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===<!--13.3.1 Measuring of Contact Resistance=-->Test Procedures for the Communications Technology==
One must differentiate between static tests (for ex. contact resistance) and dynamic ones (for ex. electrical life). In certain electromechanical components and switching devices, the contacts can be exposed to both, static and dynamic stresses (for ex. connectors, relays, switches, pushbuttons, circuit breakers). For statically stressed components, the life expectancy is usually expressed as a time period, i.e. hours, while for dynamically stressed ones the expected functional life is defined as numbers of operations or switching cycles. ===<!--13.3.1-->Measuring of Contact Resistance=== The contact resistance in the new stage is for most contact containing electromechanical components a characteristic, defining their quality. It isdefined in most standards and acceptance specifications of users. It is important however, that a clear definition of the test conditions is given with the specification. The standard IEC DIN EN 61810-2 defines the application areas of relays according to their load application categories (CC) (<xr id="fig:Schematic describing the contact load categories"/><!--(Fig. 1513.5)-->). The categories CC1 and CC2 are separated by the arc-limiting graph (DC breaking capacity). Switching operations below the graph do not generate electrical arcs, those above the graph are accompanied by electrical arcing.
*CC1 is characteristic for switching operations in control circuits at low voltages, for ex. SPS input signals
*CC2 is typical for release circuits in low voltage controls which for example actuate contactors (220 - 240 VAC)
*CC0 describes as a special application describes the voltage – current range for dry circuits<figure id="fig:Schematic describing the contact load categories">Fig[[File:Schematic describing the contact load categories. 13.5jpg|right|thumb|Figure 1:Schematic describing the contact load categoriesin in the current-voltage ranges according toIEC DIN EN 61810-2. Arc-limiting graphfor an arc duration < 1 ms]]</figure>
The measuring conditions for contact resistance measurement are defined per IEC DIN EN 61810-7 for the contact load categories as follows:
*Contact load category CC0: < 30 mV; < 10 mA <br />*Contact load category CC1: < 10 V; < 100 mA <br />*Contact load category CC2: < 30 V; < 1 A <br />
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 normally dynamically 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.
===<!--13.3.2 -->Life Testing===
Electrical life is defined as the number of operations under a defined load (make under load current, current carrying, breaking the load current). The total sequence of this switching cycle leads for the specific design parameters of a relay (bounce characteristics, materials, etc.) to the phenomena responsible for the later device failure, such as increased contact resistance, material transfer, arc erosion, and contact welding, for example ''(<xr id="fig:Relation between the breaking currents of relays and electrical life requirements of switching systems"/><!--(Fig. 13.6)''-->).
As experience shows, failures during load switching are usually related to deterioration of the contact parts. Therefore , the mechanical life must always be higher than the electrical life under the required load conditions. Typically the mechanical life is about 10 times higher than the electrical one.
===<!--13.3.3 -->Criteria for Functional Life===
In the communications technology the functional life criteria for switching devices, following conventional standards and specifications are:
*Failure to close by exceeding an upper limit for the contact resistance
*Failure to open by contact welding at a higher force than specifiedor specified or by mechanical interlocking
*Switching characteristics strongly changed by arc erosion or material transfer
As opposed to electronic components, failures in electromechanical devices can occur once and not repeat at all or at considerably later stages of the life time.
Because of this , an exact definition for the failure mode must be provided. In most cases the first failure occurrence is defined as the overall device failure, since relays are increasingly used in safety related electrical circuits. Failures to open because of contact welding must be very critically examined. In some instances a weak weld or “sticking” of the contacts can cause a delayed opening and separation of the contacts by themselves. Therefore it is useful to define weld failures as "non-opening after a specified time", approx. 1 sec, . after the actual switching-off event. ===13.3.4 Determination of Functional Life===
*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
*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 <xr id="fig:Principle and sequence of testing with electronic load simulation"/><!--(Fig. 13.8)''-->).
During accelerated life tests the following parameters from real life loads must be observed:
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:
<div class===13.3.6 Failure Analysis==="multiple-images">
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|left|thumb|Figure 6: Flow diagram for evaluation of failure cause in switching devices for communications technology]]
</figure>
<br style="clear:both;"/>
==References==
[[Testing Procedures#References|References]]
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