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Fire Alarm Circuit Design and Fire Alarm Control Units

Chapter 15

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Objectives

• List the types of initiating, notification, and signaling line circuits, and discuss the differences

• Explain the function of Class A and Class B circuits

• Demonstrate understanding of the function of an end-of-line resistor

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Objectives

• Compare and contrast the effect of an open conductor, a grounded circuit, and a wire-to-wire short

• Explain the function and advantages of an alarm verification feature

• Evaluate the differences between hardwired and multiplex fire alarm systems

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Objectives• Draw a riser diagram, a fire alarm system

plan view, or a schematic fire alarm system diagram for a system

• Calculate the required battery capacity of a fire alarm system

• Select a fire alarm circuit based upon predetermined requirements for grounded conductors, wire-to-wire shorts, open conductors, or number of devices on a circuit4

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Initiating Device Circuits

• Initiating device circuit (IDC): a circuit to which automatic or manual initiating devices are connected to the fire alarm control unit (FACU), where the signal received by the FACU identifies an alarm condition on the circuit but does not identify the specific device actuated

• Class B Initiating Device Circuits 5

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Initiating Device Circuits

Figure 15-1. Operation of a Class B initiating device circuit (part 1)

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Initiating Device Circuits

Figure 15-1. Operation of a Class B initiating device circuit (part 2)

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Initiating Device Circuits • Initiating Device Circuit Problems (Faults)

– Open conductor: IDC Class B fault that can be caused by a cut or broken wire or a loose terminal

– Ground fault: IDC fault that can be caused by a bare wire or terminal in contact with a grounded junction box or some other source of grounding

– Wire-to-wire short: two bare wires touch each other, creating an incomplete circuit

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Initiating Device Circuits

Figure 15-2. Class B circuit fault conditions (part 1)9

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Initiating Device Circuits

Figure 15-2. Class B circuit fault conditions (part 2)10

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Initiating Device Circuits

• Class A Initiating Device Circuits• Initiating Device Circuit Selection (see

Table 15-1, Page 465)• Initiating Device Circuit Alarm Verification

Features

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Notification Appliance Circuits

• Notification appliance circuits (NACs, see Table 15-2, Page 466): fire alarm circuits to which fire alarm notification appliances are connected

• Class B Notification Appliance Circuits• Class A Notification Appliance Circuits

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Signaling Line Circuits• Signaling line circuit (SLC, see Table 15-

3, Page 470): a circuit carrying multiple input and output signals of more than one fire alarm system, transmitter, or device

• Continuous software interrogation (CSI): microprocessor individually checks status of each device or control unit in sequence, then continuously rechecks devices in order 13

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Hardwired And Addressable (Multiplex) Fire Alarm Systems

• Addressable (multiplex) system: requires a central processing unit (CPU) or a computer and software that specifies and assigns the exact locations or addresses of each initiating device on the fire alarm system

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Fire Alarm Control Units

• Fire Alarm Riser Diagram• Fire Alarm System Plan• Schematic Fire Alarm System Diagram• Calculating Fire Alarm Control Unit Battery

Capacity

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Fire Alarm Control Units

Figure 15-11. Fire alarm riser diagram; two wires are shown for each circuit, a Class B system is shown 16

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Ex. 15-1: Calculation of Battery Capacity

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Ex. 15-1: Calculation of Battery Capacity

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Ex. 15-1: Calculation of Battery Capacity (cont’d.)

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Ex. 15-1: Calculation of Battery Capacity

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Ex. 15-1: Calculation of Battery Capacity

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Voltage Drop On Notification Appliance Circuits

• Standard Operating Voltage for Notification Appliances

• Wire Size, Resistance, and Gauge (see Table 15-4, Page 480)

• Calculating Voltage Drop in Fire Alarm Wiring

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Voltage Drop On Notification Appliance Circuits

Figure 15-14. Illustration of Example 15-2 (follows).23

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Ex. 15-2: Calculation of Voltage Drop

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Ex. 15-2: Calculation of Voltage Drop

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Ex. 15-2: Calculation of Voltage Drop

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Summary

• Fire alarm circuits connect initiating devices and notification appliances to the fire alarm control unit that permits the system to meet performance objectives

• Circuit logic, styles, and isolation can be chosen to increase the reliability of a fire alarm system

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Summary

• Fire alarm control unit (FACU) is the nerve center of a fire alarm system– Can display system logic in riser diagrams,

plan views, and schematic diagrams

• Battery backup for FACU is calculated by determining power requirements for all components in the alarm

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Summary

• Calculation of voltage drop in a fire alarm circuit– Important responsibility for a fire alarm system

designer – Provides evidence that notification appliances

will work in accordance with their listing

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