Breaker or Fuse for Transformer Secondary

[minesh21]

Hi,

I have a design where we need to provide a 12kV-4160V transformer. On the secondary of the transformer, since it is still HV, I'm assuming a fuse would be the cheapest form of overcurrent protection. Is there any reason to opt for a breaker? I'm guessing you wouldn't put a breaker because then you would need to install a relay to provide a protective relay with CT's and PT's which can get expensive. Any thoughts?

 

[jim dungar]

... I'm assuming a fuse would be the cheapest form of overcurrent protection. Is there any reason to opt for a breaker? I'm guessing you wouldn't put a breaker because then you would need to install a relay to provide a protective relay with CT's and PT's which can get expensive. Any thoughts?

Are you trying to minimize damage to the primary system caused by faults on the secondary? Then you could go with a fuse.
Are you trying to minimize damage to the transformer caused by faults on the secondary? Then you could go with a breaker.
Do you want to be to operate the protective device remotely or automatically? Then you should go with a breaker.

 

[minesh21]

Are you trying to minimize damage to the primary system caused by faults on the secondary? Then you could go with a fuse.
Are you trying to minimize damage to the transformer caused by faults on the secondary? Then you could go with a breaker.
Do you want to be to operate the protective device remotely or automatically? Then you should go with a breaker.

Can you please elaborate on how your statements above are achieved by a breaker versus fuse? I always thought the primary and secondary were isolated systems, so a fault on the secondary won't affect the primary.

Wouldn't a fuse protect the secondary of a transformer similar to a breaker? It's allowed by the NEC in 450, so I thought it would still provide protection.

And fuses operate automatically just like breakers, although you would have to replace them if they do...

 

[jim dungar]

Can you please elaborate on how your statements above are achieved by a breaker versus fuse? I always thought the primary and secondary were isolated systems, so a fault on the secondary won't affect the primary.

The fault current, on the secondary, has to come from the primary.

Wouldn't a fuse protect the secondary of a transformer similar to a breaker? It's allowed by the NEC in 450, so I thought it would still provide protection.

Yes fuses are allowed. Article 450 is not really about protecting the secondary side of the transformer, it's real focus is in how to size the primary side protective device.
I was just discussing a system where the secondary protective device was sized based on the max fan cooled rating of the transformer (133% overload capacity). So how is the transformer being protected if the fans aren't working correctly?

And fuses operate automatically just like breakers, although you would have to replace them if they do...

It is up to you to decide what protection you want.

 

[minesh21]

The fault current, on the secondary, has to come from the primary.

Yes fuses are allowed. Article 450 is not really about protecting the secondary side of the transformer, it's real focus is in how to size the primary side protective device.
I was just discussing a system where the secondary protective device was sized based on the max fan cooled rating of the transformer (133% overload capacity). So how is the transformer being protected if the fans aren't working correctly?



It is up to you to decide what protection you want.

How would a breaker on the secondary protect the primary better than a fuse? The primary size of the transformer has an existing fuse that we are using, so are you saying a secondary breaker would trip faster than a secondary fuse? Or are you saying the secondary breaker will provide better coordination?

 

[DrSparks]

How would a breaker on the secondary protect the primary better than a fuse? The primary size of the transformer has an existing fuse that we are using, so are you saying a secondary breaker would trip faster than a secondary fuse? Or are you saying the secondary breaker will provide better coordination?

Time delay fuses have similar trip curves as inverse time circuit breakers. However circuit breakers cannot perform like fast acting or current limiting fuses

Sent from my A574BL using Tapatalk

 

[minesh21]

Yes fuses are allowed. Article 450 is not really about protecting the secondary side of the transformer, it's real focus is in how to size the primary side protective device.
I was just discussing a system where the secondary protective device was sized based on the max fan cooled rating of the transformer (133% overload capacity). So how is the transformer being protected if the fans aren't working correctly?

I had a similar situation with a 4160-480/277V transformer where Square D said the fans allow it to run continuously in FA mode. So we sized the primary based on the FA rating not the AA. Square D recommends 133% x the FLA, so that is what we did.

 

[jim dungar]

It makes no difference to me what you choose. I dod not have enough information to make a recommendation.

What are you trying to accomplish? Make a list of your pros and cons.

If lowest acquisition cost, to meet NEC 450, is your goal, then you probably need to go with fuses.