Engineering notes
Choose the goods before you price the job
Engineer to engineer: signals, protocols, PoE, DDC — not a brochure. Also the glossary.
BMS and metering
What does BMS equipment include? Field layer and DDC panel
BMS equipment list: sensors, valves, controllers, modules, protocol gateways, panel accessories.
BMS and meteringHow to choose a sensor for a DDC: signal, range, supply
Choose a BMS sensor from the controller input: NTC, 0–10 V, 4–20 mA, dry contact.
BMS and meteringChoose a DDC from the I/O count
Size a DDC from analog and digital points, spare capacity and the protocol.
BMS and meteringBACnet and Modbus when you buy a BMS gateway
When a chiller already speaks BACnet or Modbus, and when you still need a gateway.
BMS and meteringBMS equipment lead time: order before ceilings close
Shelf sensors are not the same as a specified controller.
BMS and meteringBMS sensors: choose 0–10 V or 4–20 mA
When to use 0–10 V and when to use 4–20 mA on BMS sensors.
BMS and meteringBMS temperature sensors: NTC, PT100 or PT1000
How NTC, PT100 and PT1000 differ on a DDC.
BMS and metering24 V for a DDC panel: add the load before you buy
Add controller, sensor, valve and relay current before you buy a 24 V supply.
BMS and meteringInterposing relays and dry contacts on a BMS panel
When a DDC needs an interposing relay before a motor circuit.
BMS and meteringModbus energy meters for a BMS
Branch meters before anyone promises savings.
BMS and meteringA BACnet gateway when the chiller already has a protocol
When you still need a BACnet/Modbus converter between a chiller and the BMS.
BMS and meteringBuilding energy monitoring: main and branch meters
When the BMS must show kWh: choose main and branch meters, protocol and current transformers.
BMS and meteringATS and generator contacts the BMS should see
Mains, generator and ATS status into the BMS: dry contacts, never 230 V into a digital input.
BMS and meteringSignal cable for sensors and valves
Choose signal cable: twisted pair, screen, keep it off power tray.
BMS and meteringWhen you need both a main meter and branch meters
A main meter matches the utility bill.
BMS and meteringCurrent transformers for energy meters
Choose a CT for main and branch meters: ratio, accuracy class, never leave the secondary open.
BMS and meteringFire-alarm contacts the BMS should see
The BMS reads dry contacts from the fire panel: alarm, fault, test.
BMS and meteringHow to lay out a controller cabinet
Lay out a DDC cabinet: split valve power, signal trunking, labels, spare module space.
BMS and meteringRS-485 on a BMS: termination, node count, noise
Modbus RTU and BACnet MS/TP ride RS-485.
BMS and meteringSignal-cable screens: earth one end
A screened pair earthed at both ends can carry a loop bigger than the 0–10 V.
BMS and meteringDry contacts: NO or NC on the point list
On the same breaker, NO and NC reverse run/stop on the BMS.
BMS and meteringMeter pulses: when you do not yet need Modbus
One pulse contact and a kWh-per-pulse factor is enough for some files.
BMS and meteringOutdoor sensors: IP rating, sun, not an office head
An IP20 wall head on a metal roof dies in one wet season.
BMS and meteringFloats and level sensors: tanks and drain pans are not one part
A domestic tank, an AHU pan, a soil stack — three products.
BMS and meteringDDC or PLC: a shop-floor PLC is not an office AHU controller
A building DDC speaks BACnet and takes NTC heads.
BMS and meteringBACnet/IP or MS/TP: the same word, two kinds of cable
MS/TP rides RS-485.
BMS and metering24 VAC or 24 VDC: valves and sensors are not one brick
Many modulating valves take 24 VAC.
BMS and meteringHumidity: room or duct — a combo head is not always two points
Room humidity is not duct humidity.
BMS and meteringDuplicate Modbus addresses: two meters as slave 1, half the bus goes dark
One address per slave.
BMS and meteringBACnet device IDs: a clash and the object disappears
Each DDC and gateway needs its own device instance.
BMS and meteringkWh and kvarh: if the file asks for power factor, the meter must see Q
Many branch meters only pulse kWh.
BMS and meteringRS-485 isolator or repeater: two boxes, two jobs
A repeater adds length and nodes.
HVAC
Control valves and actuators for a BMS
Match valve size, pressure and actuator signal: 0–10 V, 4–20 mA, 24 V on/off.
HVACCO2 sensors on an AHU and in a meeting room
Where to put a CO2 sensor for demand-controlled ventilation.
HVACAHU filter differential-pressure sensors: range and tapping
Choose a filter ΔP sensor: pascal range, tappings before and after the filter, signal into the DDC.
HVACHow a drive talks to the BMS
How a drive talks to the BMS: run/fault contacts, analog speed, or Modbus.
HVACChilled-water pipe pressure sensors
Choose a chilled-water pressure sensor: bar range, 0–10 V or 4–20 mA, tap location.
HVACChoose a variable-speed drive from the motor
Size a drive from motor kW and current, pump or fan load, and EMC filters.
HVACEMC filters and chokes when you order a drive
A long motor cable over a sensitive BMS — an unfiltered drive makes 0–10 V sensors tick.
HVACSolenoid or modulating: two different orders
A solenoid is open or shut.
HVACFCUs: a room thermostat or a DDC point
Until the file says whether FCUs stand alone or report to the BMS, you have two schedules.
HVACVRV / VRF into the BMS: the maker’s gateway, not a room sensor
Per-indoor status on the BMS needs the air-con maker’s gateway.
HVACPipe temperature: strap-on, immersion, or a well
A strap-on head is slow when the insulation is thick.
HVACChilled-water flow switches: a tank float is not an AHU pipe
An AHU needs proof of flow before the chilled valve opens.
HVACDamper actuators: fire on/off is not an AHU modulating motor
A fire damper is often 230 V, two-position, spring-return.
Electrical materials
Cable tray and trunking
Choose tray or trunking by width, load and lids.
Electrical materialsMCB or MCCB: a house breaker is not an incoming
MCBs belong on small branches.
Electrical materialsSoft starter or VFD: a pump does not always need a drive
A soft starter only cuts inrush, then runs full speed.
Electrical materialsPower cable and control cable: size, cores, not one reel for both
Power follows motor current.
ELV
ELV materials in a cost plan
Split cameras, access, LAN and public address from labour.
ELVHow to size PoE for cameras
Add watts per camera, spare ports and the switch power budget.
ELVChoose a PoE switch for cameras
PoE budget, uplink, managed versus unmanaged, and where the recorder sits.
ELVStandby power for emergency-exit doors and access control
Magnetic locks, break-glass, standby power on escape doors.
ELVHow 100 V public address differs from low-impedance speakers
100 V lines for long floors and many speakers.
ELVMaglock or strike: do not mix them on an escape door
A maglock drops on power loss — right for escape.
ELVCamera network cable: Cat6, length, outdoors
Enough PoE watts and a 110 m run or indoor Cat5e in the sun — the picture still drops.
ELVAn NVR is not a PoE switch: ports, watts, retention
A 16-channel PoE NVR is enough for a small job.
Lighting
Occupancy and daylight sensors for lighting
PIR, presence and photocells: corridor off and dimming to daylight.
LightingDALI or 0–10 V for lighting control
When the file names DALI and when 0–10 V is enough.
LightingTraffic signal lamps
Traffic signals are not decorative lights: lenses, supply, pole parts.
LightingEmergency lighting: not the same as the task lights
Highbays that die with the mains are not escape lights.
LightingOffice lights and hall lights: lux, UGR, not one 40 W LED
Offices need UGR and a dimmable driver.
Plant rooms
Water-leak sensors in a plant room
Spot versus cable leak sensors under raised floors and precision-cooling trays.
Plant roomsRack UPS in a plant room
Size rack UPS from real load and required minutes, not the no-load price-list watt.
Plant roomsChoose a plant-room rack: depth, load, airflow
42U racks: 800 mm or 1000 mm depth, static load, front-to-rear airflow.
Plant roomsRack PDU: outlets, phases, contacts
Choose a rack PDU by outlet count, single or three phase, current reading and BMS contacts.
Plant roomsOnline or line-interactive: a plant room is not a workstation
Line-interactive is enough for a PC.
Plant roomsSizing UPS VA: add real watts, not nameplates in a pile
Add NVR, switch and DDC watts at infrared and at charge.
Plant roomsMetered or basic PDU: a floor comms rack is not a plant row
A floor comms rack: a basic strip is enough.
Plant roomsUPS batteries: the same VA is not the same pack
If the internals are fine, do not buy a new UPS because the batteries are old.