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Fault 13
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Fault 13 — Pt100 Open Compensation Lead

What you'll learn

**Reported symptom:** The Pt100 channel is over-range and the input card reports a sensor-break diagnostic.

Lab time: ~12 minutes.

Lesson briefing

Pt100 Open Compensation Lead

Reported symptom: The Pt100 channel is over-range and the input card reports a sensor-break diagnostic.

The circuit was commissioned from a known-good drawing. Treat every terminal as suspect, but test from source to load instead of guessing. Use voltage mode on an energised control circuit and continuity only after the simulated circuit is isolated.

Your task

Isolate the channel and compare continuity on the A, B1 and B2 conductors.

Choose the fault category only when the measurements support it.

Hints

Hint 1

A healthy three-wire RTD presents a finite resistance through both B paths.

Hint 2

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Hint 3

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This lesson uses 7 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.

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Competency and practice field guide

RTD open-lead fault lesson: practice plan

Direct answer

The learner can identify two-, three- or four-wire topology, isolate the open conductor and prove correct temperature direction and resistance after repair.

Written for instrumentation learners diagnosing an RTD channel that reports overrange, a fixed fault value or bad quality.

an instrumentation calibration bench connecting pressure, temperature, load, level and smart sensors to PLC input channels and reference measurements while studying RTD open-lead detection, lead compensation and temperature evidence
System map / 02

NODE 01observable

Scope

RTD element type, nominal resistance, two-, three- or four-wire circuit, excitation, lead resistance, terminal mapping, input type, open-wire diagnostic, quality and temperature conversion.

NODE 02observable

Signal path

Process temperature through element resistance and lead circuit to input measurement, compensation, conversion, engineering value, quality and alarm.

NODE 03observable

Baseline practice

Ambient and elevated reference conditions produce plausible resistance and temperature values with healthy quality.

NODE 04observable

Edge cases

One open lead, high-resistance terminal, unequal leads, short, wrong element type, swapped terminals, shield fault, self-heating and restart.

NODE 05observable

Fault practice

A process, element, lead, terminal, excitation, input-type, compensation, conversion or quality mismatch.

NODE 06observable

Transfer to the job

The circuit inspected and calibrated with suitable references under the approved procedure.

Answer surface / 07

What does an RTD input show when a lead is open?

Many modules report overrange or bad quality, but the exact value and diagnostic depend on the input circuit and configured RTD type.

Why do three-wire RTDs use matching lead resistance?

The input can compensate lead resistance by comparing paths under an assumed balanced lead arrangement; an open or unequal connection breaks that assumption.

Motor, wiring and VFD path

Connect the control command to physical motor behavior

Move from safe control wiring and contactors into VFD parameters, measurements, faults and PLC command paths.