The Phase-Guard 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-Guard 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-Guard models are listed with Underwriter Laboratories.

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Model P - Comes standard with the relay being energized during normal conditions.

Model PND - Comes standard with the relay being de-energized during normal conditions.

Read more information about the different models.

By adding the Normal Relay Selector feature the model of the Phase-Guard unit can be changed at any point with use of the Admin Software. Features are available both at time of purchase or as an upgrade at a later time.
Read more information about the available features.


Each Phase-Guard can be customized to match the needs of your specific environment. The Admin Software is used to tweak the settings for each device.

Instant Deployment

In addition to making setting adjustments, the Phase-Guard can be upgraded to add features and functionality at any time from anywhere. This means you can save money on shipping costs by purchasing additional Phase-Guards and having them on hand ready to be deployed at a moments notice. The Phase-Guard can be pulled off the shelf, configured or upgraded, and then put directly into use.

What to see how easy it is to upgrade a Phase-Guard with new features in the field?
Read more - How to Upgrade a Phase-Guard.


Phase-Guard 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-Guard 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.
Adjustments can be made to the various settings of the Phase-Guard with the use of the Admin Software.


Phase-Guard 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-Guard may be mounted in any position.

Designed and Made in America.


Phase-Guard 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-Guard 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-Guard 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-Guard relay can be used to protect equipment in a small plant from single phasing from an outside source.
  4. The Phase-Guard 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-Guard resets.
  5. The Phase-Guard 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-Guard 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.
See wiring diagram