IEC 60644-2009 pdf Specification for high-voltage fuse-links for motor circuit applications

4 K factor Factor which defines an overload characteristic to which the fuse-link may be repeatedly subjected under specified motor starting conditions, and other specified motor-operating overloads, without deterioration. For the purpose of this specification, the value of K is chosen at 1 0 s. Unless otherwise stated by the fuse-link manufacturer, it is valid from 5 s to 60 s, for a frequency of starts up to six per hour and for not more than two consecutive starts. For conditions different from those specified above, for example where service conditions involve inching, plugging or more frequent starts, the manufacturer should be consulted. The overload characteristic is obtained by multiplying the current on the pre-arcing characteristic by K (less than unity). 5 Withstand requirements The performance of a fuse-link for motor circuit applications is in general determined by the following criteria: – to withstand without deterioration starting pulses in rapid succession due for example to abnormal conditions, such as those occurring during commissioning of the equipment; – to withstand without deterioration a large number of motor starts in normal service conditions. This standard therefore specifies two sequences of tests representative of these conditions: 1 00 cycles corresponding to abnormal service conditions; 2 000 cycles corresponding to normal service conditions. It is expected that a fuse-link which passes these tests will have a good behaviour during a satisfactory life duration.
8 Selection of fuse-links for motor circuit applications and correlation of fuse- link characteristics with those of other components of the circuit 8.1 Selection of fuse-links The fuse-link is inserted into the motor circuit that the fuse-link is intended to protect. Some ratings of the fuse-links (e.g. rated voltage and rated breaking current) are therefore dependent on the system and others (e.g. rated current) are dependent on the motor. The ability to withstand repetitive starting conditions is an important factor. When selecting a fuse-link for a given motor circuit application, due regard should be paid to the K factor, which should be applied to the pre-arcing time-current characteristic of the fuse-link to take account of these starting conditions. The usual concept of rated current, based upon the ability of a fuse-link to carry a given current continuously without exceeding a specified temperature rise, is usually of secondary importance where the motor is started direct-on-line. The fuse-link for such applications is normally chosen by reference to the paragraphs above.