BAS peripheral devices are the relays, current sensors, power supplies, energy meters, and wireless devices that sit at the edge of a building automation system (BAS): the parts that switch a load, prove a motor is running, meter power, or carry a signal back to a controller. The controller and supervisor get most of the attention, but the peripheral layer is where a job is specified, wired, commissioned; and, increasingly, where it passes or fails a security review.
Four decisions come up most on this layer: selecting a current sensor that reads a load correctly, commissioning a networked device without losing a day to addressing, getting a connected device through IT review, and installing wireless that holds up on a retrofit. Each links to a deeper guide on its topic.
Functional Devices, Inc. manufactures across this layer: relays, current sensors, power supplies, networked BACnet devices, energy meters, and EnOcean wireless devices.
What Counts as a BAS Peripheral Device
A BAS has a hierarchy. At the top sit the supervisor and the controllers that run sequences. Below them is the peripheral layer: the field devices that turn a controller's command into a physical action and turn a physical condition into a signal the controller can read.
A relay switches a load on a command. A current sensor reports whether a motor or load is running, or how much current it draws. A power supply or transformer provides the low-voltage power the other devices run on. An energy meter measures consumption. A lighting control relay switches lighting, including the emergency lighting that has to come on when normal power is lost. A networked device joins a communication trunk and exposes its points to the system. A wireless device does the same without a home run of cable. These are the parts a contractor or panel builder selects, mounts, and wires, and the parts an OEM specifies into a design. The rest of this guide takes them by the decision that governs each one.
Relays: The Core Switching Device
A relay is the most basic peripheral device, and it does one thing: it switches a load on a command. A relay has two sides. The coil, or control, side takes a low-voltage signal from a controller or a switch. The contact, or load, side carries the circuit being switched. A small signal on the coil opens or closes the contacts, so a controller can switch a motor, a fan, or a lighting circuit without routing that load through the controller itself.
Functional Devices builds these as the RIB® (Relay-In-a-Box®) line: a relay pre-wired in an enclosure with its coil and contact terminals broken out, so it mounts and wires without building up a relay-and-socket assembly by hand. The RIBU1C is the most widely used: an enclosed pilot relay with a 10 A SPDT contact and a coil that accepts 10–30 Vac/dc or 120 Vac, which covers most low-voltage control signals from a single part number.
Two specifications decide whether a relay fits the load. The contact rating has to cover the switched load; RIB relays run from 10 A through 20 and 30 A across the Pilot and Power Series. The pilot-duty rating matters when the thing being switched is itself a coil, such as a contactor or a solenoid, because an inductive load is harder on contacts than a resistive one. Not every relay carries a pilot-duty rating at every voltage. The RIBU1C, for one, has no 120 Vac pilot-duty rating, so a 120 Vac solenoid calls for the RIB2401B instead.
Check the rating against the actual load before specifying, and when the load is a motor starter or solenoid, confirm the pilot-duty figure rather than the general contact rating.
Power Supplies and Transformers
The controllers, sensors, and relays on a panel run on low-voltage power, usually 24 V, and something has to supply it. A transformer steps line voltage down to 24 Vac. A power supply does the same and, for devices that need direct current, regulates the output to 24 Vdc. On most building automation panels this power is wired as a Class 2 circuit, a power-limited classification that allows lighter wiring methods than a line-voltage circuit, so the devices that supply it are built and listed to Class 2.
Functional Devices makes both. The TIB series is an enclosed transformer that steps 120 Vac down to 24 Vac, mounts on 35 mm DIN rail, and carries an integrated circuit breaker for overcurrent protection; it comes in 40, 50, and 96 VA sizes and replaces the foot-mount, open-frame transformers that used to sit loose in a panel. The PSH series is an enclosed 24 Vac Class 2 power source built around one or two transformers, with a breaker and an output LED, in 40, 75, and 100 VA sizes. For devices that need regulated direct current, the PSM series supplies isolated, regulated 24 Vdc from a 120 Vac or 24 Vac input, and PSH models with a -DC suffix add a switching 24 Vdc output.
Size the supply to the connected load with headroom, and account for derating. A power supply's usable output drops in a hot or crowded enclosure, so the nameplate VA is a ceiling, not a target. Adding up the draw of every device on the secondary and leaving margin is cheaper than chasing a brownout that only shows up once the panel gets warm.
Current Sensing and Monitoring
A current sensor answers one of two questions: is the load running, or how much current is it drawing? The first is a job for a current switch, which outputs an on/off contact closure to a binary input. The second is a job for a current transducer, which outputs an analog signal proportional to the current. Both are built around a current transformer (CT): the CT is the sensing element, and the switch or transducer is the finished device built around it.
The application that breaks ordinary current sensing is the electronically commutated motor (ECM). An ECM keeps its drive electronics energized whenever it has power, so it draws a small standby current at rest and only a little more at low speed. A standard current switch with a high minimum trip point never registers the running motor; one with a trip point below the standby draw never releases. Either way, the BAS gets a wrong status point.
Four specifications decide whether a current sensor reads an ECM or fan-wall load correctly. The trip point has to be adjustable and low enough to set between the standby and running currents. The hysteresis has to be tight enough that the contact holds steady near the setpoint. The core type is split-core for retrofits, where the sensor clamps around an existing conductor without disconnecting it, or solid-core for new work. The output type has to match the BAS point: a switch's contact to a binary input, a transducer's analog signal to an analog input.
On a fan wall, an array of small ECM-driven fans replaces one large fan. A single switch on the array feed proves the wall is energized; per-fan or per-branch sensing, with a trip point low enough to register one small motor, catches a single failed fan. Match the resolution to what the BAS has to know.
Functional Devices makes current switches and transducers for these loads, including the RIBXGTA-ECM, a split-core current switch with an adjustable trip point as low as 0.25 A, and the RIBXG420 series of split-core 4–20 mA transducers for analog monitoring. The full selection logic, including switch-versus-transducer and fan-wall sizing, is in the Current-Sensor Selection Guide.
Networked Devices and BACnet Commissioning
A networked field device joins a communication trunk and exposes its points to the BAS as BACnet objects. On an MS/TP trunk, commissioning has three parts: addressing, physical connection, and the point list.
Addressing uses two identifiers, and confusing them is a common error. The MAC address identifies a device on its local MS/TP segment, where master devices use 0 through 127. The device instance identifies the device across the whole BACnet internetwork and has to be unique system-wide, in the range 0 to 4,194,302. On a Functional Devices BACnet relay, the MAC address is set on the on-board DIP switches, and the device defaults its Device ID to 277 followed by that address.
The physical connection is an RS-485 daisy chain: shielded twisted pair, consistent A/B polarity end to end, the same baud rate on every device, and exactly two end-of-line terminating resistors, one at each physical end. Reversed polarity and a baud-rate mismatch are the first two things to check when a device won't appear in discovery.
The point list is the map between a device's BACnet objects and the names and logic in the BAS. Each point is an object with a type and an instance: Binary Output (BO) for a relay, Binary Input (BI) for a dry contact, Analog Input (AI) for a sensor. Oversized point lists are a time sink; map the objects the sequence needs and leave the rest. One device-specific detail to confirm against the datasheet is where object instances start, since a map built for a 0-based device is off by one on a device whose instances start at 1.
Functional Devices makes BACnet MS/TP field devices that join the trunk as one addressable node, including the RIBTW2401B-BC relay (one Binary Output, one dry-contact Binary Input). The full commissioning walk-through, including discovery in the major platforms, is in the BACnet Commissioning Guide.
Energy Metering on the BAS
Energy sub-metering puts consumption data on the same network as the rest of the controls, so a facility can see where power goes without standing up a separate metering system. A networked sub-meter reports its readings as BACnet objects, and the BAS trends and logs them the same way it reads any other point.
The reason to sub-meter is that a single utility meter at the service entrance gives only a building-wide total. A sub-meter on a feeder, a tenant space, or a large piece of equipment breaks that total down, which is what makes cost allocation, tenant billing, and measurement-and-verification possible. The same per-load data surfaces problems early: a chiller or a rooftop unit drawing more than its baseline shows up in the trend before it turns into a complaint.
Where the meter sits decides what the data answers. A meter at the main measures the whole building. A meter on a feeder isolates a floor or a wing. A meter on a single large load, such as a chiller or an elevator bank, tracks that equipment on its own. Plan the metering points to the questions the facility needs answered, the same way the point list is planned to the sequence.
For billing-grade measurement, a revenue-grade meter is built and tested to a tighter accuracy class than a general monitoring meter, which matters when the readings back a tenant invoice. Functional Devices makes the RIBPM413-BC, a revenue-grade BACnet MS/TP energy sub-meter that commissions onto the trunk with the same addressing and wiring steps as any other MS/TP device, so a metering point reaches the BAS the same way a relay point does.
Lighting Controls
Lighting is one of the larger loads a BAS switches, and part of it is governed by life-safety code: egress lighting has to illuminate when normal power is lost. The device that handles this is an automatic load control relay (ALCR), a relay that monitors normal power and forces the emergency lighting on when it drops out, so ordinary fixtures can double as emergency egress lighting instead of installing a separate emergency set.
Functional Devices makes these as the ESR series, listed to UL 924, the standard for emergency lighting equipment. The ESRN is a 20 A SPST automatic load control relay for universal 120–277 Vac, with a 0–10 Vdc dimmer override and a dry-contact fire-alarm interface. While normal power is present, the lighting is controlled the usual way, by a wall switch, an occupancy sensor, or a controller. When normal power drops, the relay's normally closed contact falls closed and drives the load to full output regardless of the switch position. The ESRB is a version that mounts in a ballast channel, and the ESRTB is a remote test button for the periodic test these systems require.
An ALCR is one accepted way to meet emergency-egress-lighting requirements; whether it suits a given project is an engineering decision, so the listing and the application should be confirmed against the design. For switching ordinary, non-emergency lighting, the RIB relays covered above and the wireless devices covered below both apply.
Cybersecurity and IT Approval
Once a field device connects to a network, it falls under a security review. An IT or security team often evaluates a connected building device before it goes on the network, and a device that can't answer the questions can stall a project. The reason is lateral movement: a device on the building network can become a path to the rest of it. In the 2013 Target breach, attackers entered through a heating, ventilation, and air conditioning (HVAC) vendor's stolen network credentials and, with no segmentation in the way, reached payment systems.
A security review asks a consistent set of questions: what can reach the device and what it can reach, whether its traffic is encrypted, whether default credentials can be changed, whether the firmware can be patched, and whether the vendor documents how the device communicates and is hardened. Knowing these in advance lets a specifier answer them at design time.
A device's security profile follows its connection type more than its function. A non-networked relay has no network identity and adds nothing to the attack surface. A serial MS/TP device sits on an RS-485 bus with no IP address and can't be reached without physical access to the bus. A BACnet/IP device has an IP address and often a configuration web page, so it has to be segmented, credentialed, and patched like any networked host. BACnet Secure Connect (BACnet/SC), an ASHRAE addendum, adds TLS and certificate-based authentication for connected devices on a managed network. The lever a specifier controls is connection type: a device that only switches a load on a local command doesn't need an IP address, and leaving it off the IP network removes it from that attack surface.
Functional Devices makes field devices across these connection types, from non-networked relays to the serial RIBTW2401B-BC and the BACnet/IP RIBTW2421B-BCIP. The full review checklist is in the BAS Device Cybersecurity Guide.
Wireless Devices for Retrofit
Wireless is worth specifying on a retrofit when running new wire is the costly part of the job: through finished walls, occupied spaces, or across a slab. The question that decides the specification is whether the device holds up after install, meaning a reliable signal and no maintenance to schedule.
Power is where wireless sensors divide into two camps. A battery-powered sensor carries its own cell, a maintenance item that has to be tracked and replaced across a building of devices. A self-powered sensor harvests the energy it needs from its surroundings: a switch press, ambient light, or a temperature difference. EnOcean, the energy-harvesting wireless standard these devices use, is built around this, so there is no battery to replace. The receivers the sensors talk to are line-powered because they switch loads anyway, which leaves a system with no batteries in it at all.
Reliability is a question of whether the signal reaches the receiver. A typical EnOcean transmitter reaches a receiver at roughly 50 to 150 feet indoors, farther in open air, with walls and metal cutting that distance. Receivers double as repeaters to extend coverage, and the 902 MHz band passes through building construction better than the 2.4 GHz used by Wi-Fi and Bluetooth. The wireless signal terminates at a line-powered receiver relay, which switches a load directly or passes a dry-contact or 0–10 Vdc output to a controller, so the wired BAS picks up from the receiver.
Functional Devices makes both halves of an EnOcean system: self-powered transmitters such as the WWS2-EN3 wall switch and the FDLTVC occupancy sensor, and line-powered receiver relays such as the RIBW277B-EN3. The full retrofit guidance is in the Wireless BAS Sensor Guide.
Specifying the Peripheral Layer From One Supplier
The decisions above span several product categories, and sourcing them from one manufacturer keeps the peripheral layer consistent across a panel. Functional Devices builds the RIB relays covered earlier, along with current sensors, power supplies, transformers, lighting controls, networked BACnet devices, energy meters, and EnOcean wireless devices.
For a specifier, that means the power, switching, proving, metering, and lighting control for a panel, plus its serial, IP, and wireless points, can come from one source with one set of conventions: the same DIP-switch addressing across the BACnet devices, the same split-core form factor across the current sensors, and the same EnOcean system across the wireless line. The deep-dive guides linked above cover how to select and commission each category.
Frequently Asked Questions
What is a BAS peripheral device?
A field device at the edge of a building automation system: a relay, current sensor, energy meter, or wireless device that switches a load, reports a condition, or carries a signal to a controller. It sits below the supervisor and controllers in the system hierarchy.
What current sensor works with an ECM motor?
A current switch with a low, adjustable trip point and tight hysteresis, set between the motor's standby and running currents. A standard switch often can't resolve that gap and reports a wrong status.
What are the two unique identifiers a BACnet MS/TP device needs?
A MAC address, unique on its local segment (master devices use 0 to 127), and a device instance, unique across the whole BACnet internetwork. Confusing the two is a common commissioning error.
Does a connected BAS device have to pass IT review?
Increasingly, yes. A connected building device often goes through a security review before it joins the network, and its connection type, encryption, credentials, and patchability all factor into approval.
Do wireless BAS sensors need batteries?
Not if they are self-powered. EnOcean sensors and switches harvest their energy from a switch press, light, or a temperature difference, so there is no battery to replace; the receivers they talk to are line-powered.
What is the difference between a relay's coil side and contact side?
The coil, or control, side takes the low-voltage signal that tells the relay to switch. The contact, or load, side carries the circuit being switched. A small signal on the coil opens or closes the contacts, which is how a controller switches a heavier load without carrying it.
What does Class 2 mean on a power supply or transformer?
Class 2 is a power-limited circuit classification that allows lighter wiring methods than a line-voltage circuit. Functional Devices power supplies and transformers for 24 V control power are built and listed to Class 2.
What is an automatic load control relay?
A relay used in emergency lighting that monitors normal power and forces the lighting on when normal power is lost, so ordinary fixtures can serve as emergency egress lighting. Functional Devices makes UL 924-listed automatic load control relays in the ESR series.
A Short Glossary of Peripheral-Device Terms
Automatic Load Control Relay (ALCR): A relay that monitors normal power and switches emergency lighting to full output when normal power is lost.
BACnet: The open building-automation communication protocol; devices expose their data as BACnet objects.
BACnet/SC (BACnet Secure Connect): An ASHRAE addendum that adds TLS encryption and certificate-based authentication for connected BACnet devices.
Class 2: A power-limited circuit classification that permits lighter wiring methods than line-voltage circuits.
Current Transformer (CT): The sensing element a current switch or transducer is built around; it reads the current in a conductor without breaking the circuit.
Device Instance: A BACnet device's identifier across the whole internetwork; unique system-wide, in the range 0 to 4,194,302.
ECM (Electronically Commutated Motor): A motor that keeps its drive electronics energized whenever powered, which complicates ordinary current sensing.
Energy Harvesting: Powering a wireless device from its surroundings, such as a switch press or ambient light, instead of a battery.
MAC Address: A device's address on its local MS/TP segment; master devices use 0 to 127.
MS/TP (Master-Slave/Token-Passing): The serial signaling used by BACnet devices over an RS-485 bus.
Pilot Duty: A contact rating for switching an inductive load such as a contactor or solenoid coil, as opposed to a resistive load.
RIB® (Relay-In-a-Box®): Functional Devices' line of relays pre-wired in an enclosure for direct mounting and wiring.
UL 924: The listing standard for emergency lighting and power equipment.
Specify Modern BAS Peripheral Devices
To match relays, current sensors, energy meters, and wireless devices to a project, contact Functional Devices at 800-888-5538, or find an authorized distributor in your region.