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.
Model P - Comes standard with the relay being energized during normal conditions.
Read more information about Model P.
Model PND - Comes standard with the relay being de-energized during normal conditions.
Read more information about Model PND.
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.
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.
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:
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
See wiring diagram