Isolated Alarm Circuit Output on Plug-in 3-Phase Monitor Relays

Three-Phase Monitor Relays


I have an application where the panel control voltage is at 120V AC, but I need to have a 24V DC alarm circuit if the three-phase monitor relay trips. How can I do it without adding a relay to the circuit?


Most standard 8 pin plug-in phase monitor relays offer a SPDT output configuration. With this configuration, the starter coil voltage and an alarm circuit voltage have to be the same because there is no electrical isolation between the N.O. contact usually used with the starter coil and the N.C. contact for the alarm circuit:

Circuit lines with connecting circles

With these standard products, if the application required an alarm output at a voltage other than the motor starter coil voltage, i.e., 120V AC for the starter coil and 24V DC for the alarm circuit, the only solution was to add a relay for the isolation output between 120V AC 24V DC as shown below:

Circuit lines with connecting circles

Of course, this adds cost in extra material and labor for installation and wiring plus takes up additional space in the panel.

Both the new PJPU-FA8X and updated PMPU-FA8X from Macromatic are 8 pin plug-in products, but each has an extra isolated SPNO contact in addition to the standard SPDT contact. With this configuration, the starter coil can be at one voltage and the alarm circuit can be at a different one without the need for the separate control relay:

Circuit lines with connecting circles

Not only do you have isolated starter coil alarm circuits, but all of this is done with a less expensive and smaller 8 pin octal socket! No more need for larger and more expensive 11 pin DPDT octal or 12 pin square base sockets.

Best of all, both the PJPU-FA8X and PMPU-FA8X products from Macromatic include all the great features available with our other three-phase monitor relays:

  • True RMS voltage sensing
  • No loss of power on any phase loss condition
  • Dual-color LED with fault codes on top of the relays for easier troubleshooting
  • Five Year Warranty

For more information on these products, click here. Or contact Macromatic technical support at 800-238-7474.