Class B Circuit

Last updated: September 2026

Quick answer

• A Class B circuit is a single path out from the panel: device to device, ending at the last one, with nothing returning.
• Everything downstream of an open is lost. The panel reports the fault, it just has no second path to route around it.
• Supervision comes from the end of line device at the far end. The panel pushes a small current through the circuit and that device, so any break stops the current and raises a trouble.
• A short is seen as a short, not an open, and on a conventional circuit it disables that circuit until cleared.
• Class B is the default configuration on most Canadian panels and the normal choice unless a specification says otherwise.
• Installation is to CAN/ULC-S524, referenced directly by the Ontario Building Code in Article 3.2.4.5.
• In residential suites the Building Code accepts Class B only where a fault at one device cannot impair the audible devices serving other suites.

What an open costs you

Picture eight horns on one circuit and a broken conductor at the third. Horns one and two still sound. Three through eight are dead until someone finds the break. The panel is not blind to it: the supervisory current stops, an open circuit fault is annunciated and the trouble buzzer sounds. But annunciating a fault and surviving one are different things, and that difference is the whole argument for Class A.

The same applies to an initiating device circuit. An open partway along a conventional detector circuit leaves the detectors past it unable to report anything, and the panel cannot tell you which ones.

A short circuit fault behaves differently again. Because the panel supervises by current, a short looks like too much current rather than none, and is reported as a short. On an addressable signalling line circuit, isolators rather than circuit class are what stop a short taking the loop down.

The end of line device is the whole supervision scheme

With no return path, the only way the panel can know the conductors are intact is to push something through them and watch it come back. That is the end of line device's job. On most Canadian conventional panels it is a resistor of a specified value, commonly 3.9 kilohms on Mircom equipment, and it belongs at the electrical end of the circuit, on the last device, not in the panel.

That is why one in the wrong place is a real defect. Land it at the second device on a run of ten and the panel supervises two devices while eight sit behind it unmonitored. The panel shows normal. Nothing is watching the wire that matters.

Where Class B is acceptable in Canada

Almost everywhere, with one well known limit. The Building Code requires installation to CAN/ULC-S524 and verification to CAN/ULC-S537. For audible devices inside dwelling units and residential suites, Sentence 3.2.4.18.(9) requires either that a single open at one device not impair the audible devices serving other suites, or that each suite sit on its own signal circuit. The City of Markham reads the first option as Class A wiring, or Class B with signal circuit isolators outside the suites, which is what a suite fault isolator is for. Our guide on residential audibility compares the routes.

Settle one more thing before pricing: which edition of the standard the project is held to, since that is set by the code in force when the permit was issued, not by the newest edition in the ULC catalogue. See which fire alarm standard applies.

What we could not confirm

CAN/ULC-S524 is paywalled and unread here, so no clause number is given for end of line placement or Class B supervision. The resistor value and the placement rule come from Canadian panel manufacturers' manuals, which vary, so use the manual for the equipment in front of you.

Fire Alarm Academy is an independent educational site. We are not affiliated with ULC Standards, CFAA, ECAO, ESA or any authority having jurisdiction, and nothing on this page is certification training. Confirm any design decision with the authority having jurisdiction and the engineer of record.