While the 3-wire connection scheme effectively enhances measurement accuracy in industrial applications, improper installation or maintenance can still lead to various faults. The following outlines the most common types of faults and their troubleshooting methods to help you quickly pinpoint the problem:
I. Wiring Errors or Poor Contact (Most Common)
Symptoms: The displayed temperature is abnormally high, fluctuates violently, shows no response, or displays an infinite value.
Primary Causes:
Wires A, B, and C are connected to the wrong ports, causing the bridge compensation mechanism to fail;
Wiring terminals are loose, oxidized, or corroded, resulting in increased contact resistance;
Wire labeling is unclear, leading to incorrect connections in the field.
Solutions:
Check the instrument's wiring diagram to confirm that Wire A (the single-wire terminal) is connected to the excitation source, while Wires B and C (the dual-wire terminals) are connected to the differential input;
Use sandpaper to clean any oxidation layers on the terminals, then retighten the screws;
Use a jumper wire to test the ports and determine whether the fault lies with the sensor or the wiring itself.
Tip: Wires B and C must be connected to two adjacent arms of the bridge circuit; connecting them to the same terminal will result in a complete loss of the compensation function.
II. Wiring Issues Causing Measurement Deviation
Symptoms: The displayed temperature reads consistently high or low, and drifts in response to changes in ambient temperature.
Primary Causes:
The three wires differ in material, length, or wire gauge, thereby disrupting the circuit's symmetry;
The extension cable is excessively long (>100 meters) and compensation wires were not used;
A single standard wire was used to replace one of the three wires, disrupting the resistance balance.
Solutions:
Replace the wiring with a 3-core shielded twisted-pair cable from the same production batch and of the same specifications;
For distances exceeding 100 meters, it is recommended to switch to a temperature transmitter to convert the resistance signal into a 4–20 mA current signal for long-distance transmission;
Ground the cable's shielding layer at only one end to prevent the formation of ground loops, which can introduce interference.
III. External Interference Causing Signal Fluctuations
Symptoms: The temperature readings jump erratically or exhibit periodic fluctuations. Interference Sources:
Routed in parallel with variable frequency drives (VFDs) or motor cables, subjecting it to electromagnetic interference within the 2–30 MHz range;
Poor grounding system, resulting in a voltage-to-ground exceeding 1V at the COM terminal.
Solutions:
Switch to shielded cables and route them away from power cables;
Install a signal isolation module;
Inspect the system grounding to ensure the ground resistance is <4Ω.
IV. Sensor or Power Supply Faults
Power Supply Anomalies:
Unstable or absent 24V supply voltage, resulting in no signal output;
Verification Method: First, check if the power supply is functioning normally; then, measure the voltage at the signal terminal.
Pt100 Sensor Faults:
Open circuit in the sensing element (reading displays infinity); short circuit in the sensing element (reading displays a value that is too low);
Verification Method: Use a multimeter to measure the resistance value at ambient temperature (e.g., approximately 109.7Ω at 25°C) to determine if it falls within the acceptable range.
V. Incorrect Instrument Configuration
Common Configuration Errors:
The instrument is not set to "3-wire" mode and continues to perform calculations based on a 2-wire configuration;
Incorrect sensor type selection (e.g., configured for Cu50 instead of Pt100);
Incorrect configuration of the measurement range (upper and lower limits).
Solutions:
Access the instrument's parameter menu to verify the sensor type, wiring method, and measurement range settings;
Refer to the equipment manual to perform a standardized configuration.
