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ISO 9001 and 17025 Certified



Transformer Winding Resistance Meter


The TRM-40 is Vanguard Instruments’ third generation transformer winding resistance meter. The TRM-40 is designed specifically to measure DC resistance values of transformer windings, rotating machine windings, or any dc resistance of an inductive device.

The TRM-40 features dual resistance-reading input channels that can measure two winding resistances simultaneously (primary and secondary windings of a transformer). It can provide a fast and stable reading of very large transformers by utilizing a 60Vdc power supply. The TRM-40 is capable of outputting a selectable test current from 1A to 40A.

Since the TRM-40 can accurately measure resistance values from 1 micro-ohm to 500 Ohms, it can be used as micro-ohm meters to measure EHV circuit breaker contact resistance, or for any low resistance measuring application. The TRM-40 can automatically demagnetize the inductive device under test, eliminating the manual task of demagnetizing the transformer core after a resistance test.

  • Auto-discharge circuit for operator safety
  • Demagnetizes transformer after test
  • Selectable test current from 1A to 40A
  • RS-232C, USB, and Bluetooth computer interfaces
  • Built-in thermal printer





Technical Specifications

TYPE Portable transformer winding resistance meter
PHYSICAL SPECIFICATIONS 21” W x 17” H x 9” D (53 cm x 43 cm x 24 cm); Weight: 33 lbs (15.4Kg)
OPERATING VOLTAGE 100 – 240 Vac, 50/60 Hz
RESISTANCE READING RANGE 1 micro-ohm to 500 Ohms
ACCURACY 1 – 19,999 micro-ohms: ±0.5% reading, ±1 count
20 – 999 milliohms: ±1% reading, ±1 count
1 – 500 Ohms: ±1.5% reading, ±1 count
TEST CURRENT 1-40A in 1A increments
TEST VOLTAGE 60 Vdc charging, 18V DC max during measurement
RESISTANCE CHANNELS Two static resistance reading channels
DISPLAY Back-lit LCD (64 x 128 dot graphic), viewable in direct sunlight and low light levels
PRINTER Built-in 2.5" 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.
SAFETY Designed to meet UL 61010A-1 and Can/CSA C22.2 No 1010.1092 standards
ENVIRONMENT Operating: -10ºC to 50ºC (15ºF to +122ºF)
Storage: (-30ºC to 70ºC (-22ºF to +158ºF)
HUMIDITY (MAX) 90% RH @ 40˚ C (104˚ F) non-condensing
ALTITUDE (MAX) 2000m (6562 ft) to full safety specifications
CABLES One 50-foot current cable set, two 50-foot resistance sense cable set,
One LTC control cable, One ground cable, one power cord,
One RS-232C cable, one USB cable
OPTIONS Transportation Case
WARRANTY One year on parts and labor
Specifications are valid at nominal voltage and ambient temperature of +25˚C (+77˚F).