Measure EHV Circuit Breaker Contact Resistance with Vanguard TRM-403

vanguard_trm_-403A three phase transformer winding resistance meter, the Vanguard TRM-403 Transformer Resistance Meter can accurately measure resistance from 1 micro-ohm to 500 Ohms, it can be used as a micro-ohm meter to measure EHV circuit breaker contact resistance, or for any low resistance measuring application.

The Vanguard TRM-403 Transformer Resistance Meter 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.

The Vanguard TRM-403 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 TRM-403 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 TRM’s built-in LTC/Voltage regulator can be used to conveniently change the LTC/voltage regulator tap position from the TRM-403 front panel.

Vanguard TRM-403 Transformer Resistance Meter 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

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

1A ¬ 40A in 1A increments

Test Voltage

60Vdc charging, 18V DC max during measurement

Input Channels

4 input channels for measuring resistance


Back-lit LCD Screen (240 x 128 pixels)

Viewable in bright sunlight and lowlight levels


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 Interfaces

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

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