The standard symbol for RTD (Resistance Temperature Detector) on a schematic is a resistor symbol bisected by a diagonal line with an upward-pointing arrow, indicating a positive temperature coefficient (PTC). However, the exact geometry of that resistor symbol—and the wire color codes used to connect the physical probe—changes drastically depending on whether your facility follows IEC (global/EU) or IEEE/ANSI (US) standards. Misinterpreting either the schematic symbol or the wire colors will result in immediate calibration failures or massive temperature offsets.
RTD Schematic Symbols by Regional Standard
When reading a P&ID (Piping and Instrumentation Diagram) or an electrical schematic, the symbol for an RTD must be distinguished from a thermistor or a standard fixed resistor. The diagonal line through the resistor body is the universal indicator for a temperature-dependent component, but the arrow direction and the base resistor shape dictate the exact component type and regional origin.
| Standard | Primary Region | Symbol Geometry | Arrow / Modifier | Standard Designator |
|---|---|---|---|---|
| IEC 60617 | EU, Global, Modern US | Rectangle (IEC standard resistor) | Diagonal line with arrow pointing UP-RIGHT | RTD, TE, or TT |
| IEEE 315 / ANSI | United States, Legacy | Zigzag (ANSI standard resistor) | Diagonal line with arrow pointing UP-LEFT | RT, TE |
| DIN 43700 | Legacy EU / German | Rectangle | Diagonal line, often with 't°' instead of arrow | Rt |
| NTC Thermistor | Global (For contrast) | Rectangle or Zigzag | Diagonal line with arrow pointing DOWN (Negative Temp Co) | TH, NTC |
What this means in practice: The upward arrow on the IEC and IEEE symbols explicitly denotes a Positive Temperature Coefficient (PTC). As temperature rises, resistance rises. For a standard Pt100 RTD, resistance increases by approximately 0.385 Ω per 1°C. If you see a downward-pointing arrow, you are looking at an NTC thermistor, which requires a completely different transmitter excitation curve and will be destroyed if connected to a standard RTD input card.
RTD Wire Color Codes & Pinout Configurations
The schematic symbol tells you what the component does; the wire colors tell you how to physically terminate it. This is where the most expensive mistakes happen in instrumentation. A 3-wire RTD relies on a Wheatstone bridge or current-source compensation circuit to subtract lead wire resistance. If you swap the excitation and sense wires due to regional color code differences, the transmitter will add the lead resistance instead of subtracting it.
| Configuration | IEC 60751 Colors (Global/EU) | US / ANSI MC96.1 Colors (Legacy) | Transmitter Terminal Mapping |
|---|---|---|---|
| 2-Wire | 1 Red, 1 White | 1 Red, 1 Black (or White) | Excitation+, Sense- |
| 3-Wire | 1 Red, 2 White | 2 Red, 1 White (Common US legacy) | Excitation+, Sense+, Sense- |
| 4-Wire | 2 Red, 2 White | 2 Red, 2 White | Excitation+, Excitation-, Sense+, Sense- |
The most frequently botched row in the table above is the 3-wire configuration. Under the IEC 60751 standard, a 3-wire RTD has one Red wire and two White wires. The single Red wire is the common/excitation side. However, many legacy US manufacturers (and older Omega or Minco probes) built 3-wire probes with two Red wires and one White wire, where the single White wire was the common.
The Failure Mode: If you wire a US-style (1 White common) probe into a transmitter expecting IEC (1 Red common), the bridge circuit measures the voltage drop across the wrong lead. Assuming 10 AWG copper leads with 1.5 Ω resistance, the transmitter will calculate an error of roughly 3 Ω. At the standard Pt100 alpha of 0.385 Ω/°C, this results in a +7.7°C false reading offset right out of the box. Always verify the manufacturer's datasheet, not just the wire colors.
4-Wire Pinout Note: In a 4-wire setup, the color pairs (2 Red, 2 White) are functionally identical within their pairs. The transmitter uses one pair purely to push a known constant current (Excitation) and the other pair purely to measure the voltage drop (Sense) using a high-impedance voltmeter circuit. Because the sense circuit draws virtually zero current, the lead resistance of the white wires is mathematically eliminated.
Safe Interpretation When Markings and Colors are Faded
In high-temperature industrial environments like extruders, autoclaves, or exhaust stacks, PTFE or fiberglass insulation bakes over time. White wires turn brownish-gray, and red wires fade to a dull pink or rust color. Manufacturer tags fall off. When you cannot trust the visual color code, you must use a multimeter to deduce the pinout based on the physical internal construction of the RTD probe.
Required Tool: A digital multimeter (DMM) capable of measuring low resistance (0.1 Ω resolution minimum). A 4-wire Kelvin measurement setup is ideal, but a standard 2-wire DMM will work if you zero out your test leads first.
Step-by-Step Diagnostic for an Unmarked 3-Wire Pt100 Probe
- De-energize and Isolate: Disconnect the RTD from the PLC or temperature transmitter. Measuring resistance on a live circuit will blow your DMM fuse and yield garbage data.
- Zero the Leads: Short your DMM probes together. Note the lead resistance (usually 0.2 Ω to 0.5 Ω). Subtract this from your final readings mentally, or use the relative (REL/NULL) button on your meter.
- Measure All Pairs: Test the resistance between Wire A-B, A-C, and B-C at the open wire ends (the transmitter side, not the probe head).
- Identify the Sense Pair: You will find two wires that show a very low resistance between them (typically 0.2 Ω to 1.5 Ω, representing just the copper wire leads). These two wires are your Sense pair. They connect to the same physical terminal inside the probe head and run parallel back to the transmitter.
- Identify the Common/Excitation Wire: The third wire will show a much higher resistance to both of the other wires. At room temperature (20°C / 68°F), a Pt100 element has a baseline resistance of 107.79 Ω. You will read approximately 108 Ω to 112 Ω (depending on ambient temp and lead length) between this third wire and either of the sense wires. This third wire is your Common/Excitation wire.
Bench Warning - Self-Heating Error: Do not leave your multimeter connected to the RTD for extended periods while on the lowest ohms range. Many DMMs push 1 mA to 2 mA of test current to measure resistance. In a small Pt100 element, this current causes internal self-heating (I²R losses), which will artificially raise the element's temperature and skew your baseline room-temperature reading by up to 0.5°C. Take the reading, record it, and disconnect.
By relying on the fundamental physics of the probe's internal bridge connections rather than faded insulation, you can safely and accurately terminate any RTD, regardless of whether the original installer followed IEC, IEEE, or no standard at all.
