This is often the case with MV starters. You do not want the contactor to interrupt current for which it is not rated for so you disable instantaneous and let the fuse take over. Note: The momentary rating of the contactor can still handle the short-circuit, it just cant interrupt it without failing. This is the same for series rated systems. The definition for Overcurrent Protective Device, Branch-Circuit says this range is between its rated current and interrupting rating (not necessarily available short-circuit current).
110.9, 240.86
The definitions for Overcurrent, Overload, and Ground-Fault Protection of Equipment do a very rough job in describing the different fault characteristics (relative magnitude, duration, failure mode). Even though its not enforceable, the informational note to Overcurrent reinforces, "the rules for overcurrent protection are specific for particular situations.". The handbook commentaries provide additional insight as well.
I'm assuming you deleted post #43 because you realized st, lt, or I couldn't be used to protect any and all three of those problems... For motor applications specifically, its not so much that its impractical, but 430.32(A)(1)/(B)(1), 430.42(D) and 430.51(C)(3) prohibit using only an instantaneous element.
you could have a motor built with higher temp insulation, bearings, cooling provisions, larger circuit equipment and use instantaneous, usually not practicle
Can you elaborate on instances where the NEC requires overcurrent protection but doesn’t require the devices to be listed? If one exists, then looking at the listing standards wouldn't help much.
I don't know of any, I didn't mean required ocpd, just things in general
I would maintain no not always. Also, LT can take a many seconds to trip for a short-circuit even though the pick-up setting is where the NEC requires... I doubt just because it will eventually trip you can get away with calling it short-circuit protection.
listing standard
If you use LT, ST or I elements to detect ground-faults on HRG or ungrounded systems, the breaker would fail to open, not because the protection device was oversized, but because LT, ST, and I are incorrect ways to detect that type of fault. Similarly, for instances where the maximum available short-circuit current is near or less than the load current (as is the case with long distribution lines and high impedance faults), you wouldn't say the protective device is oversized because it didn't trip.by oversized I mean the pickups are above the desired maximum ground fault level
None in particular.
Lets stick to NEC.
No it's only one random example.
Without getting into the definition of short-circuit (because the NEC is silent on that one) I think we can all agree a short-circuit is simply an unintentional connection. You don't actually need thousands of amperes to qualify it. Again, if a breaker fails to open in your example, it may have nothing to do with the protection device being oversized.
if its below required overload protection level then it's common sense ocpd wouldn't be required to trip for "short circuit protection", even the best relays can't pickup on all of what you are describing. and I point again to 250.4(a)(5) and (b)(4). I will check 11' if anything has changed
What about ground-faults in direct-buried cable, or faults conducted directly to earth? I'm not seeing the Effective Ground-Fault Current Path there.
correct and the typical requirement would be normal overload protection current level
2011
