What Is 4–20mA Signal in BMS?
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The Water Tap Analogy
Imagine your home water tap. When barely open — very little water flows. When fully open — maximum flow. Between these two extremes, the flow varies proportionally with how much you turn the tap.
A 4–20mA signal works exactly this way:
- 4mA = minimum (like tap barely open = 0% of measurement range)
- 20mA = maximum (like tap fully open = 100% of measurement range)
- Any value between 4 and 20 = proportional measurement
Why 4mA and not 0mA for minimum?
This is an intelligent design. If the wire breaks or the sensor fails — current drops to 0mA. The controller immediately knows this is a fault — not a valid zero reading. If minimum were 0mA, a broken wire and a true zero reading would look identical.
4mA as minimum means:
- 0mA = wire broken or sensor fault (detect immediately)
- 4mA = sensor working, measurement is at minimum range
- 20mA = sensor working, measurement is at maximum range
Temperature sensor -10°C to +60°C range, 4–20mA output:
4mA = -10°C (minimum of range)
12mA = +25°C (middle of range)
20mA = +60°C (maximum of range)
0mA = FAULT — wire broken or sensor failed0–10V Signal
Voltage equivalent of 4–20mA:
- 0V = minimum (0% of range)
- 10V = maximum (100% of range)
- Used for shorter cable runs within panels or within equipment
- More susceptible to electrical noise over long distances
- 4–20mA preferred for long cable runs in buildings
When to Use Which:
| Condition | Use 4–20mA | Use 0–10V |
|---|---|---|
| Cable length | Over 10 metres | Short runs within panel |
| Electrical noise environment | Yes — current loop ignores noise | Only in clean environments |
| Fault detection | Yes — 0mA = fault | No — 0V = valid zero or fault |
| Standard BMS field wiring | Preferred | Acceptable for local devices |
Related Topics
- What is BMS integration? — how a BMS connects with VFDs, energy meters, BACnet/Modbus devices and other building systems
- How to design a BMS system step by step — the complete BMS design methodology covering site survey, IO list, controller selection, sequence of operations
- What is a Building Management System (BMS)? — fundamentals of BMS controls and architecture for HVAC, lighting, energy and access
- What is BMS commissioning? — the disciplined commissioning process that turns a BMS install into a working building brain
- Browse all IO List & Wiring topics — more from this section of the EnSmart BMS Library
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