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Thread: Series Rating

  1. #1
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    Series Rating

    So in reviewing some panel interrupting ratings I have come across a situation where the panels are indicated as overduited, however have the ability to series rate to a higher rating with the upstream device [based on manufacturer literature]…All the literature for the manufacture testing and information I have come across indicated a breaker with a piece of wire and the downstream breaker. With that does series ratings only apply between breakers of the same voltage class in series? Can you apply a series rating between a transformer primary 480V CB and Secondary 208V secondary / panel main breaker to series rate a downstream RP?

  2. #2
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    No. But the available fault current on the transformer secondary would not be very high, unless it is a large transformer with a low impedance.
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  3. #3
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    Quote Originally Posted by Mike01 View Post
    So in reviewing some panel interrupting ratings I have come across a situation where the panels are indicated as overduited, however have the ability to series rate to a higher rating with the upstream device [based on manufacturer literature]…All the literature for the manufacture testing and information I have come across indicated a breaker with a piece of wire and the downstream breaker. With that does series ratings only apply between breakers of the same voltage class in series? Can you apply a series rating between a transformer primary 480V CB and Secondary 208V secondary / panel main breaker to series rate a downstream RP?
    Are you asking if you can use series ratings of the 208/240 volt breaker with the rating of the 480 volt breaker on the primary side of the transformer? If so I sort of doubt you would find a 240 volt breaker that has been series tested with a 480 volt breaker in the first place.

  4. #4
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    Quote Originally Posted by kwired View Post
    ... If so I sort of doubt you would find a 240 volt breaker that has been series tested with a 480 volt breaker in the first place.
    I'd bet a paycheck that you will not.

    Mike01,
    Series ratings are not allowed to be calculated, they must be physically tested. In fact you cannot even change the specific catalog number of devices in a series combination within products of the same brand. So in order for a series rating to translate through a transformer, the upstream and down stream breakers would need to have been tested WITH a SPECIFIC transformer. Way way too many combinations to test for that to be even remotely considered. Each test costs around $25k for the manufacturer, that's why they don't do it for each and every product they sell, let alone using competitor's products.

    But as dkidd said, it's a moot point anyway. The secondary of the transformer resets the AFC calculations for that side of the circuit and although based on the AFC on the primary, would not be considered affected by whatever the let-through of the primary breaker would be.
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  5. #5
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    Thanks for all the input I was pretty sure the response I was going to get, I could not find any manufacture listed combinations between voltages so was pretty confident this was a no-go but wanted to throw it out there a manufacturer also confirmed this, to dkidd’s response yes however this particular case includes actual fault current data in the study the problem is the low voltage dry type xfmr was installed in close proximity to the substation this created a compounding issue 1] a low impedance [2.7%z] resulted in a kA level greater than the 10kA and a slightly higher x/r than what the breakers were tested at caused a adjusted fault current increase [breaker kA derating] creating the perfect storm for this specific panel. The follow up to all this is I can get a manufacture listed series combination by changing out the panel main breaker, however the shop drawings list the panel AIC at 10kA so does the series rating cover the bus bracing of the panel as well, or is the series rated only inclusive of the main breaker to branch breakers in which case the bus bracing / withstand is still rated 10kA?

  6. #6
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    notsure of your exact configuration but is it possible to put fuses or breaker in between the txf secondaries and the problem panel? You may be able to find a tested configuration for this.

  7. #7
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    Series Rating

    Quote Originally Posted by Mike01 View Post
    Thanks for all the input I was pretty sure the response I was going to get, I could not find any manufacture listed combinations between voltages so was pretty confident this was a no-go but wanted to throw it out there a manufacturer also confirmed this, to dkidd’s response yes however this particular case includes actual fault current data in the study the problem is the low voltage dry type xfmr was installed in close proximity to the substation this created a compounding issue 1] a low impedance [2.7%z] resulted in a kA level greater than the 10kA and a slightly higher x/r than what the breakers were tested at caused a adjusted fault current increase [breaker kA derating] creating the perfect storm for this specific panel. The follow up to all this is I can get a manufacture listed series combination by changing out the panel main breaker, however the shop drawings list the panel AIC at 10kA so does the series rating cover the bus bracing of the panel as well, or is the series rated only inclusive of the main breaker to branch breakers in which case the bus bracing / withstand is still rated 10kA?
    Bus ratings are typically more than the panel is rated. Manufacturers don't make weaker panels just for 10kA breakers. The panel gets a lower AIC based on the lowest breaker rating.

    Call the manufacturer to check what the bus is good to. Odds are you will be fine replacing the main to series rate. Don't forget to label it.


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