cutout and fuse selection

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alberto

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hello good afternoon, i hope some of you can help me because i do not what cut out i have to choose.

it is a transformer of 1500 kva, 3 ph, 13200 volts primary, 480/277 volts secondary, %Z=5.75

the nominal current of primary side is 65.6 amperes; i calculated the fuse size as follow:

65.6 x 2.4 = 157.44 amperes, so i select a 150 amperes fuse, 2.4 is a rule of thumb that i read in a squared information, is this calculation correct for sizing a fuse?

Another question is what cutout do i have to select? a cutout of 100 amperes, 15 kv, because the nominal current of transformer is 65.6 amp. or a cutout of 200 amperes, 27 kv because the size of the fuse is 150 amperes.

i am alberto santiago from tijuana baja california, mexico.
for your help thanks

Edited out caps so the post can be read easily. Charlie

[ November 18, 2004, 02:29 PM: Message edited by: alberto ]
 
Re: cutout and fuse selection

Alberto: Please turn off the ALL CAPS feature on your keyboard. It makes your post more difficult to read, and is considered by some to be like you are SHOUTING at us.
 
Re: cutout and fuse selection

Alberto

There are many different types of HS fuses, T links, K links, etc. You need to determine which one you want to use first. Next notice that many HV fuses used in cutouts blow at 2X rated current, a 50 amp fuse begins to operate at 100 amps.

The best approach is to determine a threshold of damage curve for your transformer and plot it. Next overlay your fuse curve of choice until you find one that provides the best protection, and use it. A rule of thumb that used to be used for 15 kv 3 phase units is kva/15 = ampere fuse. In your case 1500/15 = 100 amp fuse. A better approach is to plot out your curves and determine things accurately.

Next, a 100 amp cutout won't accept a 200 amp fuse, if you decide on a fuse greater than 100 amps you'll need a 200 amp cutout. Finally the voltage rating. If you have a grounded wye system, a 15 kv cutout should be fine. If you have an ungrounded system, you might want to consider a 25 kv class fuse and cutout.
 
Re: cutout and fuse selection

Estimado Alberto: since your installation is in Mexico, you must conform to the Mexican Code NOM-001, which is part 1996 and 1999 NEC. Article 450, Table 450-3(a) provides the overcurrent protection requirements for transformers rated greater than 600 volts. In your case, assuming that the installation is not "supervised", the fuse in the primary cannot be larger than 300 percent of primary rated current. primary current = 65.7A. 3 x 65.7A = 197 amps. Note 1 allows you to go up to the next standard size which is 200 amps maximum. The overcurrent protection in the secondary cannot exceed 125 percent of secondary current. Seconday current = 1806A. 1.25 x 1806 = 2258 amps. Note 1 allows you to go up to the next higher standard size which is 2500 amps. You can always install overcurrent protection that is lower than the maximum.
Atentamente,
John M. Caloggero
 
Re: cutout and fuse selection

Lets say you have a plot of the transformer damage curve and 3 fuse curves.

Would the fuse selection be based on the value at the bottom of the graph at basically zero seconds??

You would want the line to be below the allowable primary current?

thanks
 
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