Changes

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

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">

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)">

==References==

[[Testing Procedures#References|References]]