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Changes
→13.4.2.1 Electrical Life
The electrical life of contactors, motor switches, and auxiliary current switches used in power engineering is classified into use categories which are shown in table 13.1:
The making and breaking currents for tests IEC/EN 60947-4-1 are shown in Table 13.2 for the different use categories
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.
Table 13.1: Important Use Categories and Their Typical Applications for Contactors
and Power Switches
*Contactors, Motor Starters according to IEC/N60947-4-1
bild
*Auxiliary Current Switches according to IEC/EN 160947-5-1
bild
The electrical life for the utilization categories AC-3, DC-3, and DC-5 must be at a minimum 5% of the mechanical lifetime of a switching device.
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.
Table 13.2: Verification of Electrical Life
Conditions for Make and Break Tests of Contactors
and Motor Starters by Utilization Category
Fig. 13.2:
AC-3 contact arc erosion of two differently
produced Ag/SnO contact materials 2
in a 37 kW contactor
Ag/SnO 88/12, produced by conventional 2
powder metallurgy with MoO additive, 3
extruded
Ag/SnO 88/12, powder manufacturing by 2
the reaction-spray process with CuO and
Bi O additives, extruded
====13.4.2.2 Temperature Rise====
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.
Fig.13.13:
Maximum movable bridge
temperature rise for different contact
materials in a 132 kW contactor after
high load (AC-4) switching
Ag/CdO 88/12 sintered
and extruded
Ag/SnO2 7.5In2O3 2.5
internally oxidized
Ag/SnO2 88/12
sintered and extruded
Ag/SnO2 11.5 WO3 0.5
sintered and extruded
Ag/SnO 11.6 MO 0.4
For the assessment of contact materials a temperature rise test is frequently performed after a specified number of switching operations accompanied by arcing (Fig. 13.13). The most important characteristic is the measured temeperature rise of the movable bridge contacts. If a certain upper limit of temperature is reached, adjacent plastic components may be irreversibly damaged.
