Initiating devices – Potter Fire Alarm Systems User Manual

Page 16

Fire Alarm Training • 8700055 • Rev F • 4/10

DWG# 55-1

POS. +

FACP CLASS A ZONE

NEG. -

SMOKE

DETECTORS

The Signaling Line Circuits (SLC) in addressable systems have additional devices that can isolate short circuits so the loop

will still fully operate. These types of installations are generally required in government installations, some hospitals and some

schools. These systems are more expensive to install and may require additional equipment, however there is greater redundancy

built into the system.

Initiating Devices

Smoke Detector Placement

Detector placement is critical to early warning functions. In order to provide an effective early warning of a developing fire

situation, smoke detectors should be installed in all areas of the protected premises. The location, quantity and zoning of detectors

should be engineered to provide maximum life safety.
Defining the smoke chamber

The continuous, smoke resistant perimeter boundary of a room or area to be protected between the upper surface of the floor and

the lower surface of the ceiling. The smoke barrier does not have to be a solid structure from floor to ceiling. A solid structure that

extends 18” or more from the ceiling would constitute a smoke barrier.

Open “grid” material is considered solid when:

* The openings are less than 1/4” in the least dimension

* The thickness of the material exceeds the least dimension

* The openings constitute less than 70% of the area of the perforated material

DWG# 55-2

POS. +

FACP CLASS B ZONE

NEG. -

Circuit Types

Fire alarm circuits are wired either Class A or Class B and both have advantages and disadvantages. The Class A circuit utilizes

a pair of wires to attach to all of the devices and then a pair returns back to the panel. NFPA 72 requires that a minimum distance

separates the outgoing and return wires on class A wiring. If a break occurs anywhere in the circuit, every device is still active due

to the redundant circuit paths. There are four wires, two supplying power to the front of the circuit and two supplying power from

the end of the circuit.

As described earlier, the conventional systems use the end of line resistor to monitor the status of the zones. If a wire is broken

or a device removed, the panel detects the lack of the end of line resistor and annunciates a trouble condition. Similarly most

notification appliance circuits use an end of line resistor as well. These circuits are monitored for open circuits as well as short

circuits. In addition, both circuits are monitored for ground faults or a high amount of resistance to ground potential. A ground can

impede the proper operation of a circuit.
Detectors and notification appliances must be wired to ensure supervision of the device. Removal of the detector head or

notification appliance must cause a trouble condition on the panel.
Conventional panels send a small amount of power out to the initiating devices and end-of-line resistor. The panel measures the

amount of current being consumed. If the current is in the normal or midrange the panel is normal. If the panel detects too little

current that is indicative of an open and the panel indicates a trouble condition. If the panel sees too high of a current the panel

interrupts that as a short and indicates an alarm condition. Dry contact devices such as pull stations and heat detectors put a direct

short on the initiating circuit and put the panel into alarm.
The most common installation is Class B utilizing two wires that connect to each device and an end-of-line resistor for wiring

supervision. Unlike Class A circuits, Class B circuits will not fully operate when a wire break occurs. Every device down stream

of the break will be unavailable until the wire problem is corrected. The panel will, however indicate a trouble signal.