A continuity test is a check to see if the wires conduct electricity. On a conventional fire alarm circuit, the panel automatically checks continuity. We call this supervising the wires.
The best way to check a circuit is to perform an official test of the devices on a completed fire alarm system. Remember, in order for all the devices work, the wires have to be complete (have continuity) between the panel and the devices.
However, before the system is complete, checking the continuity of the wires of a fire alarm circuit is difficult because the devices are not attached. Once the system is completely installed, using an ohmmeter to check the continuity of the wires is redundant because the panel is already checking continuity of the wires.
Another way of of looking at it is, If there are not any troubles at all showing on the panel, then using its supervision, the panel is saying that the wires have proper continuity.
The Panel's Supervision - the Panel Checking Continuity
Using the panel to check for supervision is the best way to check the continuity of the wires, and the panel will do this test automatically.
- In a Class B conventional input or conventional Class B output building circuit, one that has an end of line resistor, the panel automatically supervises the wire by checking continuity. If a wire comes loose, or breaks, the wire is not continuous, and the panel shows a trouble on the circuit.
- In a Class A conventional input or conventional Class A output building circuit, one that has an end of line resistor, the panel automatically supervises the wire exactly the same way as it supervises a Class B circuit. The exception is only on the location of the end of line resistor. The end of line resistor is located inside the panel. The end of the Class A circuit is located inside the panel so that, if the panel detects an open wire on the circuit, the panel can automatically connect both ends of the circuit together, and make two unsupervised circuits.
- In a Class B addressable building circuit, one that has addressable input and output detectors and modules attached to it, the panel automatically supervises the devices and modules. If all the detectors and modules are OK and communicating with the panel, the wires are continuous between the panel and the devices.
- In a Class A addressable building circuit, the panel also supervises the devices like a Class B addressable circuit, but will also supervise the wires themselves in a similar method to a Class B conventional circuit.
Checking the Continuity of a Wire in a Fire Alarm Circuit
It's not easy to use an ohmmeter directly to check the wires for continuity in a building wide circuit; the circuit has to be complete, but the panel cannot be attached to the circuit because it will invalidate the readings on the ohmmeter.
Sometimes, when the devices in the field are attached to the circuit, the devices will cause invalid readings.
Instead of checking the continuity of wires, check the results.
Measure the Voltage at the Panel
When a panel is powered on, it is providing a voltage to the supervised circuits in the building. Commonly, the supervised circuits are the Initiating Device Circuit
(IDC), the Notification Appliance Circuit
(NAC), and the Signaling Line Circuit
(SLC).
All of the supervised circuits have a voltage at the screw terminals of the panel. Use your voltmeter to see what voltage is at the screw terminals; the voltage at the screw terminals is your starting voltage when checking continuity of the wires.
Even when there are door holders, the auxiliary power supply is supplying voltage to the door holder circuit.
The SLC voltage will be very unsteady on the voltmeter. Get used to the unsteadiness of the voltage on the SLC.
SLC Exception - The exception here is when there is zero volts on the SLC screw terminals at the panel. Zero voltage means that the panel thinks there is a short on the SLC circuit, somewhere in the building.
On rare occasions, zero volts on the SLC screw terminals means that the panel is bad. To check this, disconnect all wires for the SLC circuit: both the Class B wires, and if they are connected, the Class A wires. Even though there aren't any wires on the screw terminals, tighten down the screws so they will make good contact. Then wait a minute for the panel to cycle through its internal testing, and then measure the voltage. If there's voltage, the panel probably is not having problems; if there's still zero voltage, the panel might be bad.
NAC Exception - Most troubleshooting on the NAC will be done when the panel is not sounding the alarm. So that the wiring can be supervised, this is the time when the NAC voltage polarity is in reverse. Take care when connecting horns and strobes. The labels on the horns and strobes are for when the alarm is being sounded. At the time that the NAC circuit is being supervised, the measured positive wire goes on the minus terminal, and the measured negative wire goes on the plus terminal of the horns and strobes.
Look for Voltage at the Devices
When there is voltage at the devices on the circuit, the IDC, the NAC, the SLC, and even the door holder circuit, that means that the wires between the panel and the devices is continuous (has continuity).
If there is a question about whether there is voltage or not, pull down the device and measure the voltage coming to the device over the building's circuit. Checking the voltage at the device shows whether or not the wires are continuous between the panel and the device.
Class A Exception - Circuits for Class A start out of the panel as Class B, and return to the panel as Class A. The panel can send voltage from both ends of a Class A circuit, so use some creativity when troubleshooting a Class A fault.
T-Taps
When using voltage from the panel to check for continuity, if voltage reaches the device, then between the panel and the devices, the wires are continuous. Whether the pathway is Class A (not T-tapped), T-tapped, or even star tapped, if there is voltage at the device, the wires for the building's circuit is continuous between the panel and the device.
Whether the pathway is Class A (not T-tapped), T-tapped, or even star tapped, if there isn't voltage at a device, the wires for the building's circuit is not continuous between the panel and the device.
When the panel is performing wire supervision on a conventional fire alarm circuit, like an Initiating Device Circuit (IDC) or a Notification Appliance Circuit (NAC), all wires have to be in the path of the supervision current. Any wire T-tapped off the wires in the supervision path are not in the supervision pathway, and cannot be checked for continuity by the panel.
In a conventional system, T-taps cannot be used in a Class A or Class B circuit.
Douglas Krantz
