Allen-Bradley ArmorStart 290E User Manual page 182

Appendix A Applying More Than One ArmorStart LT Motor Controller in a Single Branch Circuit on Industrial Machinery
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installation to be installed, as intended, in multiple-motor branch circuits. For
this example, assume all testing is done with fuses of the same class.
The UL investigation of both controllers is done according to UL 508C, Power
Conversion Equipment. The controllers are connected to the test supply through
the three-phase conductors and equipment grounding conductor and then
covered with cotton in areas that are likely to vent hot gases and sparks during
the tests. During the test, electrical faults are impressed on the output of, and
internal to, these variable-frequency AC drive-based controllers. Increasing the
ampere rating of the fuses increases the magnitude of the fault currents that flow
through and damage the controller before the fuses open. Afterwards, the damage
to the controller is evaluated to determine whether a potential shock or fire
hazard exists when protected by fuses having this ampere rating. One criterion of
the evaluation is the examination of the equipment grounding conductor that
must not open during the test, as this could leave exposed conductive parts in an
energized state (shock hazard). Another criterion is that the cotton must not
ignite, as this indicates the expulsion from the controller of hot gases or molten
metal fragments (fire hazard).
Referring to the controller on the left, UL 508C permits the individual-motor
testing to be performed with the maximum rating of fuse that can be used to
protect an individual-motor branch circuit. According to both NFPA 70 and
NFPA 79, this is 400 percent of the full-load current rating of the largest motor
that the controller can supply. In UL 508C, this is taken to be 400 percent of the
rated output current of the controller, or 6 amperes.
Referring to the controller on the right, UL 508C permits the group installation
testing to be performed with the maximum rating of fuse that can be used to
protect a multiple-motor branch circuit. According to both NFPA 70
[430.53(C)) and NFPA 79 (7.2.10.4(3)], this is 250 amperes. This value, derived
from the installation requirements of 430.53(C) and 430.53(D) of NFPA 70, is
determined by the largest size of power conductor that the ArmorStart LT
controller can accept, 10 AWG. Because the UL 508C test covers all possibilities
in NFPA 70 and NFPA 79, it permits the maximum value of 250 amperes. This
covers 7.2.10.4(2), which permits only 100 amperes. However, in this case, the
manufacturer, Rockwell Automation, chose to test and mark with the lower value
of 45 amperes. This value was chosen as the tradeoff between the maximum
number and type of controllers in the branch circuit — limited by the maximum
fuse rating — and the electrical and mechanical robustness engineered into
each controller.
Therefore, to make its use in the multiple-motor branch circuit of
practical, the ½ horsepower Bulletin 294 controller was engineered to be robust
enough to safely contain the damage when protected by a fuse having a rating
of 45 amperes, rather than just 6 amperes.
Rockwell Automation Publication 290E-UM001B-EN-P - June 2012
Figure 58