Rotor Earth Fault Resistance Monitor | AT-8000

Advanced Ground Fault Monitoring for Synchronous Generators

The AT-8000 Earth Fault Resistance Monitor (EFREM) provides continuous wireless monitoring of insulation fault resistance and field voltage on brushless exciter generator/motor field windings, combining advanced ground fault measurement techniques with the latest innovations in digital telemetry. The system also provides users with condition monitoring data, allowing operators to monitor trends over time in order to make better informed operational and maintenance decisions.

Learn more about  avoiding unnecessary downtime with Accumetrics' EFREM.


  • Continuous online monitoring of faults while rotating or at standstill 
  • Fault resistance trending 
  • Fault location indicator 
  • Continuously monitors field excitation voltage 
  • Alarm relay contact outputs


  • Rotor protection through condition based maintenance 
  • Predictive maintenance for large generators and motors

accu at-8000 mid-shaft efrem animation
Visualization of how the AT-8000 mid-shaft components are assembled.

at-8000 end of shaft animation

Visualization of how the AT-8000 end of shaft components are assembled.

Continuous On-Line Monitoring

The detection of field ground faults on generators or synchronous motors with brushless exciters has always been difficult. Conventional field ground detectors may detect the occurrence of faults but provide no advance warning or indication of the fault’s severity. The severity at the detection threshold may vary by several orders of magnitude, depending on fault location.

By combining a new generation of 16-bit digital rotor telemetry technology for the measurement of generator ground faults, Accumetrics overcomes these limitations with EFREM. Measurement of actual resistance allows users to monitor trends over time and track the progression of ground faults from their onset. This provides an early warning of impending failure and allows for predictive maintenance of a machine. The severity of ground faults can be used in making operational and maintenance decisions.

at-8000 end of shaft

End of Shaft EFREM

AT-8000 Mid shaft EFREM rotor earth fault resistance monitor

Mid-shaft EFREM

AT-8000 EFREM rotor earth fault resistance monitor receiver

EFREM Receiver

Field Voltage
     Measurement Range0 to 500 VDC (Contact factory for other ranges)
     Transient without Damage
1000 volts for 5 seconds
Resistance Measurement
     Measurement Range0 to 80 M ohms
     Accuracy (readings
     through digital interface)
Sum of ±250 ohms and ±0.5% of reading over the range of 0 to 500k ohms (exclusive of the effects of AC content and noise from the excitation system)
Earth Fault Location Factor
     Range0 to 100% representing ratio of potential at fault to total field voltage (0 at negative terminal and 100% at positive terminal)
     Accuracy (readings
     through digital interface)
±1% for a 10k ohms fault. Computation of location factor of field voltage accuracy is specified for ≥25 V
Alarm Outputs
     Earth FaultTwo independent alarm resistance, user selectable via computer interface from 500 Ohms to 1M ohms
     MalfunctionActive upon detection of a malfunction in Monitor operation or loss of receiver power
     Alarm InterfacesForm C relay; 6 A / 250 VAC
Analog Outputs
     Analog Outputs OptionOption: Dual analog outputs Field V and Log Resistance (4-20 mA) Note: this adds ±0.25% full-scale current loop inaccuracy
Digital Interface
     Computer InterfacesRS232, Ethernet
     Output DataEarth Fault Resistance, Field Voltage, Location Factor when faults occur, Alarm and error conditions
     User SettingsAlarm Resistance Thresholds, Dwell Times, Network Settings
     SoftwareEFREM view data display and .CSV format PC data archiving
Power85 to 250 VAC 50 / 60 Hz, <20 W
Transmitter Weight2.2 lb (1 kg) for shaft end mounting
Rotor ConnectionsField Positive Terminal
Field Negative Terminal
Rotor Earth/Ground
Ambient Temperature32−185 °F (0−85 °C) at Rotor Module 32−122 °F (0−50 °C) at Receiver Unit
Rotor Speed0 to 3600 RPM for center-line and mid-shaft mounting (≤16" Outer Diameter) Call factory for larger shaft diameters or specialized mounting