Short Answer
When a Class A circuit is T-tapped, the circuit isn't a Class A circuit anymore.
If the former Class A circuit is an addressable circuit, it probably is a Class B circuit: there isn't an alternate (redundant) signal path to one or more devices.
If the former Class A circuit is a conventional circuit, it is now a Class E circuit: one or more specific devices can be disconnected, and the panel won't ever show a trouble.
Long Answer - Classification of a T-Tapped Circuit
The classification of a building-wide circuit is a description of how well a circuit is supervised. The circuit can be conventional or addressable. The circuit can be two-wire, four-wire, fiber optic, wireless (RF or Radio Frequency Signals), telephone lines, local area computer network, or internet. The description also describes what the circuit can do about problems, even before the circuit can be repaired.
In a Class B circuit (pathway), if the panel can't properly communicate with a device, the panel shows a trouble. In a Class A circuit (pathway), if a wire breaks so the pathway is blocked, the panel will then also switch to using an alternate pathway, allowing the panel to communicate with all the devices, anyway.
Supervision for the two most common classifications for fire alarm circuits are Class B and Class A. Class E circuits are basically not supervised at all.
The description of how a building-wide fire alarm circuit is supervised is the circuit's classification. Changing the wiring of a circuit, for instance, is to change the circuit's classification. For instance, all devices on a Class A circuit have a primary route to get their alarm signals to or from the panel, and all the devices have an alternate route.
In essence, a technician or installer is perfectly capable of adding a T-tap, however, doing so changes the wiring of the Class A circuit. Because of the T-taped signal path, one or more devices no longer have an alternate path for signals, and the circuit is no longer a Class A circuit.
Conventional Supervision is Wire Supervision
In a conventional fire alarm system, wire supervision only checks the wires; wire supervision is basically a continuity check of the wires. In order to test wires for continuity, the end of line resistor is essential because it completes the electrical path.
Conventional Class B circuits are basic circuits that, for supervision, don't do anything other than monitor (supervise) device connection. The conventional Class B classification is a nice way of saying, "Only the wires are supervised by the panel." To be at least a Class B circuit, "If any of the devices on the circuit can't communicate with the panel, the panel will show a trouble."
When there's a T-tap on a conventional Class B circuit, the T-tapped device wiring won't be checked for continuity, and the T-tapped device wiring won't be supervised.
Yes, a T-tapped device will probably send or receive an alarm, but because it's not supervised, no one will know if is disconnected. The only way anyone will find out if it is disconnected is to notice that device doesn't work, either when performing a regular test, or when the device doesn't work during a real fire, whichever comes first.
When there's a T-tap, even though the circuit may be referred to as a Class B circuit, it's really is a Class E circuit because, if a device is disconnected, the panel won't show trouble.
Conventional Class A circuits are wired identically to conventional Class B circuits, with the exception of where the end of line resistor is located. In a conventional Class A circuit, the end of line resistor is located inside the circuitry of the panel.
The reason the end of line resistor is located inside the panel is so an alternate path (redundant path) for the input or output signals can be provided. When the panel discovers that a wire has broken or come loose, the panel automatically switches to using the Class A input as the alternate circuit.
The bottom line with a Class A circuit is that all of the devices can still send alarms to the panel, or receive alarms from the panel, even before the circuit can be fixed by a technician.
It's the alternate route to all the devices that upgrades the circuit from a Class B circuit to a Class A circuit.
Addressable Supervision is Device Supervision
Addressable Class B circuits are allowed to have signal path T-taps. The T-taps are allowed because, rather than using an end of line resistor to run a continuity check of the wires, each device, by itself, is an end of line device.
In addressable Class B supervision, the panel takes attendance (the panel polls the devices), just like a school teacher. The panel uses data on the SLC circuit, saying "Hey, device 27 are you there?" Then device 27 responds using data on the SLC circuit and says "This is device 27, I'm here and I have no alarms."
With addressable Class B SLC circuits, it is just assumed that if a device can receive signals from the panel, and can send signals to the panel, the signal pathway (in this case, the SLC wires) is good.
The panel does it's polling, or taking attendance, whether it's a Class A circuit or Class B circuit. When the panel doesn't get a response from one of the devices, or if a device says that it's in trouble when it responds, the panel turns on its trouble light.
Addressable Class A circuits cannot have signal path T-taps. Just like conventional Class A circuits, addressable Class A circuits provide an alternate signal pathway. It's the alternate route to all the devices that upgrades the SLC circuit from a Class B circuit to a Class A circuit.
In a Class A circuit, the panel includes its own Class A input terminals when it takes attendance. The panel checks the whole circuit pathway route through the building.
Basically, in addition to taking attendance of all the normal devices on the addressable Class A circuit, the panel calls out on the Class B terminals of the SLC, saying "Hey, Class A input on the panel, are you there?" Then the Class A input on the panel responds using data sent back along the SLC, saying "This is the Class A input, I'm here." That way, the pathway wires are checked to make sure they are complete, one end to the other.
The bottom line with a Class A circuit is that all of the devices can still send alarms to the panel, or receive alarms from the panel, even before the circuit can be fixed by a technician.
The problem occurs when one or more devices are on a T-tap. If a wire to the devices on a T-tap come loose or break, the devices on a T-tap don't have the alternate signal route, and are taken out of the fire alarm system . . . at least until a technician can arrive to fix the circuit.
Will a Device on a Class A Circuit Still Communicate if a Class A Circuit is T-Tapped?
T-tapping a Class A circuit takes away the Classification; the circuit is no longer a Class A circuit.
Would a T-tapped device still work? Probably. But I wouldn't count on it always working.
Douglas Krantz
