scope:

The information given in this technical report provides guidance to assist in selecting a fuse-link to ensure coordination with contactors or motor-starters (contactors with overload relay).

The coordination between motor-starters and the fuses which protect them is covered in IEC standards by test requirements such as those in IEC 947, in particular parts 1 and 4.

Overcurrent protection of other equipment, such as motors, conductors, etc., is not covered by this standard.

Tests are specified at three levels of prospective current, according to IEC 947-4-1:

a) in the region of the current /_c (see clause 5). Tests are made at 0,75 /_c when the starter shall disconnect the current without damage and the fuse does not operate, and at 1,25 /_c when the fuse shall operate before the starter (see annex A, figure A.1);

b) at the appropriate value of prospective current "r" shown in IEC 947-4-1, table XI (see table A.1 in annex A);

c) at the rated conditional short-circuit current /_q, if higher than the test current "r".

The fuse selected is capable of absorbing the surge of current on starting the motor and is normally selected from the recommendations of the manufacturer or by compliance with national installation codes and wiring rules.

Studies carried out by IEC committee "Fuses" in collaboration with motor-starter manufacturers worldwide have revealed that there is no major difficulty in achieving satisfactory coordination at the most exacting of the levels of type of coordination using selected fuses according to IEC 269-2 in coordination with modern contactors. A survey is presented in annex B of the rated currents, I²t values and cut-off currents of fuses correctly chosen according to the ratings of the starters they protect, based on the results of successful type testing throughout the world.

Examples of suitable fuse-links used for motor protection are also given in annex B.