Douglas Krantz - Technical Writer - Describing How It Works

How do we Recognize a Conventional T-Tap?

By Douglas Krantz | Descriptions

On a conventional input or output circuit for a fire alarm system, a T-tapped device is a device that has wires that aren't supervised.

QAHowToRecogniseIDCTtap


QAHowToRecogniseIDCTtap


Greetings Douglas,

I'm going to take a practical exam. My question is a simple one, I believe. One of the things they test for is to see if we recognize that a conventional input zone device is 'T' tapped.

What does this look like?

Thank You, PF

As they ask on the test, I don't know the exact phrasing used for the question, nor do I know the exact phrasing used for the possible answers on the test. However, if you know how the circuit works to supervise the wires of a conventional circuit, the answer becomes obvious.

Is the question: "On a conventional Class A or Class B input zone circuit (Initiating Device Circuit or IDC), how do you determine that a specific conventional input device is on a part of the circuit that has been T-tapped?" In essence, the T-tap is located somewhere else.

Or is the question: "How do you determine that a T-tap on a conventional Class A or Class B input circuit (Initiating Device Circuit or IDC) is located at the terminals of a specific conventional input device?"

Properly Wired Conventional Circuit

In a conventionally wired circuit, T-taps on the circuit are not an issue that can prevent alarms from being detected by the control panel, or being sent by the control panel. T-taps are a supervision issue; whether or not the circuit is capable of showing that all the devices are connected.

Understanding of how the conventional fire alarm system works, and why T-taps are so bad is a prerequisite to recognizing the T-taps.

On a building's conventional fire alarm panel, both an input zone circuit (Initiating Device Circuit or IDC), and an output zone circuit (Notification Appliance Circuit or (NAC) are wired exactly the same. Well, excluding the size of wire, that is.

The parts of the circuit are:
  • Fire alarm control panel

  • Wires

  • Devices

  • End of line resistor

Wire Supervision

The input circuit (IDC) and the output circuit (NAC) for a building's fire alarm system are classified as being either Class A or Class B. The circuits aren't classified by what kind of alarms are sent or received, the circuits are classified by how well they detect failures of the circuit itself, and how the circuits react when there is a problem with the circuit.

At least as far as the wiring goes, because the circuits are classified by how they detect and react to failures of the circuit, there's is almost no difference between a Class A circuit and a Class B circuit.
  • Class B input circuit or Class B output circuit: The end of the circuit, with its end of line resistor, can be located anywhere in the building,

  • Class A input circuit or Class A output circuit: The end of the circuit, with its end of line resistor, is located on the circuit board of the fire alarm control panel.

The circuit is tested for failure by the fire alarm control panel. It performs a continuity test of the wires in the building's circuit; the panel performs "Wire Supervision".

The end of line resistor makes a pair of wires a complete electrical path, which includes the control panel.

The panel is looking for a complete electrical path; the end of line resistor is important because it completes the electrical pathway.

Conventional input devises and conventional output devices are too dumb to be supervised. All fire alarm input devices and all output devices, including conventional and addressable devices, have to be manually tested - by actually trying them.

Wire Supervision



The complete electrical path includes each device in the circuit.

A normal zone input device on an IDC circuit has the supervision current running through its input terminals.

If a wire breaks or comes loose from a connection, the continuity path is interrupted, and a trouble shows in the panel.

However, if a wire breaks or comes loose from a connection, then the continuity of the circuit is broken, and the supervision current sent out by the panel comes to a stop. Because the current stops, the panel turns on a trouble light and trouble buzzer.

The proper way of connecting the wires is to independently connect the wires to each device.
  • If the input or output device has four screw terminals or four pigtails, one circuit wire goes under each screw or one wire is attached to each pigtail.

  • If the input or output device has two screws, one circuit wire goes under each side of each of the two device's screws.


When connecting a device to the circuit, never twist wires together and connect to one side of the screw terminal. Also, never just remove the insulation around the wire and fail to cut the wire itself. Either practice prevents the circuit from sending a trouble to the panel if the wire comes loose from the device. I've seen it happen.

By using this protocol when wiring the devices into the circuit, if even one wire comes loose, the circuit will stop conducting electricity, and the panel detects a trouble with the circuit.

A T-Tapped Circuit Does Not Have an End of Line Resistor



If a device is t-tapped, the extra wires that don't have an end of line resistor are not supervised.

At issue with a T-tap is that the added leg of the T-tapped circuit won't have an end of line resistor. Without the end of line resistor, the circuit wires won't conduct the supervision current.

At the panel, if a device the Tapped circuit is somehow disconnected, or a wire on the T-tapped circuit breaks, there will be no change in the supervision current, so the panel won't show a trouble.

When a wire break or disconnected wire is on a non-supervised T-tap, no one will find out about the fault until the next time the fire alarm system is tested, or until there is a real fire, whichever comes first.

Extra End of Line Resistors

Adding an extra end of line resistor on a T-tapped conventional Class A or Class B circuit means that extra supervision current is drawn from the control panel. One or more issues will forever plague the fire alarm system.
  • A broken wire or disconnected wire will prevent the control panel from detecting the failure.

  • There might be intermittent or continuous false alarms on the input IDC circuit.

  • The output NAC circuit might fail to turn on when there is a fire.

Follow the installation manual and installation sheets that come in the same box as the equipment. This is how the system was tested to work, and any other wiring method, or value of end of line resistor, might not work.


When the T-Tap is Somewhere Else

T-taps can be at a device, and T-taps can be elsewhere in the building, including in a junction box behind the wall.

If the T-tap is somewhere else than at the device you're looking at, the only way of detecting it is to use the panel's trouble light, Ohm's Law, a voltmeter, and an ohmmeter.

If you know that there's a T-tap somewhere on the circuit, use divide-and-conquer methods to find it. It takes lots of work, but the system will work properly after finding and fixing the T-tap.

When the T-Tap is at the Device You're Looking At

Finding T-taps at a device on a conventional Class A or Class B circuit is more of a "Count the Pairs of Wires" issue.

By the way, for the purposes of looking for a T-tap at a device, an end of line resistor is also considered to be a pair of wires.

When looking for T-taps, it's the pairs of wires that are connected directly or indirectly to the device that are important. Other wires going through the back-box can be ignored.

Inside the back-box for the detection device, the zone wires for the IDC circuit either come from the panel, or go towards the end of line resistor. That is a total of two pairs of wires. These pairs of wires are connected to the device.

If there is a third pair of wires in the back box, either connected directly or connected indirectly to the same detection device, that third pair of wires is a T-tap.

Output notification devices, like horns and strobes, are slightly different.
  • A fire horn by itself is a separate device - there will be two terminals or two pairs if pigtails

  • A fire strobe by itself is a separate device - there will be two terminals or two pairs if pigtails

  • A fire horn/strobe can be two separate devices inside the same box - there will be two terminals or two pairs if pigtails for the horn and two terminals or two pairs if pigtails for the strobe

  • A fire horn/strobe can be a combination of the two devices, controlled individually by synchronization pulses on the NAC circuit - there will be two terminals or two pairs if pigtails

The rules, though, for looking for T-taps are the same for each horn, strobe, or combination horn/strobe as the rules for the detection device.

Inside the back-box for the notification device, the zone wires for the NAC circuit either come from the panel, or go towards the end of line resistor. That is a total of two pairs of wires. These pairs of wires are connected to the device.

If there is a third pair of wires into the back box, either connected directly or connected indirectly to the same notification device (horn, strobe, or combination horn/strobe), that third pair of wires is a T-tap.



Douglas Krantz

facpdoug@gmail.com
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