Fuse or circuit breaker for transformer prot 5

[capuchi]

we have a new 175 KVA, 460/230volts, 3ph dry type transformer. following NEC art. 450 the primary side protection would be 600Amps while the secondary side would be 500Amps. Am I correct in my interpretation of art. 450? Also, would you rather recommend a fuse than a CB. Can someone pls advise? thanks

capuchi

 

[dpc]

The NEC gives you MAXIMUM values for overcurrent protection of transformers. You are free to use lower values to provide better protection.

At this voltage and size, the only reason to use a fuse instead of a molded case breaker would be cost.

 

[jraef]

...the only reason to use a fuse instead of a molded case breaker would be cost.

While I personally share your apparent bias towards circuit breakers, the "Devil's Advocate" in me wants to point out that there are several valid and compelling arguments to be made for fuses besides cost. Interrupting capacity and current limiting being the chief of them. With all the new Arc Flash considerations now, current limiting can play a big part in keeping the PPE and boundary issues under control.

Other than that, I completely agree that the NEC should not be used as a primary engineering tool, but rather as a secondary check and adjustment against what is permissible, AFTER you determine the right way to do it from an engineering standpoint.

Now, all that said capuci, you have apparently made some sort of error in either your calculations or your typing. The primary side protection, in a scheme where you have both sides protected, should be 250% MAX. of the primary current. On that 175kVA transformer it will be around 210A, so the breaker should be NO MORE THAN 500A. Notice that when considering the primary protection, Note 1 that allows for up-sizing to the next available non-adjustable trip rating does NOT apply, so you stop at the next lowest, in this case 500A, not 600A.

On the secondary side, the breaker should be 125% MAX of the secondary current, which in this case will be 439A, so the breaker should be no more than 549A; but based on Note 1, COULD be as high as 600A.

 

[davidbeach]

Current limiting fuses are great for arc-flash mitigation when they operate in their current limiting region. However there may not be that much current and you don't have to get very far below the current limiting point for the fuse time to get very long for a very small reduction in current. Fuses and transformers of this size don't often produce good results in an arc-flash study.

 

[alehman]

Fuses are great for short circuit protection, but miserable for overload and low current faults.

 

[alehman]

If the primary overcurrent protection is less than about 130% of full load, you are likely to have problems with inrush operating the overcurrent.

 

[jraef]

However there may not be that much current and you don't have to get very far below the current limiting point for the fuse time to get very long for a very small reduction in current. Fuses and transformers of this size don't often produce good results in an arc-flash study.

A very good point and one often overlooked in studies. I brought up fuses only because there are some cases where they may make sense over breakers, i.e. in an application where the available fault is 200kA as an example. Breaker technology, even current limiting, has a hard time operating in that arena.

 

[Rabalac]

Is it possible that even after adhering to to NEC 450.3(b) rule the primary breaker could still trip from inrush current?

Also how can I be sure that the circuit breaker selected can handle the inruch current?

 

[tem1234]

You have to look for the TCC curve of the circuit breaker and be sure that it will let the inrush pass. Often it the instaneous trip that you most set over the inrush. IEEE talk about 12 times the nominal current of the transformer for 0.1 second, but in most case, it's less than that.

 

[Rabalac]

tem1234,

thanks. Can you direct me to the IEEE document that specifies 12 times the nominal current of the transformer.

 

[alehman]

IEEE Red Book, standard 141, "RECOMMENDED PRACTICE FOR ELECTRIC POWER DISTRIBUTION FOR INDUSTRIAL PLANTS". It's a recommendation, not a hard specification.