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→Failure Analysis
For contacts designed to cover multiple load ranges, the measuring category for the lowest range must be used. Contact resistance measurement usually is carried out following the 4-wire method at a temperature of 27°C ± 1°C and relative humidity of 63 – 67% RH.
By adding an upper threshold value (for ex. the 90% contact resistance value of the statistical cumulative frequency), it can be assured that that those contacts are fulfilling the requirement from their new state for the respective application. The current carrying in normal dynamical operating devices is also assured by this measurement procedure. It is however not a valid indicator for the further behavior in a given application. To determine this, electrical life tests have to be performed under real electrical loads.
<figure id="fig:Relation between the breaking currents of relays and electrical life requirements of switching systems">
Electrical life, failure rate and reliability are statistical quantities. To determine the electrical life, the relays are switching the specified load in an accelerated way with higher switching frequency, until the first pre-defined failure occurs. The number of switching operations reached for the representative sample size of relays is determined through statistically valid test setups using Weibull distributions. For all switching operations, the failure criteria must be monitored and recorded.
<figure id="fig:Statistical evaluation of the electrical life of relays">
[[File:Statistical evaluation of the electrical life of relays.jpg|right|thumb|Figure 23: Statistical evaluation of the electrical life of relays (Operating parameters: 220VAC, 8A, 0.1Hz, resistive load; contact material AgCdO 90/10)]]
</figure>
The large quantity of data generated during the these 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. (<xr id="fig:Statistical evaluation of the electrical life of relays"/><!--(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 tests it is also possible to statistically predict the failure rate of relays under certain specified load conditions.
===<!--13.3.5-->Testing Technology===
*Signal Relays (Low Current Relays)
The DC load conditions are specified in the relevant standards (for ex. Telecom Specifications).
*Switching of “dry circuits” with monitoring of the contact resistance, in some cases at elevated temperatures
* 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 failure
Results of relay life testing using different contact materials are illustrated as an example in (<xr id="fig:Automotive relays under motor load"/> <!--(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 AgNiO15 was found to be the best suited contact material. <div class="multiple-images"> <figure id="fig:Principle and sequence of testing with electronic load simulation">[[File:Principle and sequence of testing with electronic load simulation.jpg|right|thumb|Figure 4: Principle and sequence of testing with electronic load simulation]]</figure><figure id="fig:Automotive relays under motor load">[[File:Automotive relays under motor load.jpg|right|thumb|Figure 5: Automotive relays under motor load: Results of electrical life testing using different contact materials]]</figure></div><div class="clear"></div><br style="clear:both;"/>
===<!--13.3.6-->Failure Analysis===
The full clarification of causes for switching device failures, for example relays, is most important for quality assurance. As a starting point, the full history of the relay, such as electrical load, environmental conditions etc. must be recorded. The process flow chart in (<xr id="fig:Flow diagram for evaluation of failure cause in switching devices for communications technology"/><!--(Fig. 13.10)-->) clearly describes a proven way to conduct a failure analysis.Following all procedures of such a failure evaluation carefully, the root cause of the defect can most likely be established in order to implement preventive measures limiting future occurrences.
<figure id="fig:Flow diagram for evaluation of failure cause in switching devices for communications technology">
[[File:Flow diagram for evaluation of failure cause in switching devices for communications technology.jpg|rightleft|thumb|Figure 56: Flow diagram for evaluation of failure cause in switching devices for communications technology]]
</figure>
==References==