Measure End-Of-Line Resistance in NAC Wiring

You should have an end-of-line resistor in all class B notification appliance circuits (NAC). To provide circuit supervision, a fire panel or NAC booster applies a reverse polarity voltage on all NAC wiring. At the end of the circuit, the installer places the appropriate resistor; the value is defined by the panel. 

The resistor creates a voltage drop in the circuit, which is how a panel or booster supervises its integrity. When that circuit is compromised, you may need to measure the End-Of-Line resistor value. You should be able to do this at any point along the circuit; however, there is a wrong way to measure the resistor.

To measure any resistance, your volt-ohm meter induces a voltage on the meter test leads. That voltage can interact with the appliance providing a false resistance value, unless you reverse the polarity of your test leads.

 Remember, the panel or NAC booster applies reverse polarity voltage for the supervision voltage and flips polarity to activate the alarms. You should flip your test leads, Red on Black and Black on Red. This will ensure you see the correct EOL.

The more devices between your test point and the EOL, the greater the measured resistance difference becomes. The make and model of the notification appliances can also determine the resistance difference. 

NOTE: Volt Ohm manufacturers are not required to apply a specific voltage polarity to the test leads.

Everything shown here could be backward, depending on the polarity of your test leads. It is primarily importance that you be aware it can make a difference in your resistance readings, allowing you to adjust as needed.