-5 V error even at low readings.
- Resolution is the smallest displayed change; it is not the same as accuracy.
- Current measurement adds burden. A 5 A meter with 0.1 ohm burden drops 0.5 V and dissipates 2.5 VA; check the CT burden.
- Use four-wire Kelvin measurement for low resistance so lead and contact resistance are excluded.
- True-RMS meters are required on distorted waveforms; average-responding meters misread harmonics.
Worked example
Measure a 0.1 ohm winding with 1 A DC. Two-wire leads have 0.05 ohm each. The meter sees 0.1 + 0.05 + 0.05 = 0.2 ohm, a 100% error. With four-wire Kelvin, the current leads carry 1 A and the separate voltage leads draw negligible current, so the meter reads the 0.1 ohm winding plus only the voltage-lead contact resistance, which is negligible.
Traps
- Forgetting burden voltage in CT circuits.
- Using a two-wire ohmmeter for busbar or contact resistance.
- Trusting an uncalibrated or CAT-mismatched meter.
- Reading peak or average values as RMS.