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ATS Short-Circuit Ratings
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Author:  bunkst [ Fri Jan 16, 2026 1:33 pm ]
Post subject:  ATS Short-Circuit Ratings
Hello all!

I'm a new forum member, but I've lurked for a long time and I've taken a few of Jim's classes, so I'm not entirely a stranger.

I have some questions about ATS short-circuit ratings that I can't find clear answers to anywhere else. But there are some pretty smart people around here, so I have high hopes.

I do have a particular situation I need a solution to, but I've phrased my questions broadly to try to get a complete answer. Here goes:

This is my understanding: If we are using the any-breaker time-based rating of an ATS, I have understood that both the fault current and the clearing time must be below the values in the WCR table. So, for an ATS that has a time-based rating of 42kA and 0.05 seconds, the fault must be below 42kA and it must clear within 0.05 seconds. Is that understanding accurate?

If that is accurate, then a follow-up: an ATS may have short-time ratings where it can withstand a lower fault for a longer duration. For example, an ATS may have a short-time rating of 7500A for 0.2 seconds. Same deal: we must show that both the fault current and the clearing time is less than the rated values. Is that understanding accurate?

Assuming I'm understanding that accurately, here are my questions that I can’t find clear answers to:
  1. Suppose you have an ATS with a short-time rating of 7500A for 0.2 seconds. Now suppose you have a fault of 7,000A. That is a lower fault current than 7500A, so we're good there. I assume that must also clear within 0.2 seconds?
  2. If the answer to #1 is ‘yes,’ then let’s take it further: Suppose we have a fault current of 2,500A. Let’s say it’s protected by a 400A breaker and it's a 400A ATS. Now we’re much lower than 7,500A, but it’s still a fault. Must that fault also clear within 0.2 seconds?
  3. If the answer to #2 is ‘no,’ then at what current between 400A (the ATS’s long-time rating) and 7500A (the ATS’s short-time rating) do we not need to worry about the 0.2 second limit?
  4. If the answer to #2 is ‘yes,’ then a similar question: how do we apply time-based ratings across the full range of a system’s possible fault current? Over any breaker’s time-current curve, there will always be a region where the time it takes to clear a fault will exceed the time-based ATS ratings. When we do fault current analysis, a system may have (hypothetically) a maximum theoretical fault current of 15,000 amps. That, though is a maximum, not an ‘always.’ For any given fault, actual current will be lower than the maximum. And at some point the fault current will be low enough to require longer than the ATS’s time-based rating.
    1. I understand, conceptually, that this is why there exist damage curves for things like cables and transformers, because thermal damage is really a function of I2t. But I’ve never seen a damage curve for an ATS.
  5. Does providing current-limiting fuse protection (from the ATSs WCR fuse tables) avoid all these questions in all cases, regardless of actual fault current? (Assuming the fuse size and max fault current is within table limits.)
  6. Does providing circuit breaker protection from the ATS’s specific breaker tables avoid all these questions in all cases, regardless of actual fault current or the breaker’s trip settings? (Assuming the breaker size and max fault current is within table limits.)
    1. For example, if the Square-D LJ circuit breaker is listed in an ATS’s specific breaker table, is the ATS always adequately protected with an LJ breaker, even if you set the breaker’s instantaneous and short-time trip settings to their maximum values (and instantaneous is not disabled)?

I really appreciate y'all's time and help!
Author:  bbaumer [ Fri Jan 16, 2026 3:05 pm ]
Post subject:  Re: ATS Short-Circuit Ratings
This document may help:

WCR-ATS_Pub-1128-R37.pdf https://share.google/MJ0V0xPCOyonBvSev
Author:  bunkst [ Mon Jan 19, 2026 6:31 am ]
Post subject:  Re: ATS Short-Circuit Ratings
Thank you for your reply! I have reviewed that document, as well as a few other manufacturer white papers and they do not answer the specific questions I posed.
Author:  JBD [ Tue Jan 20, 2026 2:44 pm ]
Post subject:  Re: ATS Short-Circuit Ratings
I believe all of the the WCR is based on short circuits, which means they occur in the instantaneous region of the OCPD.

The use of specific protection devices, both fuses and breakers does mean you do not need to do any further analysis. The ATS is protected by these tested devices.
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