Disconnect switch short circuit rating

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Grouch1980 said:
From other threads i've been on, if you use current limiting fuses to protect a downstream breaker panel, that combo has to be series rated. dynamic impedance from the breakers will cause the CL fuses to behave differently from what you see on their let-thru charts, so you have no control or idea of how the system as a whole will behave... so that forces a series rating listing needed between the fuses and the downstream breaker panel, since it's something that the manufacturer can study. At least that's what i've read.
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I dont think you have it quite right. Perhaps what people were trying to say, or what you were told but misinterpreted, is that you cannot use CL fuses to reduce the AFC downstream, at least based on manufacturer charts. You are basically stuck with whatever tested series combination there is. For the most part just ignore the concept of a CL fuse. Really the only exception is under engineering supervision, and from what I hear, good luck getting an EE to stamp a non tested series combination.
 
i see.... let me see if have this right. If i have the CL fuses upstream of a breaker, i have 2 options:
1. Have the CL fuses reduce the fault current to protect the downstream breaker, BUT that has to be a series rated combination.
2. Have the CL fuses and the downstream breaker both fully rated, and if there's enough drop in the fault current due to wire impedance, the downstream breaker can still work as a fully rated device if it's AIC rating is high enough... and the upstream CL fuses can be ignored.
 
Another good thing to take note of with series ratings, is the rating of the upstream device has a large effect on whether there is a series rating or not. For example you
Grouch1980 said:
i see.... let me see if have this right. If i have the CL fuses upstream of a breaker, i have 2 options:
1. Have the CL fuses reduce the fault current to protect the downstream breaker, BUT that has to be a series rated combination.
2. Have the CL fuses and the downstream breaker both fully rated, and if there's enough drop in the fault current due to wire impedance, the downstream breaker can still work as a fully rated device if it's AIC rating is high enough... and the upstream CL fuses can be ignored.
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I would say correct. Basically for NEC purposes, it all comes down to what the series ratings are.
 
Grouch1980 said:
Did you mean to write something? 'For example'... ?
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Oh oops, that was a thought I never finished. Its not just the difference in the actual fault current and the series rating current, that will or will not result in series rating, it is the frame size of the upstream device that is more significant. For example, you will easily find series ratings from 100k down to your standard 10k MCCB if the upstream device is 250A or smaller. It is hard to find one for 400A frame upstream devices (IIRC I looked and siemens has none, Sq D maybe had 1 or 2 that would do it.).
 
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