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  1. #1
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    Transformer Damage Curve

    I have question about the protection and coordination of the transformer. The time current curve of the primary fuse usually passes through the transformer damage curve, then why is it even required that the transformer damage curve should be plotted?

    If the curve of fuse on the primary side of transformer passes through the transformer damage curve, then will the secondary fuse be able to protect the transformer, or should an additional 50 relay be used in series with primary fuse? Thanks for help.

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    Quote Originally Posted by timm333 View Post
    I have question about the protection and coordination of the transformer. The time current curve of the primary fuse usually passes through the transformer damage curve, then why is it even required that the transformer damage curve should be plotted?

    If the curve of fuse on the primary side of transformer passes through the transformer damage curve, then will the secondary fuse be able to protect the transformer, or should an additional 50 relay be used in series with primary fuse? Thanks for help.
    The correct TCC of the primary fuse shouldn’t intersect the damage curve, where are you getting this from?

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    In the SKM application guide at the link: http://www.skm.com/applicationguides22.html

    In the second paragraph under the heading: MV Transformer Fused Switch Feeder Unit:

    It says that "typically the fuse will cross the transformer damage curve".

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    The primary fuse is often intended to remove the transformer from the system after it has failed (i.e. short circuit protection). Overload protection is usually provided the secondary protective device.

    this is why the NEC requires either the primary or the secondary, but not both, to not exceed 125% of the FLA.
    Just because you can, doesn't mean you should.

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    The damage curve is regarding short circuit protection (not overload). So if the primary fuse curve (providing short circuit protection) crosses the damage curve, it would damage the transformer. I am trying to understand why they say it is acceptable for the primary fuse curve to cross the damage curve.

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    Quote Originally Posted by timm333 View Post
    The damage curve is regarding short circuit protection (not overload). So if the primary fuse curve (providing short circuit protection) crosses the damage curve, it would damage the transformer. I am trying to understand why they say it is acceptable for the primary fuse curve to cross the damage curve.
    Looking at the reference, I see what you are now talking about... The primary fuse TCC is intersecting the long-time region and is perfectly acceptable since they are utilizing a secondary protective device to cover the long-time (overload) region on the secondary windings. Note: you don’t get overloads due to the primary windings itself so it’s ok to implement that protection on the load side of the transformer. Both primary and secondary TCC’s are superimposed.


    Sent from my iPhone using Tapatalk

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    Quote Originally Posted by timm333 View Post
    The time current curve of the primary fuse usually passes through the transformer damage curve, then why is it even required that the transformer damage curve should be plotted?
    There is no code requirement to plot the transformer damage curve. It is an industry practice to do it and show that you have selected a fuse or other protective device to do the best you can considering the application.
    Ron

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