Thermal Protection

What is Thermal Protection?

Thermal protection is a protective mechanism that switches off an electrical circuit as soon as a motor, transformer or cable is at risk of overheating from prolonged overcurrent or excessive temperature. Unlike short-circuit protection, thermal protection deliberately responds with a delay: brief starting currents are tolerated, but an overload that lasts for minutes leads to a trip. The best-known form is the bimetallic thermal overload relay, which together with a contactor forms a motor starter. There are also thermistors built into the motor winding, thermal fuses and electronic motor protection devices that pass their data to the PLC.

Thermal protection devices are often used in combination with Contactors, motor starters, soft starters or variable frequency drives in industrial installations.

🧠 How does thermal protection work?

The classic thermal overload relay mimics the way the motor heats up. It works in three steps:

1. Bimetallic element - the motor current flows through an indirectly heated bimetal in each phase. Two metals with different expansion coefficients are bonded together, so the strip bends as it warms up.
2. Time-delay effect - because heating takes time, the relay does not respond immediately. A starting current of six to eight times the rated current lasts only a few seconds and is tolerated; a persistent overload of twenty percent leads to a trip within two hours.
3. Tripping via the control circuit - the relay does not interrupt the main current itself. It opens an auxiliary contact (usually 95-96) in the control circuit, which drops out the contactor and stops the motor. A second contact (97-98) reports the fault to the PLC or the SCADA system.

After cooling, the protection usually has to be reset manually. Some relays support automatic reset, but that is only acceptable where an unexpected restart creates no hazard. Modern relays are also ambient-compensated (for example from −25 to +55 °C), so a warm control cabinet does not shift the trip characteristic, and they include a differential mechanism that trips faster on phase loss or imbalance.

🔍 What types of thermal protection are there?

One key distinction: current-based devices estimate the temperature, whereas temperature-based devices measure it. A current relay cannot see a clogged cooling fin or an excessive ambient temperature; a PTC sensor in the winding can. For critical motors both are therefore often combined.

Bimetallic relays are standard in many MCCs and can easily be set to the motor rating.

⏱️ What do trip classes mean?

The standard IEC 60947-4-1 defines how an overload relay must behave. From the cold state, the relay must not trip at 1.05 times the setting current, and it must trip within two hours at 1.2 times. The trip class gives the maximum tripping time in seconds at 7.2 times the setting current, the typical starting current of an induction motor.

Choose a class that tolerates the starting time under load, but no slower than necessary: the higher the class, the longer the winding has to endure an overcurrent.

🌡️ Why is overheating so damaging?

In practice, the life of a motor is determined by the insulation of the winding. Under IEC 60085, insulation class B has a thermal limit of 130 °C, class F 155 °C and class H 180 °C. A widely used rule of thumb is that every 10 K above the permitted temperature roughly halves the insulation life. A motor that runs ten percent overloaded as a matter of routine does not fail at once, but it does fail years early.

Typical causes of thermal overload are:

• Mechanical overload - a seizing pump, worn bearings or an overloaded conveyor
• Phase loss - the motor keeps running on two phases and draws far more current on them
• Poor cooling - fouled cooling fins, a broken fan or an overly hot room
• Too many starts per hour - every start heats the winding further
• Low speeds - a self-ventilated motor on a variable frequency drive cools much less at low speed

🔧 How do you set thermal protection correctly?

A practical approach for a standard motor starter:

1. Read the nameplate - use the rated current (In) at the actual supply voltage and connection (star or delta).
2. Set the setting current equal to In - the setting current is the rated motor current, not the desired tripping current. Never set the relay higher to work around nuisance trips.
3. Watch out with star-delta starters - if the relay sits in the winding line, set it to 0.58 × In (In divided by √3).
4. Choose the trip class - class 10 for normal starting, 20 or 30 for heavy starting.
5. Check coordination - combine relay, contactor and short-circuit protection (fuses or circuit breaker) according to the manufacturer’s tested tables for type 2 coordination.
6. Define the reset strategy - manual or automatic, in line with the machine’s risk assessment.

With a variable frequency drive or soft starter, the built-in thermal motor model usually takes over the role of the bimetal; for motors that run at low speed for long periods, a PTC sensor is the more reliable addition.

Correctly setting the thermal protection is essential to prevent both damage and unnecessary interruption.

🇪🇺 Which standards and rules apply in the Netherlands and the EU?

• IEC 60947-4-1 - overload relays, contactors and motor starters, including trip classes
• IEC 60947-8 - control units for built-in thermal motor protection with PTC thermistors
• IEC 60204-1 - electrical equipment of machines; motors above 0.5 kW should in principle have overload protection, and hazardous automatic restarting after a trip must be prevented
• NEN 1010 - the Dutch installation standard; for cables, the rated current of the protective device must lie between the design current and the cable’s current-carrying capacity (Ib ≤ In ≤ Iz)
• NEN 3140 - safe working on, and periodic inspection of, the installation, including protective devices
• IEC 60079 and the ATEX directives - protection of motors in explosive atmospheres

🔐 What should you watch for in safety and management?

• Reset procedure - include it in the Lock-out Tag-out or work permit procedure, so that nobody resets a tripped motor while work is being done on the machine
• Cause first - a trip is a symptom; reset only after investigating overload, phase loss or cooling
• Periodic testing - use the relay’s test button and check the setting during NEN 3140 inspections; wear from frequent switching mainly affects the contacts of the contactor
• ATEX motors - for an increased-safety motor (Ex e), the relay must trip from the cold state within the tE time at the starting-to-rated current ratio (IA/IN) on the nameplate. The relay itself sits outside the hazardous area, but must be certified as a protective device; for the reset function, follow the manufacturer’s instructions and the certificate, which usually require manual reset
• Functional safety - thermal protection is equipment protection, not a safety function in the sense of safety standards, unless it has been explicitly designed as one

An incorrectly set protection device may lead to motor damage or unplanned downtime.

🏭 How does thermal protection fit into IT/OT convergence?

Electronic motor protection devices reveal far more than a bimetal. Via IO-Link or ProfiNET they supply the motor current per phase, the thermal load as a percentage, time to trip, number of starts and the cause of the last trip. Operators can therefore step in before a motor fails, and maintenance teams use the trends for condition monitoring and predictive maintenance.

This also turns motor protection into a network device. A smart relay whose setting current can be changed remotely belongs in the asset inventory and behind network segmentation: anyone who can change the parameters can make a motor trip, or leave it running unprotected. Include this in your security policy for the OT environment.

❓ Frequently asked questions

What is the difference between thermal and magnetic protection?

Thermal protection responds with a delay to prolonged overload, whereas magnetic protection responds within milliseconds to short-circuit currents. A motor protection circuit breaker combines both functions. In a classic motor starter, the thermal relay handles overload and the fuse or circuit breaker handles short circuits.

What current should a thermal overload relay be set to?

A thermal overload relay is set to the rated current on the motor nameplate, at the actual voltage and connection. The relay then does not trip at 1.05 times that current and does trip within two hours at 1.2 times. In a star-delta starter with the relay in the winding line, the setting is 0.58 times the rated current.

Why does my thermal protection keep tripping on start-up?

Thermal protection that trips on start-up usually has a trip class that is too low for the starting time, or the setting current is wrong. Phase loss, a mechanically heavy load or too many starts per hour are also common causes. Never raise the setting above the motor current; choose a class 20 or 30 relay if necessary.

Is a PTC sensor better than a thermal relay?

A PTC sensor measures the actual winding temperature and therefore also detects poor cooling or a high ambient temperature, which a current-based thermal relay misses. A PTC does not protect the cable, however, and responds more slowly to a locked rotor. For motors on a variable frequency drive or with heavy duty cycles, combining both gives the best thermal protection.

Does a variable frequency drive still need a separate thermal relay?

A variable frequency drive has a built-in thermal motor model that usually takes over the function of a thermal relay, provided the motor data has been entered correctly. Where several motors run from one drive, each motor still needs its own thermal protection. For long periods of operation at low speed, a PTC sensor in the winding is advisable.

May thermal protection reset automatically?

Thermal protection may only reset automatically if an unexpected restart of the motor creates no hazard, for example with a fan behind a guard. IEC 60204-1 requires hazardous automatic restarting to be prevented. For motors in explosive atmospheres, the manufacturer’s instructions and the relay’s ATEX certificate usually require manual reset.

📌 In summary

Thermal protection safeguards motors, transformers and cabling against the creeping damage of overload. The bimetallic relay, the PTC thermistor and the electronic motor protection device each do this in their own way, with IEC 60947-4-1 as the basis for setting current and trip class. Correct settings, investigating the cause of every trip and anchoring the reset in safety procedures together determine whether the protection actually works.

Thermal protection prevents damage to electrical installations from overcurrent and overheating. It is essential for the reliable and safe operation of motors and power circuits in industry, and in its smart form it delivers valuable data for maintenance and process monitoring.