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FAQ motor protection

Is the protection of motor circuits standardized?

Partly. IEC 60 644 gives guidance HOW to protect, but does not advice WHAT current rating should be taken. Recommendation lists of the fuse manufacturers are helpful.

What do I have to consider finding the proper fuse?

At first, one has to calculate the loading current of the machine, which has to be lower than about 60 % of the fuse-link rated current. Secondly the motor starting current should be observed. The coordinate of that current and the motor run up time must be lower than the appropriate time/current -point on the fuse-curve. The distance should be about 60 %. If there are more than two starts of the machine per hour, additional safety margins have to be observed. Typically the fuse rated current is more or less 160 to 300 % the motor full load current.

Is paralleling of fuse-links for bigger machines allowed?

Yes, of course. If the necessary fuse rating is higher than the fuse manufacturers range in the catalogues, two ore more fuse-links could be mounted parallel. In this case, ask the manufacturer for details.

Are there special fuses for motor circuit applications?

Basically the typical HV back up fuse-links may be used. For temperature and power loss reasons it is better to take special fuse-links for motor circuit protection. These fuses have a special element design, which is better able to withstand the cycling load of the circuit.

Any questions?