Test Procedures for the Communications Technology

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 is defined 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) (Fig. 15.5). 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 the voltage – current range for dry circuits

Fig. 13.5: Schematic describing the contact load categories in in the current-voltage ranges according to IEC DIN EN 61810-2. Arc-limiting graph for an arc duration < 1 ms

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
  
• Contact load category CC1: < 10 V; < 100 mA
  
• Contact load category CC2: < 30 V; < 1 A
  

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 contacts are fulfilling the requirement from their new state for the respective application. The current carrying in normally dynamically 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.

Fig. 13.6: Relation between the breaking currents of relays and electrical life requirements of switching systems

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 (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 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

Electrical life, failure rate, and reliability are statistical measures. 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.

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Fig. 13.7: Statistical evaluation of the electrical life of relays (Operating parameters: 220VAC, 8A, 0.1Hz, resistive load; contact material AgCdO 90/10)

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^th occurrence analyzed. From such electrical life test it is also possible to statistically predict the failure rate of relays under certain specified load conditions.