A temperature excursion occurred in a sealed environmental chamber during a 0°C ambient temperature test. An elevated temperature in the chamber resulted in a small fire that was confined to the environmental chamber. Visual observation indicated no damage to nearby equipment, including nearby computer cables.
A committee was assembled with the task of identifying the cause of the incident. The committee concluded that the fire was caused by failure of the heater control system for the environmental chamber. The failure of the heater control system caused the chamber temperature to eventually exceed 200°C. As the chamber temperature increased, plastic materials and electrical insulation present in the chamber started to decompose. A battery scheduled for testing was also located in the chamber. As the temperature of the chamber rose, it is likely that water and possibly potassium hydroxide present in the individual cells of the battery components decomposed to yield hydrogen. At some point, these decomposition products ignited, caused either by the chamber temperature exceeding the ignition temperature or by sparks due to shorting of the electrical leads.
A subcommittee reviewed the electrical schematic and instruction manual for the environmental chamber. This group then energized the chamber to observe and troubleshoot the components crucial to temperature control and over-temperature protection. This was done by trained personnel using safe electrical "hot" work practices and appropriate lockout/tag-out procedures. Testing with the main power on, a simulated temperature was generated to exceed the set point for shut down of the heaters to the chamber. During this testing, it was observed that the over-temperature pilot light illuminated at the set point; however, the trip coil on the circuit breaker did not energize and, thus, did not shut down the chamber. It was clear at this point that there were two failures in the control circuitry; prior to the failure of the circuit breaker, there should have been a shutdown of the temperature control system. Further investigation identified a failure in the temperature controller card, allowing the chamber heaters to come on and stay on. Testing indicates that the chamber does operate, and with replacement of these failed parts, would operate properly.
- Environmental Systems
- Environmental Chamber
The manufacturer will be notified of the failed parts identified as a result of the follow-up testing plan. These results may be useful to them for their information and forwarding to others with the same equipment.