Alarm Technician Wiring & Circuits Flashcards

Ohm's Law states V = I × R. If a zone circuit has 12 V applied and the EOL resistor is 1,000 ohms, approximately how much current flows in the normal (resting) state?

Answer: 12 mA

I = V / R = 12 V / 1,000 Ω = 0.012 A = 12 mA.

Which condition describes an 'open circuit' on a supervised fire alarm zone?

Answer: A break in the conductor preventing current flow

An open circuit is a complete break in the conductor so no current can flow, causing the panel to register a trouble condition.

A notification appliance circuit (NAC) is wired in:

Answer: Parallel, with devices connected across the two conductors

NAC devices (horns, strobes) are wired in parallel so each device receives full circuit voltage regardless of how many are on the loop.

When running low-voltage alarm cable parallel to 120 VAC power wiring, the NEC recommends a minimum separation of:

Answer: 6 inches

NEC Article 725 and 760 require at least 2 inches of separation from power conductors, though 6 inches is the widely observed best practice for alarm cable.

A technician tests an IDC zone and measures infinite (OL) resistance. This most likely indicates:

Answer: An open circuit or missing EOL resistor

Infinite resistance across the zone means the circuit is broken or the EOL resistor is absent, preventing any current path.

Reverse polarity on a 2-wire smoke detector connected to a 24 VDC IDC circuit will typically:

Answer: Prevent the detector from powering up and reporting an alarm

Most 2-wire smoke detectors are polarity-sensitive; reversed polarity blocks the internal rectifier and prevents proper operation.

The primary reason for twisting wire pairs in an alarm cable run is to:

Answer: Cancel induced electromagnetic interference through differential signal cancellation

Twisting causes equal and opposite noise voltages induced in each half-twist to cancel, reducing EMI pickup on the signal pair.