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 Post subject: "No load" break cutouts
PostPosted: Sat May 12, 2012 9:23 am 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
We all know that you should never, ever open a cutout under load unless it is designed with load breaking capabilities or using a portable load break device on a hot stick, but just how much current is "no load"? It is common practice for instance to ignore the core loss "load" of smaller transformers but I'm interested in exactly what the cutoff is. In our operation it appears that at 25 kV, somewhere between 1 MVA and 2.5 MVA, we opt for installing load break switches. Below that level, we never bother putting them in. Any thoughts or practical experience on the cutoff?


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PostPosted: Tue May 15, 2012 10:13 am 
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Even MV no-load break switches will have an interuption rating on them, usually small, 100A or so. I don't consider core loss current "load".


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PostPosted: Sat May 26, 2012 8:54 pm 
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Joined: Sun May 13, 2012 5:00 pm
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Location: Amarillo, Texas
Zog wrote:
Even MV no-load break switches will have an interuption rating on them, usually small, 100A or so. I don't consider core loss current "load".

I've been in the Utility business for over forty years. My experience and the practice of the company I worked for those many years was to consider any "core loss", or line charging for that matter, greater than 5 amps should require some form of load break device.

Keep in mind when you are dealing with core loss or line charging current you are approaching a PF angle of 90 degrees - leading for line charging and lagging for core loss. Either way the recovery voltage when breaking those currents is very nearly at peak value.

That said, most pole line transformers can easily be dropped with a standard cutout. Also note that I am referring to open air cutouts, not metal enclosed equipment. That's another area - one I am going to have to become very familiar with in a very short time.


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