Wiring 22 — Three-Wire Pt100 RTD

Three-Wire Pt100 RTD

Terminate a three-wire Pt100 resistance temperature detector on a dedicated PLC temperature input. The single A conductor lands on RTD-A; the two same-colour compensation conductors land on RTD-B1 and RTD-B2.

The device in the real world

Pt100 resistance sensors are common in food, HVAC and process plants because they are accurate and stable. The sensing element is a platinum resistor that is 100 ohms at 0 degrees C and rises by about 0.385 ohm per degree near room temperature. The temperature input card passes a small current through it and measures the voltage, so the resistance of the cable becomes part of the reading unless the card can cancel it.

Why it is wired this way

A three-wire input cancels lead resistance by assuming all three leads have the same resistance (same gauge, length and material). Swapping A with a B lead breaks that compensation and gives a wrong reading. Ground the cable shield at the panel end only.

Worked example

Imagine a 50 metre cable with 2 ohms in each conductor. A two-wire connection would add 4 ohms to the reading, about 10 degrees C of error. With a three-wire sensor the card makes two measurements: one through the A lead and a B lead, and one through the two B leads alone. Because the two B leads have the same resistance, the card can subtract the cable out. This only works when the pair of identical leads land on B1 and B2 and the odd lead lands on A. In the lab, power the card first, then land the leads, then bond the shield at the panel end.

Common mistakes

• Exchanging the single A conductor with one of the B conductors, which breaks the cancellation and gives a drifting reading.
• Earthing the shield at both ends, which creates a loop that picks up noise.
• Selecting a channel configured for two-wire or four-wire when the sensor is three-wire.
• Assuming an open lead reads zero. A broken Pt100 lead normally drives the reading to over-range.

What you can now do

You can terminate a three-wire RTD, explain how the card cancels lead resistance and say which mistakes cause drift or an over-range reading.

Commissioning standard

Measure the sensor with the circuit isolated: roughly 100 ohms at ice-bath temperature and about 107.8 ohms at 20 degrees C, with equal readings from each B lead.