Auto-Ohm 200 S3 DC Micro-Ohmmeter

True DC Micro-Ohmmeter
The Auto-Ohm-200 S3 is Vanguard’s fourth generation, microprocessor-based,
true DC micro-ohmmeter. It is designed for testing EHV circuit-breaker contact
resistances, bushing contact joints, welding joints, or for any low-resistance
measuring application. This high current and very lightweight (16.8 lbs/ 7.6
Kg) micro-ohmmeter is designed to meet the IEEE C57.09-1999 (5.15)
requirement for testing circuit breaker contact resistance.
The Auto-Ohm-200 S3 can accurately measure resistance values from 1 microohm
to 5 ohms. A 0.1 micro-ohm resolution is possible with current greater than
5A. The Auto-Ohm-200 S3 applies a selectable true DC test current from 1A to
200A to the resistance load to be tested. The Auto-Ohm-200 S3 controls the test
current’s rise and fall rates. The test current rise and fall rate can be selected from
5 seconds to 10 seconds. An “Auto Test” mode is also available and can be initiated
simply by applying the sense cables’ leads across the two points of interest in
the current path. This feature is very convenient when measuring a sequence of
several resistance values in a circuit breaker contact. The Auto-Ohm 200 S3 can
also compare test results against preset limits and determine if a test passed or
failed, and a “Pass” or “Fail” flag is displayed accordingly.
Since a true DC current (with controlled rise/fall time) is passed through the
circuit breaker contact, no magnetic transient is induced into the breaker’s
current transformers. This feature greatly reduces the risk of inductively tripping
a breaker control (bus differential relay).
With the Dual Ground option, the Auto-Ohm-200 S3 can also measure the circuit
breaker contact resistance with both sides of the breaker bushing being grounded.
When a test current is applied to a circuit breaker with both sides grounded, some of
the test current flows through the safety ground cables. Using an external current sensor,
the Auto-Ohm-200 S3 measures and eliminates this current from the total test current.
The Auto-Ohm then calculates the actual resistance value of the circuit breaker.

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