Application
Fuse Links provide short circuit protection in motor circuits to both the motor starter and cables from the starter to the motor.
Overload protection is provided by the motor starter, generally by an overload relay and contactor. Combination striker tripping
may also form part of associated equipment which houses the fuse links and motor starters.
Application Procedure
For any motor the fuse current rating is determined by magnitude and duration of starting current, except in a few situations
where the starting currents are very light. The fuse current rating should therefore be selected as follows:
Direct-on-line Starting
In the absence of specific information the starting current can usually be taken to be six times the motor’s full load current.
The starting time will depend on the type of the drive but will be approximately as follows:
Pump Motors - 6 seconds
Mill Motors - 10 to 15 seconds
Fan Motors - 60 seconds
These are average values and the appropriate figures for starting current and starting time for actual installation should be obtained
wherever possible.
Multiply the starting current by 1.7 and using this value of current and the starting time, (it is recommended that a minimum
time of 5 seconds be adopted), plot this point on the time-current characteristics of the fuse link. The correct current rating
of the fuse is then chosen as being the one immediately to the right of the point so plotted. The chosen fuse must also have
a current rating of at least 1.3 times the full-load current of the associated motor.
The rating chosen will be adequate for normal applications where the associated motor is not started more than twice in given
period of one hour.
For applications using more frequent starting duties, a greater derating factor must be applied as shown in the following table.
Assisted Starting
A similar method of fuse selection may be used as for direct-on-line starting (see above), but it must be noted that the
normal running current of the motor is likely to be closer in value to the nominal current rating of the fuse than for direct-on-
line applications.
The rating of fuse chosen will need to be appreciably greater than the motor running current to allow for restricted cooling inside
control gear cubicles, particularly where multi-tier starters are involved. In case of doubt refer to Bussmann for further guidance.
Note
For applications involving more frequent starting duties than 16 times per hour or where unusual duty cycles are involved,
consult Bussmann application engineers for advice.
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Motor Fuse Links
Maximum 2 starts per hour - Derating factor 1.7 Maximum 8 starts per hour - Derating factor 2.1
Maximum 4 starts per hour - Derating factor 1.9 Maximum 16 starts per hour - Derating factor 2.4
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