AC-PRO-MP-II® Masterpact Direct Replacement Trip Unit

AC-PRO-MP-II® Masterpact Direct Replacement Trip Unit


The AC-PRO-MP & AC-PRO-MP-II are plug-in, direct replacement trip units for the STR trip units on the Merlin Gerin & Schneider Electric Masterpact MP, IEC or UL rated breakers.
The AC-PRO-MP & AC-PRO-MP-II do not use rating plugs and are user programmable to replace any versions of the STR 18M, 28D, 38S, 58U.

The AC-PRO-MP & AC-PRO-MP-II have the same protective functions, settings and time-current-curves as the original STR units.

AC-PRO-MP-II Additional Features:


  • Voltage Metering (line-to-line) rated for up to 600V three-phase systems
  • Power and energy metering (KW, KVA, KWHrs, Power Factor)
  • Modbus RTU communications via RS-485 network


60 Hz Operation

The AC-PRO-MP & AC-PRO-MP-II are designed for use in a 60 Hz environment. If you need a Masterpact replacement unit for use in a 50 Hz environment please contact Utility Relay Company.

UL and CE Certification

To ensure that the AC-PRO-MP & AC-PRO-MP-II will function when needed the AC-PRO-MP was tested to the ANSI C37.59 Standards for fault interruption. As a result of this testing the AC-PRO-MP & AC-PRO-MP-II are UL certified. This makes the AC-PRO-MP & AC-PRO-MP-II the only Masterpact replacement trip units that are UL listed.

The AC-PRO-MP & AC-PRO-MP-II also carry the CE mark.

AC-PRO-MP, Masterpact Direct & Merling Grein Replacement Trip Unit

Standard trip functions include:

(These will vary depending on the STR type selected)

  • Long-Time
  • Short-Time
  • Instantaneous
  • Residual Ground Fault
  • Ground Return Ground Fault
  • Instantaneous on Closing
  • Over Temperature Trip

All of the settings are protected by a security code to prevent unauthorized modifications.

Additional Functions:

Sluggish Breaker Detection

The AC-PRO-MP & AC-PRO-MP-II now have the ability to detect if the Masterpact breaker mechanism needs service. The first time a breaker opens after a long period of inactivity it may open slowly. This can be caused by mechanical issues, poor lubricant or other obstructions.
The AC-PRO-MP & AC-PRO-MP-II will capture the sluggish breaker event and display a warning on the screen the next time it is powered up. An external alarm can also be activated when the
AC-PRO-MP & AC-PRO-MP-II detect a sluggish breaker event.

When the breaker has detected a sluggish opening it will show this alert the next time the AC-PRO-MP &
AC-PRO-MP-II is turned on.
After the initial warning you can view the actual clearing time.

USB Port

The front mounted and electrically isolated USB port allows for easy downloading of trip data and protection settings. It can also be used to upload the trip unit settings, making commissioning the trip unit much faster. The USB port can also be used with the SAFE-T-TRIP system.

InfoPro-MP Software Compatable

InfoPro-MP is a software application that can be used with AC-PRO-MP units that shipped prior to December 2016. It offers the following capabilities:

  • Settings – upload, download, view, and save
  • Trip unit info: serial number, firmware versions, breaker name
  • Firmware updates

InfoPro-MP-II software is for MP & MP II units that shipped in December 2016 or later (these units can easily be identified by their front test port). In addition to the above features, InfoPro-MP-II includes the following:

  • Waveform capture (on-demand and for the last 8 trips)
  • Current readings
  • Voltage, power, & energy readings (AC-PRO-MP-II only)


The AC-PRO-MP & AC-PRO-MP-II can be programmed one of two ways.  The front of the unit has an OLED display that is easy to read in either low or high ambient light.  The buttons on the front of the unit are “smart” buttons that change their functions based on the displayed information.  The second way to program the AC-PRO-MP & AC-PRO-MP-II is using the USB port on the front to connect to the InfoPro-MP software application, which is supplied by URC. The program makes it very easy to commission the AC-PRO-MP & AC-PRO-MP-II.  InfoPro-MP can also be used to view historical data, upgrade the firmware, save the commissioning data and reload the AC-PRO-MP from a saved file.


The AC-PRO-MP & AC-PRO-MP-II are also compatible with Utility Relay Company’s SAFE-T-TRIP which provides a means for an operator to safely trip a breaker without having to stand directly in front of the switchgear.

AC-PRO-MP installed on a Masterpact Breaker
The AC-PRO-MP & AC-PRO-MP-II have a mechanical pop out trip indicator on the front of the unit that mimics the functionality of the original unit.  There is also a SELF-TEST OK Led, PRE-TRIP LED, QUICK-TRIP switch and an easily accessible LI-Ion battery. The SELF-TEST OK LED indicates a properly operating trip unit. The PRE-TRIP LED indicates a pending trip.  The QUICK-TRIP switch activates QT-Instantaneous and QT-Ground Fault settings in the trip unit to reduce arc flash hazard.

The battery in the AC-PRO-MP & AC-PRO-MP-II is only used to power the trip unit to review trip data or change settings when no power is provided from the CT’s.  The battery is not used for any of the protective functions of the AC-PRO-MP & AC-PRO-MP-II.  The battery is also not used to save the last trip data or settings.  All of this data is saved in non-volatile memory.  The trip unit saves the type of trip, phase, waveforms, ground currents and date/time for the last 8 trip events.  There is also a trip counter that provides the number of trips by trip function.

Additional information

Weight 20 kg


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