Open circuit fault

An open circuit fault is a break in the continuity of a fire alarm circuit: a cut conductor, a loose terminal, a device removed from the run, or a splice that has let go. On a supervised circuit the panel sees supervisory current through the end-of-line device stop, and reports a trouble rather than failing silently. It is one of the three faults CAN/ULC-S524 names in its single fault rule, alongside short circuit and ground faults, and the 2019 edition requires that any one of them not stop field devices working in more than one Building Code required fire alarm zone. On an addressable loop the same fault appears as devices dropping off the poll instead of as a resistance change.

What an open circuit fault is

The circuit is broken. Current that used to leave the panel, run out through the field wiring, pass through the end-of-line device and return has nowhere to go. On a conventional IDC or NAC the panel reads that as loss of supervisory current and posts a trouble.

The physical causes are unglamorous and repetitive. A conductor pulled out of a terminal because it was landed on the insulation. A wire nut that was never tightened. A device lifted for painting and never put back. A conductor nicked at a box edge that finally parted. A screw through a cable in a stud bay, which more often gives a short circuit fault or a ground fault but can give an open if it cuts one conductor cleanly.

Why it matters more than it looks

The reason an open is treated as a life safety event rather than a maintenance nuisance is what it does to everything downstream. On a circuit without a return path, a single break removes every device past that point. Those detectors still have power on the panel side of the break in some topologies, and none in others, but either way the panel cannot hear them and they cannot report. Protection is gone for that section of building, and the only thing telling you is the trouble light.

That is the whole justification for supervision. The Building Code requires that a fire alarm system be electrically supervised, at Article 3.2.4.9(1) in the Ontario Building Code. Without it the loss would be silent.

The Canadian containment rule

CAN/ULC-S524 does not stop at reporting. Its 2019 edition, as UL set out in a public presentation to BC building officials, requires that where a fire alarm circuit serves more than one National Building Code required fire alarm zone, a single fault, meaning one open circuit fault, one short circuit fault or one ground fault, must not prevent normal operation of input or output field devices in more than one of those required zones.

That is the sentence behind circuit class selection, isolator placement and the whole subject of fault tolerance. We are paraphrasing it. The standard is copyrighted and paywalled, the formal circuit class definitions and the exact tolerances live inside it, and the American equivalents in NFPA 72 are not interchangeable with them.

Finding one

Half splitting works the same way it does for a ground fault, and the method is set out in full in the ground fault troubleshooting walkthrough. Lift all field wiring at the panel to prove panel against field. Reconnect circuits one at a time to find the affected circuit. Then split that circuit at its midpoint in wiring order, not in room order, and keep halving.

An ohmmeter across a de-energised circuit reads infinite on an open and reads the end-of-line value on a healthy one, which makes an open the easiest of the three faults to chase. The trap is wiring order: a circuit that reads sensibly on a riser diagram frequently runs a different sequence in the ceiling, and half splitting by the wrong sequence wastes an afternoon.

A short circuit fault collapses resistance instead of removing it. A ground fault changes neither and needs a dedicated detector. All three are what supervision exists to catch, and all three are what the single fault rule is written about.

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