Method for determining a thermal endurance
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- SIEMENS AG
- Filing Date
- 2024-08-01
- Publication Date
- 2026-06-03
AI Technical Summary
Electromechanical switching and protection devices experience changes in contact resistances due to switching processes and environmental factors, leading to increased device temperature, which can result in device failure or fire if not monitored properly.
A procedure for determining the thermal lifespan of an electromechanical switching device involves measuring temperature at specific points, comparing it to predefined threshold values, and using temperature-time characteristics to predict the remaining lifespan, thereby enabling early warning and preventive measures.
This approach allows for the accurate prediction of the thermal lifespan of electromechanical switching devices, enabling early warning systems and preventive maintenance to prevent device failure and potential fires.
Smart Images
Figure EP2024071800_03042025_PF_FP_ABST
Abstract
Description
[0001] Description
[0002] Method for determining a thermal lifetime
[0003] The present invention relates to a method for determining a thermal lifetime of an electromechanical switching device and a computer program.
[0004] Electromechanical switching and protective devices switch using moving contacts. Due partly to these switching operations and partly to environmental influences, e.g., corrosion, the contact surfaces and thus the contact resistance change over time. This change tends to be abrupt during a switching operation, but more gradual due to environmental influences. An increase in contact resistance leads to an increase in power loss and consequently in the device temperature. This can cause an unmonitored device to become so hot that it fails or catches fire.
[0005] Condition monitoring of a critical temperature point (hotspot) on the device or of a thermal reference point near such a hotspot is a prerequisite for issuing an early warning of an impending failure. In addition to determining the current device status, a prediction of the device's remaining thermal lifetime is also useful.
[0006] There is therefore a need for a method for determining the thermal lifetime of an electromechanical switching and protective device.
[0007] The object is achieved by a method according to claim 1. This is a method for determining the thermal service life of an electromechanical switching device. Thermal service life is understood here to be the (still remaining) service life of the switching device which results from the thermal aging of materials which were used to manufacture the switching device. The service life, also known as durability, refers to the maximum period of time which the switching device can be used without failure. The thermal service life of the switching device ends when its thermal aging has progressed so far that it leads to failure of the switching device. The term “electromechanical switching device” covers any electromechanical switching and protective device which has movable switching contacts.Examples of electromechanical switching devices that are not to be interpreted in any way restrictively are single and double breakers, translatory or rotary.
[0008] The method comprises a step in which a temperature measurement is determined at a measuring point of the switching device. A temperature measurement is a temperature value determined by a temperature sensor. A measuring point is a position inside, i.e., in the interior of the housing, or on an external surface of the housing of the switching device, at which a temperature sensor can be positioned to measure the temperature measurement.
[0009] To determine the thermal lifetime of the switching device, the measured temperature value is compared with predefined temperature threshold values.
[0010] If the temperature measurement is less than or equal to a first predefined temperature threshold, the so-called “permissible temperature Tzu i" of the switching device, the thermal lifetime of the switching device is defined as the remaining time of a predefined maximum lifetime of the switching device. The permissible temperature of the switching device is defined by an institution that knows the materials used in the switching device, e.g. a manufacturer or an operator of the switching device. The permissible temperature forms the upper limit of a first temperature range in which the thermal aging of the switching device is not increased. In this first temperature range, the thermal lifetime decreases as time passes, e.g. in 24 hours the thermal lifetime of the switching device also decreases by 24 hours. Accordingly, the lifetime decreases as time passes.
[0011] If the temperature measurement is greater than the permissible temperature T zuiand less than or equal to a second predefined temperature threshold, the so-called “maximum permissible temperature T max" of the switching device, the thermal service life of the switching device is determined using a temperature-time characteristic curve. The maximum permissible temperature of the switching device is defined by an institution that knows the materials used in the switching device, e.g. a manufacturer or an operator of the switching device. The maximum permissible temperature forms the upper limit of a second temperature range, in which the thermal ageing of the switching device is higher than in the first temperature range. The permissible temperature forms the lower limit of the second temperature range. The second temperature range, in which the switching device thermally ages more quickly than in the first temperature range due to the increased temperature, therefore lies between the permissible temperature of the switching device and the maximum permissible temperature of the switching device.In this second temperature range, which includes all temperature measurements that are greater than the permissible temperature and less than or equal to the maximum permissible temperature, the thermal lifetime of the switching device decreases faster than time passes, e.g. in 24 hours the thermal lifetime of the switching device decreases by more than 24 hours, e.g. by 28 hours.
[0012] Every material, e.g. plastics, has a specific temperature-time characteristic curve for a material property. This temperature-time characteristic curve is defined in such a way that important material properties, such as strength or electrical insulation capacity, are guaranteed. The permissible temperature of the material can be determined from the temperature-time characteristic curve. As long as the permissible temperature of the material is not exceeded, the material can easily withstand this permissible temperature over a long period of time, e.g. 25 years, without being damaged. Furthermore, the maximum permissible temperature of the material can also be determined from the temperature-time characteristic curve. A design limit for the maximum permissible temperature can be derived from a strength limit value.
[0013] If the measured temperature is greater than the maximum permissible temperature of the switching device, the thermal lifetime is determined according to a predefined specification. The maximum permissible temperature forms the lower limit of a third temperature range, in which the thermal aging of the switching device is even higher than in the second temperature range. The thermal aging of the switching device in the third temperature range is determined, for example, by a manufacturer of the switching device who is familiar with the materials used in the switching device.
[0014] If the temperature in the switchgear exceeds the maximum permissible temperature, a material in the switchgear will be permanently weakened or damaged, and the service life of the switchgear will be reduced even more than in the second temperature range. For example, if the maximum permissible temperature of a plastic is exceeded, the flame retardant will decompose and the switchgear will no longer function. If the measured temperature is above the maximum permissible temperature, immediate shutdown of the switchgear may be necessary to protect the switchgear from further damage and to limit any further reduction in service life.
[0015] In order to prevent the maximum permissible temperature from being exceeded, one or more warning thresholds below the maximum permissible temperature can be implemented. If the measured temperature reaches one of these warning thresholds, an institution responsible for the switching device, e.g. an operator of the switching device, is warned that the maximum permissible temperature is about to be reached. After the switching device has been switched off, a defined rest period (switchgear off) can be recommended to the operator of the switching device, e.g. on a display on the switching device or on an HMI that is linked to the switching device. It is possible that the switching device can only be recommended to be switched on again after this rest period has elapsed. The exceedance of the maximum permissible temperature and the duration of the exceedance can be saved.After a manufacturer-defined number of exceedances of the maximum permissible temperature, the operator of the switching device can be automatically notified that the switching device needs to be replaced. It can also be stipulated that the switching device must be replaced after the first exceedance of the maximum permissible temperature.
[0016] The invention is based on the idea that the highest temperature to which a switching device is exposed is crucial for the thermal service life of the switching device. For this purpose, the temperature of a hotspot, i.e. the hottest point or one of the hottest points, in a switching device is recorded. To assess the thermal condition, an absolute temperature measurement at the hotspot can be compared with a permissible temperature at the hotspot and evaluated. Both the permissible temperature and the maximum permissible temperature and the length of time that a material of the switching device can withstand these temperatures are derived for the hotspot from the material values of the respective component. For example, a plastic can become brittle and a copper current path can harden. For this purpose, strength values and other relevant time- or temperature-related factors can be taken into account, in addition to temperature limit values.temperature-dependent variables of the corresponding material can be used to determine the rate of thermal aging.
[0017] The object is achieved by a computer program with software code sections according to claim 11. The computer program is used to carry out a method for determining a remaining thermal service life of an electromechanical switching device. The computer program is adapted to cause an electronic circuit to carry out the following steps: Storing, in a memory, at least one temperature measurement value that was determined at a measuring point of the switching device. If the temperature measurement value is less than or equal to a predefined permissible temperature of the switching device, setting the thermal service life as a predefined maximum service life of the switching device. If the temperature measurement value is greater than the permissible temperature and less than or equal to a predefined maximum permissible temperature of the switching device, determining the thermal service life by means of a temperature-time characteristic curve.If the temperature measured is greater than the maximum permissible temperature of the switching device, the thermal lifetime is determined according to a regulation predefined for this case.
[0018] Advantageous embodiments and further developments of the invention are specified in the dependent claims. The method according to the invention can also be further developed in accordance with the dependent device claims, and vice versa.
[0019] According to a preferred embodiment of the invention, the temperature-time characteristic curve follows an Arrhenius relationship. The Arrhenius relationship describes a temperature dependence in physical and chemical processes. The thermal aging of materials used in a switching device, especially plastics and metals, can be quantitatively described using the Arrhenius relationship. Changes in the material properties over an exposure time of materials used in a switching device can also be quantitatively described using the Arrhenius relationship. According to a preferred embodiment of the invention, an original temperature-time characteristic curve, which determines the service life LD of the switching device, is defined by the equation K_0 (T) = e nT+b, where n is a negative real number. In a temperature-time diagram (Tt diagram), in which the temperature T is plotted linearly on the x-axis and the time t is plotted logarithmically on the y-axis, the original temperature-time characteristic curve K_0 forms a straight line. In the Tt diagram, the temperature-time characteristic curve K_i was shifted at the recorded temperature measurement value parallel to the original temperature-time characteristic curve K_0 along the t-axis by a time interval that corresponds to the length of time that the recorded temperature measurement value prevailed at the measuring point of the switching device.
[0020] According to a preferred embodiment of the invention, the temperature-time characteristic curve is a step function which assigns a recorded temperature measurement value to a temperature class, with each temperature class being assigned a thermal service life. The advantage of this step function is that a calculation of the service life does not have to be carried out for each recorded temperature measurement value. Instead, a recorded temperature measurement value is assigned to a temperature class. Since a unique service life value is stored for each temperature class, e.g. in an assignment table, this service life value can be easily read out according to the respective temperature class. This saves calculation steps, which means that the service life can be determined more quickly.
[0021] According to a preferred embodiment of the invention, a temperature sensor for detecting a temperature measurement in the switching device is positioned on a current path of the switching device. The temperature measurement is thus determined on a current path of the switching device through which a load current is conducted. In this case, the measuring point is on the current path.
[0022] If the current sensor is electrically insulated, the measuring point can be located directly on the surface of the current path; otherwise, it can be located at a sufficiently large distance from the current-carrying current path to prevent electrical flashover from the current path to the temperature sensor. The advantage here is that the temperature measurement can be taken at a primary heat source of the switching device, namely a current path.
[0023] According to a preferred embodiment of the invention, a temperature measurement value is determined at a measuring point on the switching device, said measuring point being a thermal reference point of the switching device. The thermal reference point is a component near a hotspot or the hotspot itself, the hotspot being the warmest point on the switching device or one of the warmest points on the switching device. To determine the thermal reference point, the temperature-time characteristic curve of the component material as well as the temperature conditions of the component must be taken into account. The thermal reference point must be determined individually for each switching device. If the actual hotspot or the actual thermal reference point cannot be selected in a switching device, a suitable measuring point nearby can be selected as an alternative.
[0024] According to a preferred embodiment of the invention, the original temperature-time characteristic curve of the switching device is determined empirically. This can be done by measuring one or more test samples of the switching device (calibration). In this way, the temperature-time characteristic curve describes the thermal aging of the switching device relatively accurately.
[0025] According to a preferred embodiment of the invention, an estimate is made of when a measured temperature value will exceed the maximum permissible temperature of the switching device. This estimate is based on a trend analysis of measured temperature values recorded at two or more previous points in time. This gives a user of the switching device sufficient time to plan a timely shutdown of the switching device.
[0026] According to a preferred embodiment of the invention, a warning is issued before the measured temperature exceeds the maximum permissible temperature of the switching device. This allows a user of the switching device to initiate a timely shutdown of the switching device before the switching device suffers irreversible damage due to excessive temperatures.
[0027] According to a preferred embodiment of the invention, if the measured temperature value is greater than the maximum permissible temperature of the switching device, the switching device is switched off, and a predetermined rest period is observed after the switching device is switched off before the switching device is switched on again. This allows the switching device to cool down before it is put back into operation.
[0028] According to a preferred embodiment of the invention, an ambient temperature measurement value in the area surrounding the switching device is recorded. The temperature at a hotspot of the switching device is derived from the temperature measurement value at the measuring point of the switching device, the ambient temperature measurement value and the difference between the temperature measurement value and the ambient temperature measurement value. Finally, the thermal lifetime is determined on the basis of the derived temperature at the hotspot using a temperature-time characteristic curve. The highest temperature to which the switching device is exposed, the so-called hotspot temperature, is crucial for the thermal lifetime of a switching device. All too often, however, this absolutely hottest point of the switching device is not accessible for measurement; only another measuring point on the switching device, the so-called thermal reference point, is accessible.In this case, one can attempt to derive the hotspot temperature from the temperature at the reference point, the ambient temperature, and the temperature difference between the two aforementioned temperatures. From this derived hotspot temperature, the characteristic curve method is then applied to determine the lifetime LD.
[0029] A preferred embodiment of the invention is a computer program product which comprises the computer program described above.
[0030] In the following, the invention is explained using several embodiments with the aid of the accompanying drawing. It shows schematically and not to scale
[0031] Fig. 1 shows a switching device;
[0032] Fig. 2 is a Tt diagram showing temperature-time characteristics;
[0033] Fig. 3 is a Tt diagram showing temperature classes; and
[0034] Fig. 4 is a flow chart.
[0035] Fig. 1 shows a switching device 10 for switching a load current which flows through a load current line 16, starting from a voltage source, e.g. a mains transformer, to an electrical load, e.g. an electric motor. The switching device 10 is shown as a relay by way of example and not by way of limitation. The switching device 10 has a housing 20 with load current connections 15. The load current line 16 is electrically connected to the load current connections 15 such that the switching device 10 is connected between two ends of the load current line 16. Inside the housing 20, the load current connections 15 are each electrically connected to a switching contact 11 of a fixed switching contact pair 11 by means of a current path 14.The switching contacts 11 of the fixed switching contact pair 11 are electrically connected by a movable contact bridge 13, which carries switching contacts 12 of a movable switching contact pair 12, if the contact bridge 13 is in a first, closed position; in this position there is a continuous electrical connection of the load current connections 15. In a second, open position of the contact bridge 13, the two switching contacts 11 of the fixed switching contact pair 11 are electrically separated; in this position the load current connections 15 are galvanically separated. An actuator unit 17 is able to move the contact bridge 13 back and forth between the two positions; such a change in position of the contact bridge 13 represents a switching event. Such a switching event is triggered by the actuator unit 17 being controlled to change position.The actuator unit 17 can be controlled internally in the switching device 10, e.g., in the case of a circuit breaker, by thermally or magnetically induced tripping. Such control of the actuator unit 17 can also be effected by an external control signal, e.g., in the case of a contactor, by a control command that is transmitted via a control line 19 to a control current connection 18 of the actuator unit 17.
[0036] A temperature sensor 23 is arranged inside the housing 20 at a first measuring point P. The first measuring point P is located on the current path 14 of the switching device 10. This is the warmest point of the switching device 10, the so-called "hotspot." The temperature measurements recorded by the temperature sensor 23 are transmitted to an evaluation unit 30 via a data line 24.
[0037] Inside the housing 20, a current sensor 21 is also arranged at a second measuring point Q. The second measuring point Q is located on the current path 14 of the switching device 10. The current measured values detected by the current sensor 21 are transmitted to the evaluation unit 30 via a data line 22.
[0038] The evaluation unit 30 has a computing unit that can determine a thermal lifetime LD (T) of the switching device 10 based on the received temperature measurements. A thermal lifetime value determined by the evaluation unit 30 can be transmitted to an HMI 32 via a transmission medium 31 and communicated to a user of the switching device 10 via the HMI 32.
[0039] Fig. 2 shows a Tt diagram 40 in which the temperature T is plotted linearly on the x-axis and the time t is plotted logarithmically to the base ten on the y-axis.
[0040] To simplify writing and improve readability, the subscript of a letter (w) can also be expressed by an underscore > followed by the letter (w), ie the notation applies: V w = V_w.
[0041] In diagram 40, an original temperature-time characteristic curve K_0 of a switching device is defined as K0(T) = K_0 (T) = e nT+b, shown, the so-called "output function". It is specifically tailored to the aging behavior of the switching device and indicates the service life LD of the switching device as a function of the temperature T to which the switching device is exposed: LD = f (T) . The higher the temperature T to which the switching device is exposed, the shorter the service life LD of the switching device; therefore the factor n of the output function K_0 (T) is a negative real number, i.e. the higher the temperature T, the shorter the service life LD of the switching device. The temperature-time characteristic curve K_0 shown in Fig. 2, which follows an Arrhenius relationship, is that of a plastic, e.g. a thermoplastic or a duroplastic.
[0042] In addition, two straight lines parallel to the t-axis are entered in the Tt diagram 40, a first straight line 41 at T=T_zul = T zu i, the permissible temperature, and a second line 42 at T=T_max = T max, the maximum permissible temperature. For temperature measurements T_m=T m , which are less than or equal to the permissible temperature T_zul, a first temperature range 43 in which the thermal lifetime LD of the switching device simply decreases with the elapsed time. To do this, the elapsed time is counted down along the straight line 41, and the original temperature-time characteristic curve K_0 is shifted parallel by the corresponding distance in the direction of the t-axis, for example, from point A on the straight line 41 to point D on the straight line 41.
[0043] For temperature measurement values T_m that are greater than the permissible temperature T_zul and less than or equal to the maximum permissible temperature Tjnax, there is a second temperature range 44 in which the thermal lifetime LD of the switching device decreases significantly more than the elapsed time. Assume that the temperature measurement value T_m is Ti = T_i. The output function K_0 is then shifted parallel by a time period Δt, during which the switching device is exposed to the temperature T_i with T_zul < T_i <= Tjnax, along a straight line 46 parallel to the t-axis, which intersects the T-axis at T_i, for example from point B on the straight line 46 to point C on the straight line 46.
[0044] At temperatures T_m that are greater than the maximum permissible temperature Tjnax, there is a third temperature range 45 in which permanent weakening or damage to the material of the switching device occurs. If T_m > Tjnax, the switching device is immediately switched off in order to protect it from further damage and to keep the reduction in service life to a minimum. The switching device may only be put back into operation after a cooling-down period specified by the switching device manufacturer. If the limit value Tjnax is exceeded a number of times specified by the switching device manufacturer, the switching device must be replaced.
[0045] In the following, a concrete example is described using Fig. 2: The output function K0(T) is K0(T) = LD0(T) = e~ O, O8 +22, 38. The temperature is plotted on the x-axis and the storage time at this temperature is plotted on the y-axis. The two perpendiculars 41 and 42 limit the range in which the thermal lifetime undergoes a significant change when the point A (T zui = 126 °C) is used as a reference, which indicates a thermal lifetime LD of approximately 25 years. The thermal lifetime LD of the switching device is read as the y-value of point A.
[0046] The measured temperature value T_m is now recorded at the measuring point in the switching device, e.g., at fixed, specified intervals. As long as the measured temperature values T_m do not exceed the permissible temperature T_zul and are within the first temperature range 43: T_m <= T_zul, the thermal lifetime LD (LD0 = approx. 25 years) is simply reduced ("counted down") according to the elapsed time: the calculation follows line AD by a parallel shift of the output function K_0.
[0047] If the measured temperature values T_m exceed the permissible temperature T_zul and are in the second temperature range 44: T_zul < T_m <= T_max, the thermal lifetime LD is reduced more sharply: In this example it is assumed that the switching device is stored at Ti=150°C for At=35,000 hours; therefore the material of the switching device loses its ability to fulfil its intended function after 35,000 hours at 150°C. To calculate the reduction in thermal lifetime LD, the initial function K_0 is shifted parallel by this time interval At=35,000 hours at the temperature Ti=150°C towards the origin of the y-axis T=0°C, i.e. from B to C, where the shifted function Ki is located. This parallel shift from K_0 to K_i is necessary so that at lower temperatures, e.g. at D, the thermal lifetime (functional capacity) remains higher, but the loss of lifetime at the higher temperature is taken into account.
[0048] However, to give the operator of the switching device a better understanding, the service life is always given at point D, i.e. at the permissible temperature T_zul. Early warning, particularly before Tjnax is reached or in the event of high continuous temperatures, is enabled by defined warning thresholds. Fig. 3 shows a Tt diagram 41 in which the temperature T is plotted on the x-axis and the time t on the y-axis. Five adjacent temperature intervals TI to T5, which form so-called temperature classes, are plotted along the temperature axis. A step function Si is drawn in the diagram 41. It assigns the same service life value LD1, ..., LD5 to all temperature values within a temperature interval, i.e. within a temperature class TI, ..., T5. The higher the average temperature T in a temperature class TI, ..., T5, the lower the service life LD1, . . . , LD5 assigned to this temperature class TI, ..., T5.
[0049] The advantage of this step function Si is that a service life calculation does not have to be performed for each recorded temperature measurement. Instead, each recorded temperature measurement is assigned to a temperature class TI, ..., T5. Since a unique service life value LD1, ..., LD5 is stored for each temperature class TI, ..., T5, e.g., in an assignment table, this service life value LD1, ..., LD5 can be easily read out according to the respective temperature class TI, ..., T5. This saves calculation steps, which means a faster service life determination.
[0050] Fig. 4 is a flow diagram of a method according to the invention. In a first step 310, at least one temperature measurement value Tm determined at a measuring point of the switching device is stored in a memory. If the temperature measurement value Tm is less than or equal to a predefined permissible temperature Tzul of the switching device, in a further step 320 the thermal lifetime LD(T) is defined as a predefined maximum lifetime LDmax of the switching device. If the temperature measurement value Tm is greater than the permissible temperature Tzul and less than or equal to a predefined maximum permissible temperature Tmax of the switching device, in a further step 330 the thermal lifetime LD(T) is determined using a temperature-time characteristic curve Ki, Si. If the temperature measurement value Tm is greater than the maximum permissible temperature Tmax of the switching device, in a further step 340 the thermal lifetime
[0051] LD(T) is determined according to a rule 350 ("Manual") predefined for this case.
Claims
Patent claims 1. Method for determining a thermal lifetime (LD) of an electromechanical switching device (10), comprising the following steps: - detecting a temperature measurement value (T_m, T_i) at a measuring point (P) of the switching device (10); - If the temperature measurement value (T_m) is less than or equal to a predefined permissible temperature (T_zul) of the switching device (10), setting the thermal lifetime (LD) as a predefined maximum lifetime (LD_max) of the switching device (10) and reducing the lifetime according to the elapsed time; - If the temperature measurement value (T_m) is greater than the permissible temperature (T_zul) and less than or equal to a predefined maximum permissible temperature (Tmax) of the switching device (10), which is greater than the permissible temperature (Tzul), determining the thermal lifetime (LD(T)) by means of a temperature-time characteristic curve (Ki, Si); - If the measured temperature value (Tm) is greater than the maximum permissible temperature (Tmax) of the switching device (10), determining the thermal lifetime (LD(T)) according to a rule predefined for this case.
2. The method according to claim 1, wherein the temperature-time characteristic curve (K0, Ki) follows an Arrhenius relationship.
3. Method according to one of the preceding claims, wherein an original temperature-time characteristic curve (K_0) of the switching device (10) is defined as K_0 (T) = e nT+b , where n is a negative real number, and where in a Tt diagram (40) the temperature T is plotted linearly on the x-axis and the time t is plotted logarithmically on the y-axis, so that the original temperature-time characteristic curve (K_0) forms a straight line; and where the temperature-time characteristic curve (K_i) in the Tt diagram (20) at the recorded temperature measurement value (T_m) is parallel to the original temperature-time characteristic curve (K_0) along the t-axis was shifted by a time interval (At) during which the recorded temperature measurement value (T_m) was present at the measuring point (P) of the switching device (10).
4. The method according to claim 1, wherein the temperature-time characteristic curve (S_i) is a step function which assigns a detected temperature measurement value (T_m) to a temperature class (TI, ..., T5), wherein each temperature class (TI, ..., T5) is assigned a thermal lifetime (LD1, ..., LD5).
5. Method according to one of the preceding claims, comprising the following step: positioning a temperature sensor (23) for detecting a temperature measurement value (T_m) on a current path (14) of a switching contact of the switching device (10)- 6. Method according to one of the preceding claims, comprising the following step: empirically determining the original temperature-time characteristic curve (K_0) of the switching device (10).
7. Method according to one of the preceding claims, comprising the following step: estimating when a temperature measurement value (T_m) will be greater than the maximum permissible temperature (T_nax) of the switching device (10) on the basis of a trend analysis of temperature measurement values recorded at two or more past points in time.
8. Method according to one of the preceding claims, comprising the following step: issuing a warning before the temperature measurement value (T_m) is greater than the maximum permissible temperature (Tjnax) of the switching device (10).
9. Method according to one of the preceding claims, comprising the following step: If the temperature measurement value (T_m) is greater than the maximum permissible temperature (Tjnax) of the switching device (10), switching off the switching device (10) and maintaining a predetermined rest period after switching off the switching device (10) before the switching device is switched on again.
10. Method according to one of the preceding claims, comprising the following steps: - detecting an ambient temperature measurement value (T_um) in the vicinity of the switching device (10); - Deriving the temperature at a hot spot of the switching device from the temperature measurement value (T_m) at the measuring point (P) of the switching device (10), the ambient temperature measurement value (T_um) and the difference Delta = T_m - T_um between the temperature measurement value (T_m) and the ambient temperature measurement value (T_um); - Based on the derived temperature at the hotspot, determination of the thermal lifetime (LD) using a temperature-time characteristic curve (Ki, Si).
11. Computer program with software code sections for carrying out a method for determining a thermal lifetime (LD) of an electromechanical switching device (10), comprising the following steps: - storing (310) in a memory at least one temperature measurement value (T_m) which was determined at a measuring point (P) of the switching device (10); - If the temperature measurement value (T_m) is less than or equal to a predefined permissible temperature (T_zul) of the switching device (10), setting the thermal lifetime (LD) as a predefined maximum lifetime (LD_max) of the switching device (10) and reducing the lifetime according to the elapsed time; - If the temperature measurement value (T_m) is greater than the permissible temperature (T_zul) and less than or equal to a predefined maximum permissible temperature (Tmax) of the switching device (10), which is greater than the permissible temperature (Tzul), determining the thermal lifetime (LD(T)) by means of a temperature-time characteristic curve (Ki, Si); - If the measured temperature value (Tm) is greater than the maximum permissible temperature (Tmax) of the switching device (10), determining the thermal lifetime (LD(T)) according to a rule predefined for this case.
12. A computer program product comprising a computer program according to claim 11.