Overcurrent protection 4160V soft start

[ckinney02]

I working on a concept here. Say I have a 1000HP 4160V soft start that has a load break disconnecting means THEN, as the manufacture calls them, 'motor protection fuses' immediately downstream. The fuses are not an integral part of the disconnecting means. The disconnect and the fuses are installed in the same gear but in seperate compartments, one above the other. The 1000HP soft start is the only load. This would be feed from an outside oil-filled transformer. I plan to meet either NEC 240.21(C)(3) or (4). Is there any issues with this?

What if I can't meet any conditions in NEC 240.21(C), would the disconnect and fuses of the soft start meet code? I'm trying to research what type of fuses are provided with the soft start unit but I haven't received this information yet. The fuses would be rated lower than the requirements in NEC Table 450.3(A).

 

[Jraef]

I working on a concept here. Say I have a 1000HP 4160V soft start that has a load break disconnecting means THEN, as the manufacture calls them, 'motor protection fuses' immediately downstream. The fuses are not an integral part of the disconnecting means. The disconnect and the fuses are installed in the same gear but in seperate compartments, one above the other. The 1000HP soft start is the only load. This would be feed from an outside oil-filled transformer. I plan to meet either NEC 240.21(C)(3) or (4). Is there any issues with this?

What if I can't meet any conditions in NEC 240.21(C), would the disconnect and fuses of the soft start meet code? I'm trying to research what type of fuses are provided with the soft start unit but I haven't received this information yet. The fuses would be rated lower than the requirements in NEC Table 450.3(A).

1) 240.21 is trumped by the fact that it is a dedicated motor circuit. Read the ENTIRE section, starting from the beginning. 240.3 says that if one of the listed other sections apply, then use those. 430 is one of them.

2) The fact that they are in separate compartments has to do with worker safety (albeit in the days before Arc Flash prevention). The fuses are still part of a "Fused Disconnect" arrangement.

3) For MV motors, the rules are slightly different from LV circuits, they start at 430.221.
To whit:
"Fuses of a suitable type and rating placed in each
ungrounded conductor. Fuses shall be used with suitable
disconnecting means, or they shall be of a type that can
also serve as the disconnecting means. They shall be
arranged so that they cannot be serviced while they are
energized."

4) The type of fuses used is up to the soft starter manufacturer, not you. The fuses must be coordinated, BY THEM, with the withstand capabilities of the devices down stream. When they got UL listing on it as a "Class E-2 Controller" (assuming such), then all of that coordination has been done for you already and you are actually not permitted to make changes in the field without retesting. But if you must know, I can all but guarantee you that they will be "R" rated MV fuses, which are for motor circuits. I don't think you can use the fuses in the motor controller as secondary protection for the transformer, that would be "E" type fuses. But if this transformer doesn't feed anything else and you have primary protection, I believe that will suffice.