Microhmmeter QMOM 200 S3

Microhmmeter QMOM 200 S3

The QMOM-200 S3 is Amperis’ 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 (19.8 lbs/ 8.9 Kg) micro-ohmmeter is designed to meet the IEEE C57.09-1999 (5.15) requirement for testing circuit breaker contact resistance.

The QMOM-200 S3 can accurately measure resistance values from 1 micro-ohm to 5 ohms. A 0.1 micro-ohm resolution is possible with current greater than 5A. The QMOM-200 S3 applies a selectable true DC test current from 1A to 200A to the resistance load to be tested.

The QMOM-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 30 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 QMOM-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).

Dual Ground Option

With the Dual Ground option, the QMOM-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 QMOM-200 S3 measures and eliminates this current from the total test current. The QMOM-200 S3 then calculates the actual resistance value of the circuit breaker.

Test Record Storage

The QMOM-200 S3 can store 128 records of 64 readings internally, and up to 999 test records on an external USB Flash drive. Test header information (Company, Substation, circuit breaker ID’s) can also be entered using the 44-key keypad and is stored with each test record.

User Interface

The QMOM-200 S3 features a back-lit (128 x 64 pixels) LCD screen that is viewable in both direct sunlight and low-light levels. The resistance readings are displayed on the LCD screen in micro-ohms or milliohms. The unit is operated via a convenient 44-key “QWERTY” keypad on the front panel.

Included Cables

The QMOM-200 S3 is furnished with a 30-ft test cable set. A 15-ft test cable set is also available as an option. Test cables are terminated with heavy duty welding type clamps. The test current and voltage sense cables are isolated and fastened to the clamp jaws. This feature allows for a simple connection to the circuit breaker bushing. An optional voltage sense cable and probe can be used to measure resistance in small access locations. Optional heavy duty, welding type C-clamps are also available allowing the user to connect the test leads to a wide variety of bushing sizes, bus-bars, or large conductors.

Built-in Thermal Printer

The QMOM-200 S3 features a built-in 2.5" wide thermal printer that can be used to print test reports in the field.

Computer Interface

Windows®-based analysis software is provided with each unit and can be used to retrieve test records (from the unit’s memory via the RS-232C port or from a USB Flash drive), analyze test results, and print test results on a desktop printer. Test records can also be exported to PDF, Excel, and XML formats for further analysis.

QMOM-200 S3 Specifications


Portable micro-ohmmeter

Physical specifications

18”W x 7”H x 15” D (45.7 cm x 17.8 cm x 38.1 cm); Weight: 19.8 lbs (8.9 kg)

Input power

100 - 240 Vac, 50/60 Hz

Resistance reading range

1micro-ohm a 200A - 5 ohms a 1A


1A to 4.99A: 1% ±10 micro-ohms
5A to 9.99A: 1% ±2 micro-ohms
10A to 200A: 1% ±1 micro-ohm

Test current range

1 Ampere to 200 Amperes (selectable in 1A steps); Thermally protected DC power supply


Back-lit LCD screen (128 x 64 pixels); viewable in bright sunlight and low light conditions


Rugged, 44-key "QWERTY" membrane keypad

Internal test record storage

128 test records. Each record can contain up to 64 readings

External test record storage

Up to 999 test records on external USB flash drive.

Computer interface

One RS-232C pc interface, one usb flash drive interface


Built-in 2.5" wide thermal printer

Pc software

Windows®-based analysis software included with purchase price


Designed to meet IEC 61010 (1995), UL 61010-a, and CAS-C22.2 standards


Operating: -10°C to +50°C (+15°F to +122°F); Storage: -30°C to +70°C (-22°F to +158°F)


90% RH @ 40°C (104°F) non-condensing


2,000 m (6,562 ft) to full safety specifications


30 ft (9.1 m), #1 AWG test cables, power cord, ground cable

Optional accessories

Shipping case, 15 ft test cables, c-clamp set, hand spike set, dual ground option


1 year on parts and labor

NOTE: the above specifications are valid at nominal voltage and ambient temperature of +25°C (+77°F). Specifications are subject to change without notice

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