Moving-Head Thermal Shutdown During a Live Launch
During a two-day product launch, one of eight identical LED moving-head fixtures on a 6 m truss shuts down after 15–20 minutes. The fixture’s fan runs at high speed, its display shows a temperature warning, and it resets only after cooling. The truss is fed from a 32 A distro; the other seven fixtures are stable. The DMX chain is daisy-chained with 5-pin cable, and the last fixture has no terminator. Ambient temperature near the truss is 34 °C. The fixture is still under warranty. A spare identical unit is in the truck, but replacing it requires lowering the truss and a 30-minute rigging pause. The next show block starts in 50 minutes. The show is being livestreamed, and the lighting designer wants no visible gaps.
1. How would you isolate the cause without opening the fixture?
Read the worked answer
First separate fixture-specific from common-cause. Seven fixtures are stable on the same distro, so a site-wide power failure is unlikely, but check the suspect fixture’s plug, breaker, and DMX cable. The temperature warning plus high fan speed points to a thermal or airflow problem: blocked filter, failed fan, dust, or 34 °C ambient near the truss. Do not open a warranty unit. Do a five-minute external check: clean accessible vents, confirm the fan is actually moving air, reseat DMX in and out, and fit the missing terminator. Swap the DMX cable and address with a known-good fixture to rule out data. If the warning returns after 15–20 minutes, treat it as a fixture fault. Record the error, serial number, ambient temperature, and runtime. Do not simply reset it repeatedly; a repeated thermal shutdown can damage LEDs or drivers and it will fail during the show.
2. Would you swap the fixture or keep troubleshooting, and why?
Read the worked answer
Given a 50-minute window, the spare and warranty status favor replacement, but only after a quick external check. Lowering the truss and swapping takes about 30 minutes, leaving 20 minutes for refocus, DMX re-addressing, and a test-tight but feasible if the rigging crew is ready. Use lockout/tagout, inspect the truss and clamps, and use a two-person lift. If the truss cannot be lowered safely in time, disable the fixture, re-patch the remaining seven, and refocus to cover the gap; do not leave a known thermal fault hanging above the audience. Tell the lighting designer and show caller the decision and the new cue. The failed unit goes back in its case with a tag, not into the spare’s position.
3. What documentation and preventive action should follow the event?
Read the worked answer
Write a service report: fixture serial, error message, ambient temperature, time to failure, DMX termination status, and the fact that seven identical units were stable. Send it to the supplier for warranty repair and ask for a loaner. Add a preventive check to the event checklist: clean filters, verify fan operation, confirm terminator, and record truss temperature. On future shows, keep one spare per eight moving heads and schedule a thermal test during focus. If the same model fails again at high ambient, derate or add ventilation rather than treating it as a one-off.