They where the first modern Fire Alarm Systems to come to market they used 120 volts AC power to operate and ring the bells and had a 2nd power leg from a different phase which acted as a back up trouble signal if fuse on primary blew.
Some systems where pretty simple Pull Stations and bells or horns and some times Heat Detectors. many where installed between 1930s and early 70s
No warning lights to tell you something was wrong only way to tell was a small bell which came on when a fuse blew on primary power leg .
The bells and horns where wired in series which means if one went bad or 1 simple wire break entire line of horns or bells did not work .
Then there where fancy complicated systems which punched out pull stations and codes but again where AC power only with series bells and horns
But one good thing these systems where considered bullet proof but this is not the case now that they are getting over 70 years in some cases protecting apartment buildings and town halls they are starting to silently fail.
The problem the simple plunger switch in many of these pull stations or simple snap switch
and go to a grounding situation and since many of these older systems are all in conduit a violent short circuit develops and damages the panel and in many cases the back up power phase does not turn on the trouble bell. So now you have a non working system which everyone just assumes is working when it is not.
Systems are suppose to be tested annually but many times they are not .
I just recently took over an old Faraday System in an apartment building and this is exactly what happened and only god knows how long the system was not working as previous building owner was a slum lord.
Yes the twilight of many of these old style panel is now upon us and its just a matter of time when a serious fire breaks out and building occupants will not be notified.
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