Allen-Bradley ArmorStart 290E User Manual page 189

Applying More Than One ArmorStart LT Motor Controller in a Single Branch Circuit on Industrial Machinery Appendix A
current between 5000 and 10000 amperes, the class of the fuse must be CC, J
or T. Since the electrical supply has an available fault current of 9000 amperes,
selecting a Class CC, J or T fuse with a rating of 45 A or less ensures each motor
controller is applied within its own ratings.
Supplementary Note 1: The rating of the fuse must not exceed the rating
permitted by 7.2.10.4(2) to protect the smallest conductor in the circuit.
Selecting a Class CC, J or T fuse with a rating of 45 amperes, being less than 60
amperes, also protects the conductors (see Requirement 2). Although the
ArmorStart LT products presently have a maximum fuse rating of 45 A, future
controllers may have maximum fuse ratings that exceed 60 A. In this case, the
maximum rating of fuse is limited by the rating to protect the 14 AWG
conductors, 60 A. The maximum rating permitted for the controller, 45 A,
is a maximum rating and can be reduced, for more conservative protection,
provided nuisance opening of the fuses do not occur.
Supplementary Note 2: In this appendix, a fuse having a rating of any class
means a fuse having the let-through characteristics of an Class RK-5 fuse. Class
RK-5 fuses are assumed to have the maximum let-through of any class of fuse. For
this reason, the ArmorStart LT motor controllers that are marked for use with
fuses, without a restriction to a particular class, have been tested with and are
intended to be used with fuses having a class of RK-5. Of course, fuses of a class
that have lower let-throughs than Class RK-5, such as Class CC, J or T, are also
acceptable. A fuse having a rating of any class also restricts the fuse to those that
have been evaluated for use as branch-circuit protection devices. This means that
semiconductor fuses, used to protect power electronic equipment, or
supplemental fuses cannot be used to protect the multiple-motor branch circuit.
Supplementary Note 3: There are four complementary ratings relevant to the
"specified maximum branch-circuit protection" of 7.2.10.4(1). They are: the fuse
class, the maximum fuse rating, the voltage rating and connection of the source
(480Y/277 V), and the available fault current of the source. Applying the
controllers according to these four ratings means that a fault on the output of all
the controllers, and internal faults for Bulletin 294 controllers, will not result in a
shock or fire hazard.
Supplementary Note 4: In this example, the assumption is made that the
available fault current at the controller is that of the source on the line side
of the fuses. Although it is true that the wiring impedance between the fuses
and the first controller reduce the fault current available at the controllers, this
reduction is neglected by assuming the first controller, the ½ horsepower
Bulletin 294 controller, is very close to the fuses.
4. Requirement Four: Overload Protection — The motors, conductors and
controllers must be protected against motor overload conditions.
Text:
"7.3.1 General. Overload devices shall be provided to protect each motor, motor
controller, and branch-circuit conductor against excessive heating due to motor
overloads or failure to start."
Rockwell Automation Publication 290E-UM001B-EN-P - June 2012 189