I was on a flight out of DFW a week ago, that was on-time, until...

Ice build-up on the power line from the ground to the airplane was alleged to have gotten heavy enough that gravity pulled the plug out, literally. This caused a short on the aircraft, making it lose all lights and power. It took 28 minutes for power and lights to be restored on the aircraft.

This next announcement is what I am inquiring for more information about:

Since the lights went out in the aircraft, apparantly the emergency exit lights automatically came on in the aircraft. When regular lighting was restored 28 minutes later, it was "too late".

Apparantly the emergency exit lights only have a rated lifetime of 30 minutes. (?!) Since that lifetime had been expended, the aircraft was considered to not have any emergency exit lights, and so had to be taken out of service. They said they were pulling one of six similar planes out of a hanger to replace it, we switched gates (from C3-something to A3something), and took off about 2 hours late.

My question is what is the story behind emergency exit lights only having a rated lifetime of 30 minutes? Are they chemical/bioluminescent driven?

Thanks,

Steve