Isolator

An isolator is a module that opens a faulted segment of a fire alarm circuit so the rest of the circuit keeps operating. The 2019 edition of CAN/ULC-S524 recognises six of them at Section 10.2: field device data, network data, audio buss, field device power buss, equipment power buss, and suite. They exist to satisfy the single fault rule, which does not allow one open, short or ground fault to stop field devices working in more than one fire alarm zone the National Building Code requires. How many devices may sit between isolators is set by the control unit manufacturer, not by any Canadian code.

What an isolator does

A fire alarm circuit that crosses zone boundaries has a problem built into it: one fault anywhere on the run can take down everything on it. An isolator is the hardware answer. It watches the segment of circuit on its far side, and when that segment collapses, most often into a short circuit fault, it opens and drops the segment out. The panel posts a trouble, the faulted section stops working, and everything on the healthy side keeps polling and keeps sounding.

Recovery matters as much as isolation. Once the fault is cleared and the trouble reset, the isolator has to close again and restore the segment without anyone visiting it.

The six types

UL's May 2023 presentation to the Building Officials Association of British Columbia sets out six categories of isolator recognised at Section 10.2 of the 2019 installation standard:

Field device data
Network data
Audio buss
Field device power buss
Equipment power buss
Suite

The first five are circuit and network hardware placed by design. The sixth is the one most technicians meet first, because it is one per dwelling unit and it carries its own placement rule. See the suite fault isolator entry and the guide to where they actually go.

Where the Canadian requirement comes from

No Canadian code says "install an isolator". The chain runs the other way. The Building Code decides how a building is divided into fire alarm zones. CAN/ULC-S524 then requires that a single fault, meaning one open circuit, one short circuit or one ground fault, not prevent normal operation of input or output field devices in more than one of those required zones. That is the fault tolerance rule, and an isolator is one of the two ordinary ways to satisfy it. The other is circuit class.

That is a paraphrase. S524 is copyrighted and paywalled, so we do not reproduce its text, and we do not substitute the NFPA 72 equivalents, which are not interchangeable with the Canadian ones.

What the manufacturer decides, not the code

The number of devices permitted between isolators is not a code figure. It comes from the control unit listing and the installation manual. Mircom's FX-3500 manual, for one Canadian panel, publishes a device limit between isolators on its addressable loops. Read the manual for the panel actually in front of you rather than carrying a number over from the last job, because exceeding it means a single short takes out more of the loop than the design assumed.

What goes wrong

Isolators fitted and never recorded. A technician arriving for an annual has to find every one of them. An undocumented module above a corridor ceiling is a problem handed to somebody else, quite possibly to yourself.

Isolators that will not pass an alarm after a trouble. Some devices will not let a signal sound if the alarm arrives after a short has already put the circuit into trouble. City of Vancouver's bulletin on audible signal devices states that this is not acceptable. Short first, initiating device afterwards: the signal still has to sound.

A design that assumes isolators the drawings never located. Zone boundaries and isolator positions are one decision, made at design stage, not on site with a bag of modules.

Isolation contains a fault. Supervision reports it. Those are two different jobs, and satisfying one does not satisfy the other.

Fire Alarm Academy provides educational content only. It does not confer any certification, licence or credential, and it is not affiliated with, endorsed by, or accredited by the Canadian Fire Alarm Association, the Electrical Contractors Association of Ontario, ULC Standards, UL Standards and Engagement, CSA Group, the Electrical Safety Authority, any authority having jurisdiction, or any manufacturer. Always work from the edition of any standard adopted by your authority having jurisdiction.