The control chain: three suspects
Every heating zone is a chain of three links: the heating element introduces the power, the sensor measures the temperature, the control compares against the setpoint and switches the power. If the zone fails, any link can be the cause — and the symptoms point to the culprit with surprising reliability.
On suspicion, it is almost always the heating element that gets replaced — yet in practice the sensor and the sensor cable are at least as often the cause.
Reading the symptoms
Before any measurement, it is worth looking at the behaviour of the zone. The assignment is not conclusive, but it sorts the order of testing:
| Symptom | Probable cause | First check |
|---|---|---|
| Zone stays cold, reading plausible | Heating element or its connection | Measure the resistance of the element |
| Reading jumps or drifts | Sensor or sensor cable | Check sensor resistance or thermoelectric voltage, move the cable |
| Reading permanently implausible (e.g. room temperature on a hot zone) | Sensor break or reversed polarity | Disconnect the sensor and measure it directly |
| Zone overheats despite a correct reading | Power controller or contactor does not switch off | Switch off immediately — safety-relevant, qualified electrician |
| Zone heats too slowly, just misses the setpoint | Partial damage to the element or one of several cartridges defective | Measure the resistance and compare with the target value |
| Fuse or RCD trips | Insulation fault in the element or in the wiring | Insulation measurement by a qualified electrician |
Two measurements bring certainty
Both measurements are made on the disconnected component with the power off. The control cabinet and measurements under voltage are exclusively a matter for the qualified electrician.
Element: resistance against target value
Measure the resistance of the disconnected heating element. Open circuit = defective. The target value is calculated from rated voltage and rated wattage.
Sensor: check the reference value
Pt100: 100 Ω at 0 °C, about 108–109 Ω at room temperature. Thermocouple: a small thermoelectric voltage rising with temperature; type and polarity must match the controller and the compensating cable. Values jumping when the cable is moved expose a cable break.
Which measuring principle suits which application — thermocouple or Pt100 — is explained in the comparison thermocouple or resistance sensor?.
The dangerous special case: zone overheats
If a zone overheats although the reading and the setpoint are correct, the power stage no longer switches off — a shorted solid-state relay or a sticking contactor lets the current through permanently. This is not a comfort problem but a safety-relevant fault: the zone heats without control until the material, the tool or the insulation is damaged.
Replacing the right part
Once the culprit is identified, it is worth looking at the why: a burnt-out element has a cause (band heater burnt out: identify the cause), a broken sensor often sits at a vibration point (vibration on heating elements), and an aged controller belongs to the installation, not to the wear parts. For the enquiry, nameplate data, rated voltage, rated wattage, sensor type and a photo of the installation situation are helpful.
Note: This guide is a general orientation aid for machine operators, setters and maintenance staff. Work on the electrical installation may only be carried out by qualified electricians. Since we can neither influence nor determine the operating conditions and the condition of the installation in each individual case, no legal claim can be derived from this article; the assessment in each individual case lies with the operator.
Enquire about element, sensor or controller Give us the nameplate data, sensor type and installation situation — we check which link in the control chain needs to be replaced, directly from the manufacturer.
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