Electrical Theory and Installation for Fire Alarm
Ohm's law and circuit behavior
Fire alarm work rests on basic electrical theory. Know Ohm's law (E = I × R), power relationships (P = I × E), and how voltage drop affects NAC performance. A NAC that is long or heavily loaded may not deliver rated voltage to the last appliance - this is the classic voltage-drop trap.
Series and parallel
Series circuits share current; parallel circuits share voltage. Fire alarm circuits are typically parallel (or loop) configurations so that one device failure does not disable all. Know how a short differs from an open: a short bypasses load (high current, blown fuse); an open breaks the path (no current, trouble signal).
Ground faults and supervision
A ground fault occurs when a conductor unintentionally contacts ground. The FACU must detect and annunciate it. Know that supervision detects opens, shorts, and grounds, and that end-of-line resistors allow the panel to see circuit integrity.
Traps to expect
- Confusing series and parallel voltage/current behavior.
- Forgetting that voltage drop grows with distance and load.
- Assuming a ground fault is always a short.
- Ignoring conductor ampacity and insulation requirements.
Worked example / rule of thumb
A question gives a NAC with a known current draw and asks whether the last appliance gets enough voltage. Rule of thumb: voltage drop = current × resistance of the run, and the drop is doubled for a two-conductor loop. If the resulting voltage at the end device is below its listed minimum, the circuit fails. Compute drop first, then compare to the appliance rating.