MRM 100
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INTRODUCTION
The MRM-100 is a portable micro-Ohmmeter used to test circuit breaker contact resistance, or
anywhere low resistance measurements at currents up to just over 100 Amps are required. The
MRM-100 uses a low voltage, high current power supply to run a user-defined test current of
between 10 Amps to 105 Amps to any resistive load that requires no more than 2.5 Volts across it
to test. The MRM-100 does detect if the test is able to reach desired current, and allows for
immediately stopping the test.
THEORY OF OPERATION
The MRM-100 uses a two-voltmeter method where each voltage reading implements a four-wire
Kelvin connection. One voltmeter monitors the voltage across a known, highly stable shunt that
passes the test current through it, while the other voltmeter monitors the voltage across the unit
under test. A test current is then applied (see ANSI C37.09 and IEC 62771-100 for test current
requirements). This current is ramped up, held for a period of time and then ramped down. While
the current is relatively constant, the voltages across the known resistance and unit under test are
amplified and filtered. Then they are sampled several times by analog to digital converters to
reduce any noise, and processed by a microcontroller. The microcontroller calculates the ratio
between the test current and sensed voltage to determine the resistance. The micro-Ohmmeter is
designed to test resistances of 1 micro-Ohm to 240 milliohms.
