Terms on quotations and drawings

These words appear on HTECH equipment lists. We spell them out the first time on each page; this page collects them.

Controls and protocols

DDC

Direct Digital Controller — a panel-mounted controller. It reads sensors and commands valves and contacts. It is not a desktop PC.

BMS

Building management system: software and network that watch HVAC, power and sometimes extra-low voltage on one screen.

BACnet

An open protocol common on central plant and BMS. MS/TP on twisted pair; IP on Ethernet.

Modbus

An industrial protocol common on energy meters, drives and some chillers. RTU on serial; TCP on Ethernet.

Gateway

A protocol converter — for example Modbus to BACnet — when two devices do not speak the same standard.

Field signals

0–10 V

Analog voltage. Sensors or modulating valves. Short runs; the controller needs a voltage input.

4–20 mA

Analog current. Longer runs, better noise immunity. Below 4 mA usually means an open wire.

NTC / PT100 / PT1000

Resistance temperature sensors. They must match the controller input. Do not swap them because the probe looks the same.

Dry contact

A contact that does not carry its own voltage — it opens or closes a circuit powered by the controller or another device. Used for run/fault and float switches.

Interposing relay

A relay between a weak controller output and a power circuit (starter, 230 V valve).

ELV

ELV

Extra Low Voltage — cameras, access control, LAN, public address.

PoE

Power over Ethernet — camera or wireless power on the network cable. Add watts per port and the switch budget.

100 V PA

A high-voltage public-address line for many loudspeakers and long runs. Not the same as an 8 Ω meeting-room system.

Door on power loss

On power loss the door unlocks (escape route) or stays locked (store). Do not mix them on an emergency exit.

HVAC and plant rooms

VRF / VRV

Variable-refrigerant systems with many indoor units. Not a domestic multi-split.

AHU

Air Handling Unit — filters, fan, chilled-water coil. Filter differential sensors sit on the AHU.

UPS / PDU

Battery backup; rack power strip. Runtime follows real load, not the watt figure on a price list with no load connected.

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