Model P
Model PND

Phase Failure Relay

The Phase Failure Relay is a phase-voltage-balance monitoring device designed for use with magnetic controls to automatically prevent three phase motors or other equipment from operating or attempting to start up under open phase or single-phase conditions, thereby eliminating burnouts and consequent down-time frequently incurred by phase failure.

Phase Failure Relay is not a thermal device. It operates on the principle of phase voltage unbalance which may occur in a three phase system. It also protects against phase loss. All Phase Failure Relay models are listed with Underwriter Laboratories.


Phase Failure Relay is a self contained power sensing device. In normal operation the incoming phases of the three phase voltage applied to our power-sensing network are in balance. When any phase becomes more than 12% unbalanced from the other two (either low or high), or a phase loss occurs, the sensing network will deliver an output voltage to activate the transistor circuit. This will cause the master output relay on the Phase Failure Relay to trip (either dropout or pickup depending upon model). A time delay of approximately 1 ½ seconds is incorporated to eliminate the possibility of nuisance tripping.


Phase Failure Relay electronic components are enclosed in an isolated cover of rugged plastic construction. The complete unit is mounted on a 3/8” thick plastic base.
Base dimensions 5X7"
Mounting holes 4 ¼ x 6 ¼ "
The Phase Failure Relay may be mounted in any position.


Phase Failure Relay can be used to protect any magnetically controlled three phase equipment that would be damaged if subjected to abnormal phase conditions such as single phasing. It is designed to monitor any three phase supply line, and can be used with any type of actuating controls such as push button stations, thermostats, pressure or float switches. It may also be used in a trip circuit utilizing manually operated starters in conjunction with shunt or capacitive trip devices. Typical applications are as follows:


  1. Phase Failure Relay operation is not affected by variations from base voltage (within acceptable limits), provided phase voltages are balanced
  2. It can be used with motors of any horsepower rating at a given voltage. Standard models are available for operation on the following voltages: 120, 208, 240, 480, 550, 600 vac.
  3. The Phase Failure Relay can be used to protect individual motors or a group of motors collectively, when they are fed from a common contactor or magnetically latched circuit breaker. One Phase Failure Relay relay can be used to protect equipment in a small plant from single phasing from an outside source.
  4. The Phase Failure Relay incorporates an automatic reset feature that makes it ideal for protecting unattended motors actuated by sensory controls. When phase failure occurs, the motor is disconnected from the line and when normal line conditions are restored, the Phase Failure Relay resets.
  5. The Phase Failure Relay is easy to install and requires no adjustments in the field.


Wiring of all models requires only the connecting of the three phase power to Phase Failure Relay at Ll, L2, & L3. The contacts of the master relay (DPDT) are provided for customer use. The contact ratings (30 amp. @ 277Vac.) allow for diverse applications, either opening or closing a circuit to effect the desired action to protect the three phase equipment.