Power Limited wiring carries signals to and from devices. Life Safety wiring adds supervision to the wiring to make quick repairs, keeping the system working.
Why is Supervised Wiring So Important?
By Douglas Krantz
Based on Life Safety considerations, supervision makes the difference between building low voltage wiring and building fire alarm system wiring.
Low voltage (Power Limited) Wiring Is Function Wiring
The idea with power limited
wiring is to make sure that power or signals get through, and they aren't interfered with. If something goes wrong with the wiring, the wiring can be fixed the next business day, or week, or whenever it's convenient. No one is going to be hurt if the problem exists a little while.
Fire Alarm Wiring is Function Wiring --- with Added Life Safety Considerations
Fire alarm system wiring takes the concept of low voltage wiring one step further. Not only does it have to function, but because it's there for Life Safety it has to function all the time; if the wiring doesn't work, people can die.
When something goes wrong, Life Safety means that the problems have to be fixed --- now --- before anything bad happens.
Continual Electronic Supervision
For anyone to know there's a problem, when fire alarm technicians aren't on site with their meters, continual electronic supervision of the wiring is the only method that can be used. This type of supervision lets people know there's a problem (trouble), and then the problem can be fixed.
Supervision - the Idea of Life Safety Wiring
For example, if a telephone extension goes bad, especially in a room that's not used very much, that problem may have been caused by something bumping into it. The next month, the problem with the telephone might finally be noticed and then the telephone would be fixed.
The telephone isn't continually supervised.
Instead of bumping the telephone, that object may bump the smoke detector. Without continual supervision
, the problem with the smoke detector wouldn't be noticed until someone went into the room and noticed it was knocked down. Being on the ceiling, it might not be noticed for years, if ever.
But if, before someone noticed the problem with the smoke detector, a fire started in the room, the smoke detector wouldn't detect the smoke. By the time the fire progressed into the next room where there was a working smoke detector, the fire would be out of control.
All this, of course, is unless using the fire alarm system's supervision the missing detector was noticed and fixed right away.
Fire Alarm Wiring Supervision
The whole idea of fire alarm system wiring is centered around life safety. The wiring is continually supervised by the fire alarm control panel (continually checked electronically for faults --- open or broken wiring, ground faults, sometimes wire to wire shorts), and when a problem occurs, the panel shows a "trouble" with the fire alarm system.
Life Safety --- Fixing the Trouble Right Away
When there's trouble with the system, the fire alarm panel shows an amber light and sounds its local buzzer; It gets people's attention. It indicates that something needs to be fixed. It annoys the building's occupants and the building owners until they call for service. The problem then gets fixed.
That's life safety supervision.
More information on Supervised Wiring and Life Safety can be found in the book "Make It Work - Conventional Fire Alarms
" by Douglas Krantz.
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