Note for the uninitiated: A fused switch is a switch where the fuses form part of the moving contact mechanism, whereas, a switch fuse is a switch with separate fuses in the same enclosure.
The electricity supply voltage will determine the voltage rating of the starter and all of the other power circuit devices.
Of all the devices within a starter the main fuses are the key components, their purpose is to protect the starting equipment, the motor and the motor cables. The rating of the fuses is determined by the motor full load current, starting method, run up time and frequency of starting. The rating of the fuses will determine the size of the fuse switch, the cable sizes, the contactor rating, and main terminal sizes.
The main fuses must have a rupturing capacity equal to or greater than the equipment short circuit level at their position within the electricity supply network.
The fuse ratings should be no more than 40% of the upstream protective device rating for fuses or the time current settings for circuit breakers. Where circuit breakers are used upstream of fuses it is important to ensure the fuse curve is below the circuit breaker trip curve. The protection curves of fuses and circuit breakers are so diverse, with those of fuses following no mathematical formula that it is imperative that the curves are plotted on graphs to ensure discrimination.
Most manufacturers of fuses and circuit breakers state a major to minor ratio of 50%, practical experience show this to be, on occasions, optimistic which is why 40% is recommended in this article.
In section 02-09-05-00 entitled motor starting current the following starting to running current ratios were given, direct-on-line Is=7xIn, star delta Is=2.5xIn and Korndorffer Is=1.75xIn. The tables for fuse ratings against various three phase 400V 50Hz motor run up times that are appended to the pdf version of this section reflect those starting current ratios. Where Korndorffer starting is used with variable tap setting the ratings for the 80% taps should be used; this is to ensure that all the taps of the auto-transformer within the starter can be utilised, thus the starting current ratio for the highest tap should be used in selecting the rating of the fuses. It is also necessary to take into account the auto-transformer inrush current.
The tables have been derived from the time/current characteristic graphs published by Lawson Fuse Ltd and typical current/speed graphs for the starting characteristics of three phase squirrel cage motors.
The fused switch must be able to physically mount the selected fuse and have an AC23A (breaking capacity of 8xIn at a power factor of 0.45, where In is the full load motor current) current rating compatible with the motor full load current and have a short circuit rating when the selected fuse is installed that is equal to or greater than the equipment short circuit level.
The fused switch must also be fitted with 4NO and 4NC auxiliary contacts each with an Ith rating of 10A and an AC-15 rating of 6A so that the control, alarm and indication circuits within the starter are isolated when the switch is off. The terminations of the auxiliary switches must be capable of being shrouded as they will be live even when the fused switch is off.
The auxiliary contact that controls the electricity supply to the starter control circuit must open before the fused switch main contacts open and close after the main contacts close to ensure that the main contacts operate on no load.
The pdf version of this section has tabulated data for fuse sizing against starting method, run up time and motor full load current.