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Thermal protectors – Frequently Asked Questions

Thermal protectors

Single pole or multi-pole contact systems that are used to reliably interrupt the power supply to an electrical device when it reaches a specified temperature are called thermal protectors. The typical fields of application of thermal protectors are electric motors, transformers, heating devices and common household appliances such as mixers and kettles. In this regard, there is a difference between opening and closing thermal protectors (Normally Closed NC and Normally Open NO). For classic device protection only thermal protectors with opening contacts can be used. The closing thermal protectors, on the other hand, are suitable for operational safety, which is the interruption-free and application-safe operation of the device, like connecting a signal generator or a fan.

Nominal switching temperature

The nominal switching temperature (short NST) is the most important function parameter of a thermal protector. The nominal switching temperature specifies at what temperature a thermal protector should be activated. In the case of an NC, the nominal switching temperature corresponds to the temperature at which the contact of the switch opens and thus interrupts the flow of electricity. Accordingly, the nominal switching temperature of an NO shows at what temperature the contact is closed. The nominal switching temperature is generally quoted in degrees Celsius (°C). In conjunction with the nominal switching temperature, the switching tolerance is usually also stated and this in Kelvin (K).  This parameter describes the spread of the actual switching temperature. The standard tolerance is ±5K, however, tolerances of ±2.5K are also possible.

Reset temperature

The counterpart to the nominal switching temperature is the reset temperature. The reset temperature determines the temperature at which a switch switches back into its starting position. Thus for opening thermal protectors the reset temperature is the temperature at which the contact is closed. Accordingly, closing thermal protectors are opened again when the reset temperature has been reached. Compared to the nominal switching temperature, the reset temperature plays a subordinate role. However, when defining the reset temperature, it must always be assured that this is above the ambient temperature of the final application in order to ensure that for instance an automatic reset switch can switch back again after activation. Therefore, the reset temperature is generally chosen based on the highest expected ambient temperature.

Contact resistance

Neben den zwei Parametern Nenn- und Rückschalttemperatur, gehört auch der Kontaktwiderstand zu den wichtigsten Eigenschaften eines Temperaturbegrenzers. Aside from the two parameters nominal switching temperature and reset temperature, contact resistance is also an important feature of a thermal protector. In data sheets, the upper limit of the contact resistance is always stated. In real applications, however, the switches show significantly lower contact resistance than that stated.  The reason for this is the change in the contact resistance over its service life, which is caused by electric load conditions such as power surges, inductive and capacitive reactance. The contact resistance of a thermal protector measured during an application is made up of several individual resistors connected in series. Depending on the setup of the switch the number of single or dual electric contact systems adds up, as rivet, welding or soldering connections, active resistance, current-bearing parts as well as the resistance of the attached cables and their connectors. Although It is often difficult to separate the contact resistance from the other resistances, it is essential in order to determine the shift in the switching temperature caused by inherent warming.

Contact bounce

Contact bounce is a repeated unintentional opening and closing of contacts at high speed. Contact bounce occurs for a short time when the thermal protector is activated and reset until a stable state has been reached. With mechanical switching systems, this contact bounce is in principle not completely avoidable. The time of the contact bounce is also a quality feature of a thermal protector. The shorter the time of the contact bounce is during the opening and closing process, the higher the quality of the switch because this results in less contact burn caused by electric arcs when switching occurs under load.

Switching cycles

The essential performance feature of a thermal protector is the number of switching cycles that it achieves under the least favourable load conditions during the respective application without leaving the defined parameter range (nominal switching and reset temperatures as well as contact resistance). Switching is understood to be the change between opening and closing the contact.

Resistance to impregnation

One of the most frequent fields of application of thermal protectors are coils. In order to increase their service life they are protected with insulating materials, impregnation varnishes or resins. Normally, the thermal protectors are already attached to the coil before impregnation and pass through the impregnation process with the coil. For this reason, a thermal protector must possess resistance to impregnation which means it must be sealed. The switch must prevent insulating varnishes with low viscosity penetrating the inside of the switch. If the thermal protector has no resistance to impregnation and liquids enter the switch, it might not function in the event of an incident and thus could not protect the end device from overheating. Hereby the vacuum impregnation process places the greatest demands on the thermal protector‘s resistance to impregnation. Ideally, the thermal protector’s resistance to impregnation should be in place beforehand.  Subsequent insulation of switching systems is achieved by using epoxy resins or silicone.


PTCs (Positive Temperature Coefficient) are resistors whose main characteristic is to conduct the electricity better at low temperatures than at high temperatures due to resistance increasing as the temperature rises.  These mainly act as over current or over heating protection because of the non-linear resistance curve of the PTC. The temperature range of the Thermik PTCs lies between 70°C and 180°C. The main difference between PTC-thermistors and thermal protectors with bimetal system is that the PTCs require additional evaluation electronics to be functional. Higher and individual switching cycles are the advantages of the PTCs.

VDE Approbation

All electrical and electronic devices, including all components used, must generally be tested for their risk-free usage. VDE (Verband der Elektrotechnik Elektronik Informationstechnik e.V.) tests all electro-technical devices, components and systems in its certification institute for this and issues permits, which is the VDE approbation. Thermik has VDE approbation for almost all types and styles of its switches. By obtaining the VDE approbation, the manufacturer is given permission to use the VDE approbation mark and other conformity verifications. VDE approbations are of particular importance for the German market.

UL Approbation

In the US market, UL approbations are the most important proof for the safety of a product. Aside from the VDE approbations, Thermik products are to a large extent also UL approved. The UL (Underwriters Laboratories) also tests and certifies products, components, materials and systems as an independent agency. Proof that the requirements have been fulfilled that is the UL approbation has been issued, are the UL-Listing mark or the UL Recognized Component mark.

CSA Approbation

The CSA approbation is to Canada what the VDE approbation is to Germany and the UL approbation to the USA. The Canadian Standards Association (in short CSA) is a recognized and accredited test and certification institute for the Canadian market. If a product meets all the safety-relevant standards, it receives the CSA approbation and the CSA approbation mark.

CQC Approbation

A CQC approbation identifies the compliance with standards and the quality of products on the Chinese market. The CQC (China Quality Certification Center) is comparable to the VDE, UL and CSA and is also available for many of the thermal protectors from Thermik.

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