3-Phase winding resistance meters ATRM 20/3 - 40/3

3-Phase winding resistance meters ATRM 20/3 - 40/3

The ATRM-20/3 and ATRM-40/3 are three phase transformer winding resistance meters that allow the user to connect all test cables to the transformer bushings. The unit will then measure the transformer resistance value for each of the phases without the need to disconnect and reconnect cables for each phase. The ATRM-20/3 and ATRM-40/3 can provide a fast and stable reading of very large transformers by utilizing a 60Vdc power supply. The ATRM-20/3 is capable of outputting a selectable test current from 1A to 20A while the ATRM-40/3‘s test current is selectable from 1A to 40A. Since both units can accurately measure resistance from 1 micro-ohm to 500 Ohms (up to 2000 Ohms for the ATRM-20/3), they can be used as micro-ohm meters to measure EHV circuit breaker contact resistance, or for any low resistance measuring application.

For a Delta transformer, the ATRM-20/3 and ATRM-40/3 can measure the phase resistance readings and provide the individual Delta winding resistance values. The ATRM-20/3 and ATRM-40/3 can also provide the individual winding resistance values for a Wye transformer without the neutral terminal.

If the transformer winding resistance temperature is available at the time of testing, the ATRM-20/3 and ATRM-40/3 can calculate the equivalent resistance value at any temperature value. This useful feature can be used to compare the field readings against the factory test resistance values. The ATRM-20/3 and ATRM-40/3 can perform a special test to collect data automatically for up to 90 minutes (at 60-second sampling intervals) or 45 minutes (at 30 second sampling intervals).

The test data is recorded with a time stamp. All test results can be printed on the unit’s built-in 2.5” wide thermal printer.

Test record header information including the Company, substation name, transformer information, and operator information can also be entered using the rugged, 44–key “QWERTY”-style membrane keypad. The ATRM-20/3 and ATRM-40/3 can automatically demagnetize the inductive device under test, eliminating the manual task of demagnetizing the transformer core after a resistance test.

The ATRM-20/3 and ATRM-40/3 also has a “make-before break” test mode that can be used to test the Load Tap Changer (LTC) or Voltage Regulator contact test sequence. The ATRM-20/3 and ATRM-40/3 produces a “Dynamic-Resistance” graph of the LTC or Voltage regulator contact under operation. An opened contact can be detected visually from this resistance chart.

The ATRM’s built-in LTC/Voltage regulator can be used to conveniently change the LTC/voltage regulator tap position from the ATRM-20/3 and ATRM-40/3 front panel.


  • Computer Interface
  • User Interface
  • Safety Features
  • Test Record Storage

ATRM-20/3 and ATRM-40/3 Specifications


Portable transformer winding resistance meter

Physical specifications

21”W x 17”H x 9” D (53 cm x 43 cm x 24 cm); Weight: 35 lbs (15.8 kg)

Operating voltage

100 - 240 Vac, 50/60 Hz

Resistance reading range

ATRM-20: 1 micro-ohm - 2000 ohms; ATRM-40: 1 micro-ohm - 500 ohms


1 - 19,999 micro-ohms: ±0.5% reading, ±1 count;

20 - 999 milli-ohms: ±1% reading, ±1 count;

1- 2000 ohms: ±1.5% reading, ±1 count

Test current

ATRM-20: 1A ¬ 20A in 1A increments; ATRM-40: 1A ¬ 40A in 1A increments

Test voltage

60Vdc charging, 18V DC max during measurement

Resistance channels

4 input channels for measuring resistance


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


Built-in 2.5-inch wide thermal printer

Internal data storage

256 static test records (each can contain up to 111 readings) and 120 dynamic test records

External data storage

Up to 999 test records on external USB Flash drive

Computer interface

RS-232C, USB, and Bluetooth


Designed to meet UL 61010A-1 and CAN/CSA C22.2 No. 1010.1-92 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


Four 50-foot test cables, one LTC control cable, one ground cable, one power cord, one USB cable

Optional accessories

Transportation case


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

Controls 3-Phase winding resistance meters ATRM 20/3 - 40/3Connections 3-Phase winding resistance meters ATRM 20/3 - 40/3Thermal printer report 3-Phase winding resistance meters ATRM 20/3 - 40/3

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