Most fire alarm systems fall into two categories: conventional or addressable. Conventional fire alarm systems are a simple, common, time-proven technology, which protect a large percentage of commercial buildings today. Their reliability and low cost make them ideal for small to medium size properties. Conventional fire alarm systems are characterized by a fire alarm control panel, which holds the entire system’s intelligence. Connected to this panel via hard wires are a number of detectors or initiating devices such as smoke, flame or heat detectors. Additionally, the control panel is wired to notification devices such alarm bells, strobe lights and automatic dialers.

Fire alarm cable, just like networking cables, comes in either shielded or unshielded varieties. Shielded fire wire is usually needed only for noisy EMI (ElectroMagnetic Interference) environments or for extremely long runs. One should note that excessive capacitance becomes an issue here two especially in addressable systems. In shielded fire cable, a capacitor is formed not only between the conductors, but also between each conductor and the shield. While the capacitance values are typically around 30 to 75 pF per foot for each of the two previously listed capacitors, this value adds up quickly to the low hundred μFs with cables runs in thousands of feet.

Another distinguishing detail between different fire wire is the number of conductors. Fire alarm cable is available with two to six conductors, all of which are normally rated for up to 300 VRMS. The number of conductors required is dependent upon the type of system (conventional, addressable or hybrid) and the device type.

This can be a little confusing as fire alarm cable ranges in size from 18 AWG up to 12 AWG (American Wire Gauge; the smaller the value, the larger the wire diameter). The size of the wire required for the job is dependent upon each individual detector or notification device within the circuit receiving sufficient voltage to operate. This is due to the wire itself causing a voltage drop to its own internal resistance. The larger the wire gauge, the less resistance and associated voltage drop.

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