ATS SCCR etc

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Isaiah

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Is there any difference between an ATS and a ‘normal’ disconnect switch in terms of SCCR, amps city rating etc? This will be a closed transition auto transfer switch between a generator and service panel for a small building


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I think that would certainly depend on the specifics and your definition of a "normal switch".
 
SCCR ratings are not dependent on the type of switch. Put simply, equipment must be rated for the amount of fault current available at its line side, or source, terminals.
For a transfer device you need to consider the amount available from each source.
 
jim dungar said:
SCCR ratings are not dependent on the type of switch. Put simply, equipment must be rated for the amount of fault current available at its line side, or source, terminals.
For a transfer device you need to consider the amount available from each source.
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Thanks Jim. Since only one source can be in service at a given time, would the ATS need to be rated for the combined available fault currents or just the larger of the two?


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Isaiah said:
Thanks Jim. Since only one source can be in service at a given time, would the ATS need to be rated for the combined available fault currents or just the larger of the two?


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From each source individually.
 
For a closed transition ATS, you add the two sources.
ATS's have SCCR's generally higher than a normal disconnect switch, but they are specific to the manufacturer, and need to be checked with their literature.
 
electrofelon said:
Ok , right for closed transition, as Ron said that makes sense from a theoretical standpoint, but it does seem like an astronomically small chance of having a bolted fault right during that
brief transition period.
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Very true, which is why many short circuit studies contain statements saying very short closed transition periods were not considered. A two position ATS might be evaluated for the worst single source, however a transfer scheme of two breakers is usually considered as being more likely to have a long closed transition period, especially if it has an option for manual operation.
 
An open transition UL1008 ATS ensures that the 2 sources are not connected together.
A closed transition ATS typically has the 2 sources paralleled for 100ms or less, but there is no guarantee that during a malfunction that the once both contactors (or breakers) are closed, that one of them will open.
So from a functional standpoint, those that have a disclaimer stating that the study only includes one source for a closed transition application, are assuming the equipment operates correctly, as we do thinking a breaker will open when it is told to open, but a solid state ATS controller is not a trip unit, so I add the sources together.
 
electrofelon said:
Ok , right for closed transition, as Ron said that makes sense from a theoretical standpoint, but it does seem like an astronomically small chance of having a bolted fault right during that
brief transition period.
Click to expand...
Although I might have a hard time finding a specific example, I think its possible that a fault could cause a transfer, or vice versa. So I think the chance is a little higher than most people would assume.
 
steve66 said:
Although I might have a hard time finding a specific example, I think its possible that a fault could cause a transfer, or vice versa. So I think the chance is a little higher than most people would assume.
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Don't forget it would have to be a bolted fault, as that is what the SCCR is based on.
It is pretty hard to have a bolted fault on a system that is up and running.
 
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