Changes

The main requirement for low voltage power switches is the withstanding of high short circuit currents. The short circuit switching capacity of power switches is determined in tests according to IEC/EN 60947-2 ''(Table 13.3)''. These test differentiate between the maximum short circuit current switching capacity (ultimate current limit) I_cuCU and the operational (or service) short circuit current capacity I_CS . 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. Table 13.3: Testing for the Short Circuit Breaking Capacity of Low VoltagePower Switches According to IEC/EN 60947-2 (Shortened Summary)  ===13.4.3 Testing According to UL and CSA=== 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.