Changes

The testing of electrical contacts for power engineering applications serves on the one hand the continuous quality assurance, on the other one the new and improvement development efforts for contact materials. To optimize the contact and switching performance contact materials and device designs have to complement each other. The success of such optimizing is proven through switching tests.

The following chapters are limited to metallurgical analysis and the testing of the most important properties of switching devices such as electrical life, temperature rise, and switching capacity.

The main characteristic for the appraisal of contact materials for power engineering is the optical evaluation of their microstructure in a metallographic mount. This provides a picture of the internal structure of the materials. It allows detecting structural in-homogeneity, grain boundary enrichments, cracks, material separations, or defects in the brazing interface. The metallographic view is however limited to the one two-dimensional plain in which the mounting cut was made.

<figure id="fig:Microstructure of a powder metallurgical Ag CdO material">

[[File:Microstructure of a powder metallurgical Ag CdO material.jpg|right|thumb|Microstructure of a powder metallurgical Ag/CdO material after being affected by intense electrical arcing]]

</figure>

The electrical life of a motor switch is influenced primarily by arc erosion which is generated during make and break arcs on the contact surface. During AC-3 testing, for which the make current is six time the nominal rated current, the arc erosion is mainly caused by the make arcs, especially if frequent contact bounces > 2 ms occur. Therefore the bounce characteristic of switching devices primarily used for “normal” use in switching on and off electrical motors is of critical importance. If make and break currents are the same, as in the ultilisation categories AC-1 and AC-4, the break erosion dominates the arc erosion so much that make erosion can be neglected.

<figtable id="tab:Important Use Categories and Their Typical Applications for Contactors

and Power Switches">

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The conditions for make and break tests of auxiliary current switches and control circuit devices are described in IEC/EN 60947-5-1. Usually the

electrical life of auxiliary switches is of lesser importance since these devices see only smaller loads. Under certain conditions however requirements for make and beak capacity can be as high as 10 times the nominal current. This results in very severe requirements on the dielectric strength and recovery voltage of the arc affected region immediately after arcing.

<figtable id="tab:Verification of Electrical Life Conditions for Make and Break Tests of Contactors and Motor Starters by Utilization Category">

{| class="twocolortable" style="text-align: left; font-size: 12px"

[[File:AC3 contact arc erosion of two differently produced Ag SnO2 contact materials.jpg|right|thumb|AC-3 contact arc erosion of two differently produced Ag/SnO₂ contact materials in a 37 kW contactor <b>1</b> Ag/SnO₂ 88/12, produced by conventional powder metallurgy with MoO₃ additive, extruded <b>2</b> Ag/SnO₂ 88/12, powder manufacturing by the reaction-spray process with CuO and Bi₂ O₃ additives, extruded]]

Testing for temperature rise is required only for switching devices in the new stage. During use however over the entire life of the device no damages due to temperature rise are allowed in the device or at ist terminal points.

sintered and extruded]]

</figure>

In switching devices, which are actuated by AC actuator magnets, the contact parts can close and open synchronously at a specific phase angle relative to the voltage-zero of the supply voltage. Of similar importance is the sequence of closing and opening of the contacts with regards to the three phases. The closing and opening delays define at which time delay after the first phase (or pole) the other phases close and open.

lead to extreme damage, especially on at least one of the phases or poles. They are the cause of early failure of this phase and therefore the complete switching device and can happen as early as after only 30% of the normally expected lifetime. Because of variations in the mechanical characteristics of switching devices from manufacturing processes life testing cannot be performed on one device alone. Only statistical analysis of tests from multiple device samples can be used as reliable results. Such a procedure is however time consuming and costly. If however every single switching operation during a test is monitored for bounce behavior, on- and off-switching synchronization and related phase sequencing and phase delays, the arc moving behavior, and especially arc energy which is transferred during make and break arcing to the contact pieces, and then these data are properly analyzed, is it possible to assess a specific contact material from a test in only one device alone. Only statistical analysis of tests from multiple device samples can be used as reliable results. Such a procedure is however time consuming and costly. If however every single switching operation during a test is monitored for bounce behavior, on- and offswitching synchronization and related phase sequencing and phase delays, the arc moving behavior, and especially arc energy which is transferred during make and break arcing to the contact pieces, and then these data are properly analyzed, is it possible to assess a specific contact material from a test in only one device.

When specifying I_CU it must be guaranteed that short circuit current up to the maximum limit value can be interrupted safely. After its occurrence it must be possible to switch on one additional time onto the not yet eliminated short circuit and again interrupt this short circuit current again safely. The switch does not have to be functional any more after this second interruption. A switch specified for I_CS must still be capable to protect the circuit and be further usable within certain limitations.

To safely withstand short circuit currents high requirements are imposed on the weld resistance of the materials used for the mating contacts. During short circuit switching the contact force between the contacts pairing is reduced by electromagnetic forces. Above a certain device specific current value the contacts will separate. This generates an electrical arc with contact material melting at its root points. During the next closing of the contacts this can cause contact welding, prohibiting the opening of the contacts during a subsequent short circuit and therefore eliminating the safety function of the switching device.

<figtable id="tab:Testing for the Short Circuit Breaking Capacity of Low Voltage Power Switches According to IECEN 60947-2 (Shortened Summary)">

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|Subsequent isolation test

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|Subsequent temperature rise test

CO = Switch on and off

The test standards for North America according to UL (USA) and CSA (Canada) differ in part substantially from those of the IEC and harmonized European EN standards. In the US and Canada the standards differentiate between switchgear for power distribution, for example low voltage circuit breakers and power switches covered by UL 489 (UL = Underwriters Laboratories) and CSAC22.2 No. 5-02 (CSA = Canadian Standard Association) and those for industrial switching devices, for example contactors covered by UL 508 and CSA-C22.2 No. 14 respectively. For industrial controls contactors and other switching devices are often classified in the USA according to NEMA (National Electrical Manufacturers Association) current rating. North American standards emphasize the prevention of fires and therefore have high limit requirements on temperature rise. They also require larger air and creep gaps than those of IEC, which leads to significant differences in the design of the switches and their contact systems.

==References==

[[Testing Procedures#References|References]]