Control and Relay Modules in Fire Alarm Systems: A Practical Guide

Almost every addressable system you commission will need to talk to something that is not part of the fire alarm system: a conventional sounder, a lift controller, a smoke damper, an access-control door, a voice alarm rack. That conversation happens through interface modules — and the two you will meet most often are the control module and the relay module.

Engineers mix these up constantly, on site and in exams. They both sit on the loop, they both take an address, and they both “switch something on when there’s a fire”. But the electrical behaviour is different, the monitoring behaviour is different, and picking the wrong one produces either a non-compliant unmonitored sounder circuit or a burnt-out relay contact. Let’s separate them properly.

The Core Distinction: Powered Output vs Volt-Free Contact

A control module (often badged as a sounder control module or output module) provides a switched, powered, supervised output. When the panel’s cause-and-effect logic calls it, the module passes an external 24 V DC supply out to the connected devices. Because it is energising a load itself, it needs that external supply brought to its terminals — and because it is driving alarm devices, its output circuit is monitored end to end.

A relay module provides a volt-free (dry) changeover contact — common, normally open and normally closed. It does not supply power. It simply makes or breaks a contact that some other system reads as a signal. That is why it needs no external supply and why its output wiring is, by default, not monitored.

One sentence to carry into the exam hall: the control module feeds devices, the relay module tells devices. If the thing on the far end needs power from you, use a control module. If it needs only a trigger, use a relay.

Where the Control Module Earns Its Place

The classic use is conventional sounders on an addressable system. Many loop-powered sounders draw more current than a designer wants to hang off the loop, or the project simply specifies conventional sounders, flameproof sounders or bells in hazardous or industrial areas. The control module becomes the bridge: it takes its address on the loop, it takes a separate 24 V DC feed from a monitored supply with its own standby capacity, and it switches that feed out to the sounder circuit.

Two details matter here and both come up in assessments:

1. The end-of-line device is not optional. Fit the manufacturer’s specified end-of-line resistor at the far end of the sounder circuit, across the last device’s terminals — not looped back in the module enclosure to make the fault light go out. Fitting it at the module defeats the entire point, because the cable between the module and the sounders is then unmonitored. This is the single most common installation fault found on interface circuits.
2. Polarity matters on the output. Conventional sounders are polarised. Reverse a pair and that device will be silent in alarm while the circuit still reads healthy, because the monitoring current still finds its way to the end-of-line resistor. A functional test, not a circuit-integrity test, is what catches it.

BS 5839-1 requires that circuits serving alarm sounders are monitored for open and short circuit, and that a fault on one circuit does not disable alarm sounders throughout the whole building. So when you plan control modules, think about how many you need and where the boundaries sit — not just whether the sounders work. On larger buildings the standard’s requirements around sounder circuit segregation push you towards more modules, not fewer.

Where the Relay Module Earns Its Place

Relay modules are the workhorses of ancillary control. Typical outputs on a UK project:

• Lift homing — sending lifts to a designated floor and parking them with doors open.
• Fire and smoke dampers — de-energising hold-open or driving closed to stop smoke passing between compartments.
• Air handling and smoke extract — shutting down general ventilation, or starting extract fans and pressurisation to the smoke control strategy.
• Door release and access control — releasing hold-open devices and unlocking escape doors so that means of escape are not compromised.
• Voice alarm and PA — triggering a pre-recorded message or handing over to a live announcement.
• Plant and power isolation — signalling a switchgear trip through the plant’s own control circuit.
• Remote signalling — firing a dialler or ARC transmitter.
• BMS interface — passing status to the building management system for monitoring and coordinated control.

The hard rule: a relay module is a trigger, not a power supply. Manufacturer ratings for these contacts are modest — figures in the region of 24 V DC at 2 A are typical, with a lower rating for inductive or mains AC loads. You switch a contactor coil, a controller input, or an interposing relay in the plant panel. You never run a fan motor or a damper actuator through a loop relay. Where the third-party plant sits on mains, the interposing relay and any segregation between fire alarm cabling and other services need to be agreed with the electrical contractor and reflected in the design.

Monitoring, and the Honest Conversation About It

Because relay outputs are usually unmonitored, a cut cable between the module and the lift panel produces no fault on the fire alarm system. Everything looks healthy right up until the day the lifts do not home.

This is a design decision, not an accident — and it needs to be a documented one. On projects where an ancillary function is critical to the fire strategy, the specification may call for monitored interfacing, a monitored input proving the plant actually responded, or regular functional testing recorded in the log book. Some manufacturers offer modules combining a relay output with a monitored input specifically so the panel can prove the far-end state.

Ancillary interfaces are the most frequently untested part of a fire alarm system. If nobody has operated that damper since handover, nobody knows whether it still closes. Build ancillary functions into the servicing regime and record them.

Cause-and-effect logic is what makes all this coherent. Each module holds a unique address, so the panel can decide precisely which output operates for which input — sound the alarm in the zone of origin and its adjacent zones, home the lifts, close the dampers on that floor only, hold the plant on elsewhere. Getting that matrix agreed, written down, demonstrated and signed off is a certification requirement, not an optional extra. A cause-and-effect matrix that was never witnessed is a snag waiting to appear at the first fire drill. Our UK installation walkthrough covers how this fits into the wider project sequence, and the system categories guide explains how the chosen category shapes what ancillary control you are obliged to provide in the first place.

Wiring in Practice

A typical control module for a conventional sounder circuit looks like this. Two terminals take the loop in and out, so the module sits in the loop like any other addressable device. A second pair of terminals takes the external 24 V DC supply — from a monitored PSU with standby batteries sized for the load, not from a convenience socket or a general-purpose supply. A third pair forms the switched output to the sounders, observing polarity, with the manufacturer’s end-of-line resistor at the far end. In alarm, the module LED changes state and the sounder circuit energises.

A relay module is simpler. Loop in and out on one pair. Common, normally open and normally closed on the output side, wired into the third-party controller or a contactor coil that has its own supply. Nothing else. No external 24 V, no end-of-line device.

Some practical habits that separate a tidy installation from a troublesome one:

• Mount modules close to the devices they serve. Long runs on a 24 V output mean volts dropped and sounders that under-perform at the far end. Do the volt-drop calculation rather than guessing.
• Use a proper enclosure. Modules in plant rooms, car parks or external locations need an appropriately rated box. A module cable-tied above a ceiling grid is a future fault.
• Site for access. Every module will be tested periodically. If it lives behind fixed plant or above a sealed ceiling, it will quietly stop being tested.
• Label everything on site and on paper. LIFT-RLY-01, FAN-CTL-03. The label on the enclosure must match the as-built drawing, the address on the loop and the device text on the panel. Three-way consistency here saves hours on every future service visit.
• Record the interface in the certification. Ancillary connections, and whether they are monitored, belong in the design, installation and commissioning documentation.

Exam and Specification Caveats

Two warnings before you carry any of this onto a live job.

First, terminology drifts between manufacturers. “Output module”, “control module”, “relay module” and “supervised output module” do not map cleanly across brands — one manufacturer’s “control module” may offer a selectable supervised or dry-contact output. Always read the specific data sheet for the part you are fitting rather than assuming behaviour from the name on the box. Terminal numbers, end-of-line resistor values and relay contact ratings are manufacturer data, not standard values, and they vary between product generations.

Second, check which standard applies. Some manufacturer literature is written to North American requirements. In the UK your design, installation, commissioning and maintenance sit under BS 5839-1, alongside product standards in the BS EN 54 series, and the electrical installation under BS 7671. Always work to the current published edition of the standard and to the project specification, which may be more onerous than the standard’s recommendations. If in doubt, the specification and the fire strategy win.

How Fire Regs Pro Fits Into This

Interface modules are exactly the kind of topic that separates engineers who can recite a definition from engineers who can specify and defend a design. Fire Regs Pro gives you 1462 questions across 71 topics and 7 exam paths, so you can drill addressable system architecture, cause-and-effect logic, sounder circuit monitoring and ancillary interfacing until the distinctions are automatic rather than half-remembered.

Alongside the question bank, the six calculators handle the numbers that surround this work — including standby battery capacity, which is precisely what you need when a control module adds another 24 V load to the system. The nine quick-reference areas let you practise navigating BS 5839-1 the way you will need to in an open-book assessment, and the field records — certificates, service log and CPD tracking — mirror the documentation you are expected to produce on a real project.

If you are not yet sure which qualification suits you, the exam paths overview and the comparison tool lay out the options side by side. Or go straight to the free quiz and find out how solid your addressable-systems knowledge really is.