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→13.3.4 Determination of Functional Life
The large quantity of data generated during the tests can be only analyzed with computer based test systems. After all the relays failed a failure statistic is calculated and the expected electrical life is calculated based on the specified switching and operating criteria. Fig. 13.7 shows in form of a Weibull diagram the results of a relay life test for a sampling of switching relays under a resistive AC load with failures after the first and the 10<sup>th</sup> occurrence analyzed. From such electrical life test it is also possible to statistically predict the failure rate of relays under certain specified load conditions.
===13.3.5 Testing Technology===
The test set-ups for relay tests consist of a load part, which allows the selection of specific electrical loads, and a control / monitoring part, which typically includes the measuring devices with data collection and mostly also a computer for data analysis ''(Fig. 13.8)''. Variations for testing of different relay types concern mostly the load circuits, the sample size of test specimens, and the frequency of testing.
*Signal Relays (Low Current Relays)
The DC load conditions are specified in the relevant standards (for ex. Telecom Specifications).
Fig. 13.8:
Principle and
sequence of testing
with electronic load
simulation
*Switching of “dry circuits” with monitoring of the contact resistance, in some cases at elevated temperatures
*Two load conditions at 30 W (125 VDC, 0.24 A and 37.5 VDC, 0.8 A
*Two wiring load conditions
*Two different overload conditions at 60 W (250 VDC, 0.24 A and 37.7 VDC, 1.6 A)
The number of test samples varies between 3 and 20 pieces, depending on the load; the switching frequency varies between 0.3 and 10 Hz.
The test set-up has to follow the regulations of IEC DIN EN 61810-2.
*Elementary Relays (or General Purpose Relays / Switching Relays)
For these relays electrical life tests according to their various DC as well as AC load conditions are required. These load conditions are also defined in well known VDE standards for power engineering products (IEC/EN 60947-4-1, IEC DIN EN 61810-2).
The number of samples (mostly < 10 relays, and testing frequency are defined. Often tests for these load characteristics are used for the selection of optimum contact materials.
*Automotive Relays
For these relays electrical life tests are conducted under the DC voltage used in the on-board circuitry. Since no standardized tests have been established for automotive relays, the electrical loads are agreed upon between the supplier and the end user. The multitude of load conditions, i.e. resistive, inductive motor, and lamp loads require a flexible set-up of suitable test equipment ''(see Fig. 13.8)''.
During accelerated life tests the following parameters from real life loads must be observed:
*Lamp loads are strongly temperature dependent resistive loads. Therefore a sufficient cooling time of the bulb filaments must be allowed.
*The actual loads used in test experiments wear over time and must be replaced at regular intervals.
*Tests at low temperatures require large cooled climate chambers and extended cool-down times.
These requirements lead to the need to simulate the real life loads through electronically controlled load circuits. Such computerized electronic load circuit simulations easily allow the test sequences to be controlled and monitored:
Fig. 13.9:
Automotive relays under motor load: Results of
electrical life testing using different contact
materials
* Time saving test runs for lamp loads by reducing interval times Monitoring of each switching operation for electrical life criteria Allowing to recognize non separation of the contacts after a pre-set time limit to be classified as a relay fallure
Results of relay life testing using different contact materials are illustrated as an example in Fig. 13.9. For each contact material 10 relays were tested under the prescribed motor load until a failure due to non-opening was detected. For this specific load condition AgNiO. 15 was found to be the best suited contact
