A hydrogen alarm sounded when hydrogen buildup occurred in an unmanned switching room containing backup lead acid batteries after the exhaust ventilation fans failed to start at the 1% hydrogen trigger level. Failure of the ventilation fans to vent the normal off-gassing hydrogen from the lead acid batteries resulted in the hydrogen concentration in the room increasing to 2%, which triggered the hydrogen alarm. The alarm was automatically sent to an alarm-monitoring company that alerted the local fire department as well as company personnel of the condition. The fire department was dispatched to the scene and, along with company personnel, provided secondary ventilation to lower the hydrogen concentration to normal conditions. Hydrogen leakage from lead acid batteries is normal, and the procedures in place to respond to increased concentrations of hydrogen worked as designed and lowered the concentration of hydrogen gas before it became a potential problem. No injury of personnel or property damage occurred from this event.

The hydrogen alarm worked as designed and the problem was resolved without incident. Normal operations resumed when the primary ventilation fan was changed to run continuously (24 hours/day, 7 days/week). Another safety precaution was added, consisting of a pressure switch to remotely alert personnel if the room ventilation is lost (i.e., ventilation fans stop working).


Incident Date
Nov 27, 2009
  • Safety Systems
  • Measurement / Sensing Device
  • Ventilation System
  • Exhaust Fan
  • Batteries and Related Equipment
  • Batteries
Damage and Injuries
Probable Cause
When Incident Discovered
Lessons Learned

Redundant safety systems prevented this event from becoming an incident. The 1%-hydrogen-concentration-level-triggered fan was backed up by a 2%-hydrogen-concentration alarm. The alarm is continuously monitored (24/7) by a remote Network Operations Center (NOC).
Since this event, a pressure switch has been added to alarm in case of a fan failure and is also continuously monitored by the remote NOC.
Future standards will require two ventilation fans, one running continuously and the other triggered to start when a 1% hydrogen concentration is reached.

Adequate ventilation of battery charging facilities is addressed in the Lessons Learned Corner on this website.