IEC 60751 Digital and PID Temperature Controllers

A standard is a set of tests, not a property of a shape. IEC 60751 is one of the documents digital and pid temperature controllers are built to; the sections below say what it fixes, what it does not, and what has to be named on the enquiry for the approval to be usable.

IEC 60751 — Scope

Platinum resistance thermometers: the Pt100 and Pt1000 resistance-temperature relationship, and the class AA, A, B and C tolerance bands that a sensor is ordered by.

What IEC 60751 Does Not Cover

It fixes the sensing element's tolerance, not the accuracy of the finished measurement — the lead resistance, the wiring configuration and the transmitter all add error after it.

This is the division of labour to carry away from any heating standard: safety standards decide whether the part can hurt somebody, and say nothing about whether it holds a temperature, lasts five years or matches the last batch. Performance is a specification you write; safety is one the standard writes.

How a Made-to-Drawing Heater Is Certified

A construction is built and tested to a standard. The insulation system, the creepage and clearance distances, the earthing, the lead anchorage, the temperature under fault and the marking are what the test covers — and all of those are properties of the construction rather than of the outline it is die-cut to.

So a custom outline does not have a certificate of its own, and no supplier can honestly hand you one. What is available is the construction file: the standard the build is tested to, the materials list with their own ratings, and the test reports. That is what an approvals engineer needs, and it is what the end product's own certification is built on.

The practical consequence: name IEC 60751 on the enquiry, not after approval. A construction chosen for cost and then asked to pass a standard afterwards usually has to be rebuilt, because the insulation thickness, the creepage distances and the lead anchorage are decided at design time and are not adjustable later.

What a Digital and PID Temperature Controllers Is

Panel controllers to close the loop around the heater: on/off for simple holding, PID where the set point has to be reached without overshoot.

The instrument that closes the loop: it reads a sensor, compares it with a set point and switches the heater. It is what gives a heating system an accuracy figure — no heater has one on its own.

The heat leaves through whatever the component is clamped or bonded to; a sensing component adds no heat of its own and simply has to be in thermal contact with the thing it is reporting on.

The continuous limit of 400 °C is set by the insulation system. The element alloy would run far hotter; it is never the element that decides.

The Other Standards This Construction Is Built To

A heater is rarely governed by one document. This construction is also built and tested to:

• IEC 60730
• IEC 60584
• IEC/EN 60335
• RoHS & REACH
• CE Marking

Safety, materials, substances and quality system are four separate questions with four separate documents behind them. A part can satisfy all four and still be the wrong heater for the duty, which is what the rest of this page is about.

Control and Protection

This heater has no way of limiting its own temperature. Resistance heating draws roughly the same power whatever the temperature, so with the load removed, the fluid gone, the fan stopped or the sensor open circuit, the element keeps producing watts until something stops it.

The design must therefore contain a control device and, separately, a protective one: a controller with a sensor to hold the set point, and a bimetal thermostat or a one-shot thermal cutoff wired in series with the element to open the circuit if the loop ever fails. The two are not interchangeable — a controller that has lost its sensor cannot protect anything, which is exactly the fault the cutoff exists for.

Leads and Terminations

Termination is specified on the drawing: lead length, gauge, insulation and the connector or tag at the end of it.

The lead exit is a thermal discontinuity as well as an electrical one. The joint between a fine element and a stranded lead is the hottest point of many heaters, so it is kept out of the highest watt density zone, strain relieved, and — where the exit sits in a hot area — insulated in PTFE rather than in silicone.

How to Specify One

This is a made-to-drawing part, so there is no catalogue number to quote. A quotation is built from the following, and the more of it that arrives with the enquiry the fewer rounds it takes:

• The temperature to be sensed or switched, and the tolerance the application actually needs.
• Where the component sits — against the element, in the potting, on the load, or in the airflow.
• The electrical duty: for a switching device, the voltage, the current and whether the load is AC or DC.
• Lead length, insulation and termination, and whether the component is fitted by us into a heater assembly or supplied loose.
• The standard the end product is certified to, so the component chosen is one that fits inside that approval.

Lead time: Samples 7-10 days from approved drawing; production 15-20 days.

Digital and PID Temperature Controllers — Capability Table

These are process limits for the construction, not the specification of one stocked item. Every line is what the factory can hold; what your part holds is fixed on your drawing.

Figures are the published tolerance and rating bands of the standard sensor and control components fitted to these heaters. Sensor tolerance classes follow IEC 60751 for RTDs and IEC 60584 for thermocouples; thermostat and thermal cutoff ratings are the component manufacturers' own.