A DC Resistance Tester (also called a Micro-ohmmeter or Low-Resistance Ohmmeter) is a precision instrument designed to measure extremely low electrical resistances (typically 0.1 µΩ to 2 kΩ) using direct current (DC). It eliminates inductive/capacitive effects, providing accurate readings for quality control, diagnostics, and maintenance of electrical components.

Key Features & Working Principle

1. 4-Wire Kelvin Measurement (Essential)

Current Leads (C1, C2): Inject a stable DC current (e.g., 1A–100A).

Voltage Leads (P1, P2): Measure the voltage drop directly across the test object, bypassing lead/contact resistance.

Accuracy: ±0.1% or better, critical for sub-ohm measurements.

2. High Test Current

Uses DC currents (1A–1000A) to overcome thermal EMF/noise and improve signal-to-noise ratio.

Example: Measuring a 500 µΩ busbar requires ≥10A to detect 5 µV changes.

3. Automatic Stabilization & Averaging

Compensates for inductive "ramp-up" in windings (transformers/motors).

Averages multiple readings to cancel noise.

Primary Applications

Transformers: Measure winding resistance per phase (detects loose connections, broken strands).

Motors/Generators: Test armature/field windings (identify shorts, opens, imbalances).

Switchgear: Check contact resistance in breakers, busbars, joints (target: <50 µΩ for HV contacts).

Cables/Busbars: Validate bonding resistance and joint integrity.

Welding/Manufacturing: Quality control of electrical joints.