By Douglas Krantz
It's assumed that fire alarm systems
have to work all the time because fires happen anytime... but, then again, this is the real world. Problems with the fire alarm system do occur, and that's when it may not go into alarm. To know when it needs to be fixed, the whole fire alarm system needs to be supervised.
If there's trouble inside the fire alarm panel, the panel does its self-diagnostics and indicates trouble. However, when there's a problem in the wiring or devices outside the panel, that's another story.
Polling is an I'm OK Check
Conventional Class A and Class B wiring
directly supervises only the wires
; it does not really supervise the devices. The Signaling Line Circuit (SLC), on the other hand, is different; it supervises the devices.
Unlike the conventional methods, the SLC is a kind of two way street. To supervise, the SLC uses Polling of the Devices. Polling is similar to an I'm OK survey of the entire fire alarm system outside the panel.
To Poll the devices, the panel uses a signal to say 'device 27 report', and device 27 reports back with a signal that says essentially, 'I'm OK.' If device 27 doesn't report back, or if it does report back but says it isn't working correctly, the message on the fire alarm panel indicates trouble on device 27.
Failure of the device to report back could be trouble with the device, or trouble with the wiring. However, because the panel specifies the device in trouble, the troubleshooting process of the fire alarm system is sped up.
Supervising the Wire
With conventional Class A
or Class B
wiring methods, in order to directly supervise the wire, a current is passed through the entire wire loop.
The devices connected to the loop aren't supervised; the panel never checks the devices to see if they work. Unless a detector in the field opens the loop, or a horn or strobe shorts (almost never happens), the fire alarm panel won't indicate trouble.
On the other hand, with the exception of Styles 6 and 7 (equivalent to the conventional Class A), the wires in an addressable loop aren't directly supervised.
Instead of passing a current through the wire, the SLC wiring is indirectly supervised. If, during the polling process, the returning I'm OK signal is received at the panel, the panel assumes that the wiring is complete. That's the indirect supervision of the wire in the loop.
Alarm Signals -- Input and Output Device
For input devices, of course, if smoke detector 27 goes into alarm
, the panel gets the signal from device 27, and using words on the display, identifies smoke detector 27 as the source of the alarm.
Then again, to turn on the horns or strobes is a specific area, the panel can also send signals along the same SLC to the specific output modules it needs to turn on.
Addressable I'm OK Supervision -- It's All About Confidence
Using the "I'm OK" signals of the polling process, the addressable fire alarm system directly supervises the devices on the SLC. With this direct supervision of the devices, the fire alarm system has confidence the building wiring is intact.
You can see how the fire alarm system works by ordering the book "Make It Work - Conventional Fire Alarms
" by Douglas Krantz.
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