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deltawhy (Electrical) (OP)
8 Feb 12 12:30
Hi there,
I have a couple of questions regarding available fault currents.  I have a 208Y/120V 1200A distribution with an available fault current of ~30kA.  I thus had planned to used current limiting fuses to knock down the available current so all my downstream equipment could be much cheaper.  Unfortunately, by the looks of it, it won't save much money.
 
My first question is:  I am looking at a bussmann T-Tron fuse current limiting curve.  With a 1200A fuse, it knocks the available ASYMMETRICAL peak down to 30kA (without it, could be up to 70kA).  Now, do current limiting fuses also knock down the symmetrical rms current as well?

Second question:  Do you typically rate gear to the available symmetrical current or the average asymmetrical current (~2.3xsymm)?  I can't find it in the code (CEC).

Thanks for the help.
resqcapt19 (Electrical)
8 Feb 12 13:12
You can't use fuses to permit the use of downstream equipment with a lower short circuit rating unless you are using series rated sysem.
dpc (Electrical)
8 Feb 12 13:46
There are a lot of misconception about current-limiting fuses.  The only way that they limit fault current is by melting within the first 1/2 cycle of the fault before the first peak of the fault current.  If you look at a fuse curve and the fuse is taking longer than 1/2 cycle to clear, it is not doing any current-limiting.

Further to what resqcapt19 said, the old "up-over-down" method of determining max fault current downstream of a current-limiting fuse has been rejected by the NEC for many years.  You have to apply a listed series combination of fuse and breaker that has been test by the breaker manufacturer.  

I assume the CEC is similar, but really don't know.   

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