Motor Protection

inter-teknik HV current-limiting fuses are used for MV motor circuit protections for decades. Proper selection is essential in this application.

What should you expect from the fuses in MV motor circuits?

For direct-on-line starting MV motors, fuses are selected to provide short-circuit protection only. Major requirement for fuses in motor circuits is to prevent contact welding of contactors under locked rotor conditions and short-circuit faults. As MV motors usually operate in powerful industrial networks, high breaking capacity is a vital requirement for fuses in MV motor circuits.

What do you need to consider?

The fuse does not need to provide overload protection as an interlock circuit that triggers a mechanical switching device (e.g., a vacuum contactor commonly cares for this).
Apart from the co-ordination with the contactor, the fuse has to withstand frequent motor starting currents without deterioration. Figure 1 highlights all these major parameters on a graph.

Selection
Standard motor operating conditions for fuse selection are as follows

  • Motor starting current: 6 times the motor rated current
  • Duration of starting current: 10 s
  • Number of consecutive starts: 2
  • Frequency of starts: 6 per hour
  • Deviations from these standard starting conditions may require different fuse ratings. Please see the Fuse Selection Table for MV Motor Circuit Protection.


IMPORTANT

Thermal protection feature is highly adopted and recommended in this application; particularly due to frequent motor starting currents. Please visit Thermal Protection section for more details.

Related Links:

Switch-Fuse combinations
Fuse use in Air Insulated Switchgears (Industrial)