Whether the signal pathway is wire based, fiber optic based, or RF (radio frequency signals, AKA wireless), the rules are really more concerned with signal paths than they are concerned with electrical current in wires.
Even though the National Fire Protection Association (NFPA) rules and the International Fire Code (IFC) rules do touch on wiring issues, the main intent of the rules and codes in these publications is more that of a pathway function -
Using signal pathways, does the fire alarm system detect fires and alarm people?
Signal Path
The electrical current route through a circuit, and a signal path through a building have to be thought about two different ways. The electrical current route shows where the electrons flow around a circuit; the signal and power flow show the pathway between the source of the signal or power, and the load.
However, a signal doesn't exist on its own; a signal is always carried by power. A TV signal is carried by the power of the transmission, a cellphone conversation is carried by the transmissions of cellphone towers, a singer's voice is carried by sound pressure changes the air, etc. Even a person waving to show where they are is using the power of light to carry the signal of the waving.
As far as a Fire Detection and Alarm System (FDAS) goes, the method of carrying a signal isn't the important part, the ability for the detection devices to detect fire and then sound the alarm is the important part.
When looking at the classification of fire alarm circuitry, the wires, or the fiber optics, or radio waves (RF or Wireless) are not the important parts, how the signal can get around a break or fault in the circuit is the important part.
T-Taps
In a fire alarm's two-wire electronic circuit, there are three kinds of T-taps: Wire T-taps, Circuit T-taps, Pathway T-taps.
For the code writers, however, T-tapping a fire alarm circuit is about T-tapping a pathway. Even though a junction box may have both wire T-taps and circuit T-taps inside it, the junction box does not necessarily have a pathway T-tap in it.
No Pathway T-Tap
Wire T-taps: There are eight (8) wire T-taps. Each device on the circuit is T-tapped to the positive circuit wire, and also to the negative wire. That's a total of two wire T-taps per device.
Circuit T-taps: There are four (4) circuit T-taps. Each device is T-tapped to the two-wire circuit. That's a total of one circuit T-tap per device.
Pathway T-taps: This circuit has zero (0) pathway T-taps. The pathway just continues on through the junction box.
Even though there are both electrical and circuit T-taps, because the signal has only one place to continue-on, there are no pathway T-tap.
Detail of a Smoke Detector at a Junction Box
Looking at a detail drawing of a smoke detector shows us all three types of T-taps for a device. Many times, the wire T-taps and the circuit T-taps are visible, but the T-taps are still in the detector.
This shows the wiring inside a junction box.
The positive circuit wire has a wire T-tap going to the actual smoke detector inside the detector's case, and the negative wire has a wire T-tap going to the actual smoke detector inside the detector's case. The signal splits; that's a total of two (2) wire T-taps.
The circuit, with its four wires, is T-tapped to the detector inside the case. The signal splits; that's one circuit T-Tap.
The signal goes past the detector, connects to the detector, and continues on; it does not go anywhere else. That's zero pathway T-taps.
One Pathway T-Tap
This Class B SLC circuit shows all three types of T-taps.
Wire T-taps: There are twelve (12) wire T-taps in this circuit. That's two (2) wire T-taps for each device, plus two (2) wire T-taps where the circuit and the pathway is split.
Circuit T-taps: There are six (6) circuit T-taps in this circuit. One (1) circuit T-tap for each device, plus one (1) circuit T-tap where the circuit and the pathway are split.
Pathway T-taps: There is one (1) pathway T-tap in this circuit. The signal comes out of the panel at the screw terminals, past the first device, then is split two different ways.
Isolators
Isolators are devices that are inserted into a signal pathway. However, when an isolator is inserted into a pathway, the pathway is not split to go two different directions. The pathway is not T-tapped.
If there's an electrical short in the circuit, the isolators shut off the electrical current route in order to isolate the building's circuitry from the shorted circuitry. By separating the shorted circuit from the rest of the circuit, the short won't affect the rest of the circuit.
Another way of looking at it is that a circuit isolator separates the building's signal path from the shorted signal path. This becomes an interruption in the signal path.
In a two-wire Class B circuit, an activated isolator separates the good part of the circuit from the shorted part. That way, even though the devices further from the panel are cut off from the panel, at least the devices connected to the good part of the Class B circuit are still connected to the panel.
In a Class A circuit, when the wires are shorted together, an isolator on each side of the short is activated. This makes a wiring sequence:
When there's a short on the circuit, it's as if there are now two not-supervised Class B circuits instead of one Class A circuit.
Because both ends of the Class A circuit are connected to the panel, the devices on both sides of the isolated short circuit are still connected to the panel.
T-taps on the Isolators
All devices in a fire alarm system are required to be "Listed for Use". That means that someone other than the manufacture (Like UL, ULC, FM, CE, CCC, etc.) has confirmed that the devices, control panel, and wiring scheme follow the Fire Alarm Guidelines, like the guidelines shown in the National Fire Protection Association (NFPA 72) or the International Fire Code (IFC).
In the installation sheets that come with the isolators, the exact circuit that was used for confirming that the isolators worked as advertised is shown, exactly.
If the installation sheet that comes with the isolator says that the isolator can be used in a Class A pathway system, then, if the wiring shown in the installation sheet is exactly followed, the isolator can be used in a Class A pathway fire alarm system.
Follow the instructions in the installation sheet that come with the isolator. When the installation sheet is followed, the installation is "Listed for Use". Also, follow the instructions shown in the installation manual for the control panel.
Douglas Krantz
