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Fast Acting Class RK1 vs. Semiconductor Fuses
Copyright © 2003 Francis J. Martino

Use of a Class RK1 fast acting fuse installed in a wall mounted
standard fusible disconnect switch will give adequate protection for
a VFD. (Install fuse rejection clips to prevent replacement with class
K-5 and H fuses.)

A drive with internal semiconductor fuses will be in an oversized
enclosure and, therefore, the use of RK1 fuses will result in a savings
in the cost of the drive. A typical 60 HP, 480 volt drive that is rated at
77 FLA will cost an extra $1400 with 100 Amp semiconductor
fuses installed.

Current vs. Melting Time curves in the Ferraz Shawmut catalog #106,
pages D34 and B25, give the characteristics in the Table below.
The VFD must be operating a low inertia load. A high inertia load may
require a larger fuse size if there is a prolonged acceleration.

With an overload condition, the drive's electronic overload
protection must always trip before the fuses blow. The fuses are
for fault protection only.

Response times for 100 Amp fuses are as follows:

Amps Semiconductor
Fuse
Shawmut
A70QS100
Class RK1
Shawmut
A6K100
VFD
Programmed
Overload
Parameter
130 > 100 seconds > 1000 seconds 16 seconds
300 16 seconds 15 seconds < 16 seconds
600 .02 seconds .36 seconds < 16 seconds

The following characteristics of the A70QS100 Semiconductor fuse and
the A6K100 fuse are identical:

Current Limiting Maximum Instantaneous Peak Let-through Amperes:
2.3 times Symmetrical RMS Amps

Interrupting Capacity: 200,000 Amps

The objective of the drive's electronic overload protection is
to prevent thermal damage to the drive and motor.

The objective of the current limiting capability of the fuses is to
limit the damage to the drive when an IGBT or other component fails.
Thus, damage to the drive will occur prior to the opening of either a
semiconductor or RK1 fuse.

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