Which supervisory circuit can distinguish an open from an alarm condition in a closed loop circuit?

Which supervisory circuit can distinguish an open from an alarm condition in a closed loop circuit?

• A. Double end-of-line resistor
• B. Single end-of-line resistor
• C. Voice verification
• D. Interface device

Answer: (A)

Supervisory circuits rely on what the ends of a field loop look like to report both device status and line integrity. A single end-of-line resistor can indicate the loop is intact or faulted, but it doesn’t reliably tell apart an open (broken wire) from an alarm condition when the loop is closed, because the resistance changes can overlap in those states. Placing a resistor at both ends of the loop—double end-of-line resistors—gives the controller two reference points. The supervisor measures the overall loop resistance (and sometimes the individual end values) and can map three distinct conditions to three signatures: normal, alarm, and open. In this setup, an alarm condition produces one signature while an open circuit produces a clearly different one, so the system can distinguish between the two reliably. That’s why the double end-of-line resistor approach best satisfies the requirement of distinguishing an open from an alarm in a closed loop circuit. Other options don’t provide that end-to-end, dual-reference monitoring needed for clear differentiation.

Supervisory circuits rely on what the ends of a field loop look like to report both device status and line integrity. A single end-of-line resistor can indicate the loop is intact or faulted, but it doesn’t reliably tell apart an open (broken wire) from an alarm condition when the loop is closed, because the resistance changes can overlap in those states.

Placing a resistor at both ends of the loop—double end-of-line resistors—gives the controller two reference points. The supervisor measures the overall loop resistance (and sometimes the individual end values) and can map three distinct conditions to three signatures: normal, alarm, and open. In this setup, an alarm condition produces one signature while an open circuit produces a clearly different one, so the system can distinguish between the two reliably.

That’s why the double end-of-line resistor approach best satisfies the requirement of distinguishing an open from an alarm in a closed loop circuit. Other options don’t provide that end-to-end, dual-reference monitoring needed for clear differentiation.