ARES 200D Static and Dynamic micro-ohmmeter

A micro-ohmmeter is a measuring instrument for very low resistances (from a few nanohm to milliohm). It uses a 4-wire (Kelvin) measurement method to measure cable and contact transitions without measurement error of measurement leads.

Two lines send a known current (I) through the object, two separate lines measure the voltage drop (V) directly across the object. The resistance is R = V/I. Since no measuring current flows through the voltage lines, cable and terminal resistances do not affect the measurement.

• Contact resistance of switch/disconnector contacts and busbars

   

• Weld and bolt connections, bus bars, earth connections

   

• Transformer windings (fast DC resistor, not for extended WRT)

   

• Motors: winding and connections, rotor/stator transitions

   

• Quality control of cables and feedthroughs

Resolution: 0.1 μΩ to 1 μΩ. Ranges: from ~0.1 μΩ to u0026gt;200 mΩ (sometimes up to a few ohms). For each model, check the combination of resolution, accuracy and maximum test current.

Higher test current gives a larger voltage drop and thus a better signal-to-noise ratio-essential for very low resistors. For contact resistors in high-current applications, 100-200 A is common; for general connections, 10-20 A is often sufficient.

No. Too high a current can generate heat and temporarily increase the resistance (self-heating). Choose a current that suits the application and follow the duty-cycle of the instrument.

Usually specified as ±(percentage of readout + number of digits). Note that for low absolute values, the “digits” component weighs relatively heavily. Also check noise filtering and repeatability.

Copper and other metals have a temperature coefficient (Cu ~0.00393/°C). For comparisons over time, correct to a reference temperature (usually 20°C). Some instruments do this automatically with an internal/external temperature sensor.

Yes, but note that the current builds up more slowly and energy can be stored. Use instruments with suitable protection/discharge function. Wait for “discharge complete” before disconnecting.

• Measuring 2-wire instead of 4-wire

   

• Clamps too far apart/no fixed position

   

• Dirt/oxidation between clamp and conductor

   

• Insufficient discharge of inductive objects

   

• Not waiting for thermal equilibrium or stabilisation

Compare with factory values or baseline measurements. Trends are leading: increasing resistance often indicates wear, contamination or insufficient contact pressure. Set limit values per asset.

Micro-ohmmeters are optimised for low resistances with higher test currents and fast measurements. Winding resistance testers typically offer controlled excitation and longer measurement cycles for inductive components, including demagnetisation and temperature modelling.