The URC Model B-291-MP is designed specifically for testing AC-PRO-MP and AC-PRO-MP-II microcontroller-based trip units—the direct plug-in retrofit units used to replace legacy Merlin Gerin / Square D STR trip units on Masterpact MP low-voltage air circuit breakers.
Target Trip Units: Exclusively for AC-PRO-MP and AC-PRO-MP-II. (Note: It is not compatible with standard AC-PRO or AC-PRO-II breaker kits, which require the standard B-291 / B-292 test sets).
Test Functions: Calibrated secondary current injection to test pickup thresholds and trip time delays for:
Long-Time (LT)
Short-Time (ST)
Instantaneous (I)
Ground Fault (GF)
Output Drive: Single-phase current output capable of driving up to $40\times$ CT rating.
Displays: Dual digital LCDs displaying elapsed trip time ($0.01\text{ s}$ resolution) and test current/multiple ($0.01\times\text{CT}$ resolution).
Supply Power: $120\text{ VAC}$, $50/60\text{ Hz}$, $3\text{ A}$ nominal.
Safety & Isolation:
Ensure the Masterpact MP breaker is in the DISCONNECTED or TEST position with primary stabs de-energized.
Harness Connection:
Plug the multi-pin circular connector of the test cable into the AC-PRO-MP test port on the face of the B-291-MP.
Connect the keyed interface connector to the front-panel test jack of the AC-PRO-MP trip unit.
Power On & Parameter Verification:
Switch on the test set. Verify the AC-PRO-MP powers up through the harness and reads the breaker’s programmed CT rating ($I_n$).
Current Preset:
Hold the Current Preset push button. Rotate the Amps Coarse and Amps Fine potentiometers to the target test multiple (e.g., $3.0\times\text{CT}$ for Long-Time delay verification).
Execution:
Select the target phase (Phase A, B, C, or GF).
Press Clear to zero the timer.
Press Start. The test set injects current until the trip unit fires the actuator or reaches the timing setpoint, at which point the timer automatically freezes the trip duration.
