Traffic Signal Conflict Monitor MMU Testing Procedure and Field Acceptance
Introduction
A conflict monitor (MMU or CMU) is the safety device that forces an intersection to flashing red if it detects conflicting green indications or other illegal states. The MMU is the last line of defense and must be tested regularly. This guide covers the test procedure.
The MMU Function
The MMU monitors every green output of the controller. If it sees:
- Two conflicting greens energised at the same time (e.g., NB green and SB green when they should not run together)
- A green and a yellow on the same phase (illegal sequence)
- A green on a phase that should be red (cabling fault)
- A voltage on an output that should be off (controller fault)
...it forces the intersection to flashing red within 250 ms.
GAOQIAO cabinets ship with an MMU pre-configured to the intersection's phase diagram. The field test confirms that the MMU recognises every conflict scenario.
Field Test Procedure
The standard MMU test sequence:
1. Power-on test: confirm MMU self-test passes; note the firmware version 2. Voltage sense test: confirm AC voltage present at the sense input 3. Program card verification: confirm the program card matches the intersection's phase diagram 4. Conflict test (every pair): manually energise each conflicting green pair; MMU should trip to flashing red within 250 ms 5. Restore test: clear the fault; MMU should reset and return control to the controller 6. Diagnostic log: pull the MMU event log; verify no unexpected faults
GAOQIAO MMUs log every fault with timestamp, conflicting phase, and voltage at the time of trip.
Annual Certification
Most US states and many EN12368 markets require annual MMU certification:
- Test every conflict scenario
- Verify program card matches current intersection geometry
- Replace the MMU battery (for the memory backup)
- File a certification report with the authority
GAOQIAO can supply a pre-printed test sheet for each common phase diagram. Match the sheet to the cabinet and walk through the test cases.
Troubleshooting MMU Trips
Common causes of unexplained MMU trips:
- Program card mismatch: intersection was re-phased but the MMU program was not updated
- Wiring fault: a green output is bleeding voltage through to a phase that should be off
- MMU input failure: the sense input or the green input to the MMU itself has failed
- External voltage: a backfeed from a neighbouring circuit
Always start by checking the program card against the as-built phase diagram. A mismatch is the most common cause of unexplained MMU trips.
Featured Products
GAOQIAO supplies conflict monitors and controller cabinets with pre-tested MMU.
Conclusion
Test the MMU annually and on every re-phasing. GAOQIAO supplies MMUs and pre-printed test sheets. Send us your phase diagram for a test sheet package.
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Frequently Asked Questions
How long does the MMU battery last?
3-5 years. Replace at the annual certification.
What happens if the MMU fails?
The intersection goes to flashing red or all-red. No unsafe state is possible.
Can I test the MMU myself or do I need a technician?
The field test is straightforward and can be done by a trained signal technician. GAOQIAO can provide training and test sheets.
Relevant GAOQIAO products
These public GAOQIAO catalogue pages are related to the guide. Open them for the representative description, then confirm the latest drawing and configuration before ordering.
- Traffic signal controllersWired and solar controllers with 6, 22, 44 and 56 output options.
- LED traffic signal heads200mm, 300mm and 400mm signal heads with MUTCD, EN12368, GB14886 compliance.
- Signal polesVertical, signal arm and double-arm poles for any intersection geometry.
- Spare parts and accessoriesSpare LED modules, lenses, visors, surge arrestors and anchor bolts.