Be careful refurbing old Generators in Schools they could be putting out 120 Volts of DC Power not AC

About 20 years ago a customer of mine bought an old school building and there had been numerous tenants and work done in it. In the basement was an old emergency generator set. The owner said to me you like to tinker with this stuff why don't you see if you can get it to work . Well I check everything out and find there are 2 dead battery's wired in series making it a 24 volt starting system and it needs spark plugs etc. So I get the set ready to use.
The owner fires the generator up and it comes on line only one problem There is smoke in the hall we turn off the generator and we find one of the emergency lights which has a back up battery pack has burned up.
Thats strange I wonder if where over voltage I pull out my rugged Square D Wiggy tester to check what voltage is coming out when the DC- Direct Current flag indicator comes up . Hey wait a minute this generator should be putting out 120 VAC- Alternating Current like all the other ones I have worked on.
Wrong after extensively looking it over and finding prints I find out this generator puts out 120 volts DC not AC For what ever reason it seems generators in schools in PA. where designed for DC power when running in emergency mode not AC.
Well then how did the light bulbs in the signs work ? How can they be AC power one minute DC the next which is what you read when you test them? Well the bulbs are standard Edison based incandescent bulbs and Incandescent bulbs can work with AC or DC So when the generator is not working the buildings 120 VAC power feeds them when the power goes out and generator comes on there feed with 120DC
power in this case with a Shunt Wound DC generator . But Why?
To this day I have not been able to get a good explanation why this was done and the very serious danger this poses to any one remodeling and operating a business in an old school here in Pa. is very real because typically these old exit signs are replaced with modern ones which have transformers and electronic boards and when you hit them with the DC it fries them and they can explode and catch fire.
If any engineer can explain to me why such a dangerous practice was allowed with little to no warning of these systems changing voltages I would really like to know why it was done and why more warnings where not posted.To warn school janitors changing the light bulbs especially since I have found these generators in other schools where the 2 light bulb signs have 1 socket AC only and 1 socket is AC/DC during an emergency the right AC/DC bulb only comes on.
Since many of these schools where built from early 1900's -40' this way there is really no one around to talk to who knows about it.