With an end-of-line resistor of 4.7 ohms and the circuit powered down, the positive leg of a fire alarm circuit measured to ground on a 10 kΩ setting should read ______.

• A. 0
• B. infinity
• C. 4.7k
• D. 15 or less

Answer: (B)

End-of-line resistance sits across the two fire alarm loop conductors at the far end to supervise the circuit. When the system is powered down, there’s no active reference to ground on the loop. Measuring from the positive lead to ground with a high-impedance meter (10 kΩ range) finds no complete path to ground, so the reading is an open circuit—essentially infinite resistance. The end-of-line resistor value is seen only when you measure across the two conductors themselves, not to ground, which is why the reading to ground comes out as infinity.