800 amp fuses on 500 KVA transformer

Status
Not open for further replies.
X

xguard

Senior Member
Location
Baton Rouge, LA
I'd like to use 800 amp (time delay) fuses on the primary of a 480:208/120 500 KVA transformer. I'm concerned about nuisance tripping. The Load is 377 KVA

Following the rule I found on the net:
601 amps x 12 = 7,212 amps @ 0.1s
601 amps x 25 = 15,025 amps @0.01s

800/600 = 1.33 (I will have secondary protection)

Several 800 amp time current curves (melting pre-arcing) I've looked at are okay at the 0.1s mark but not the 0.01s.

I'd prefer not to go up to 1000 amps, as this would also push me to a 1000 amp transfer switch (fuses would be ahead of transfer switch), require ground fault protection, bigger wires, etc.

Appreciate any recommendations.
 
Last edited:
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I don't know where you came up with the 1.33 number. The code would allow you to go up to 250% if you have secondary protection.

Personally I think you might well end up with nuisance tripping. Have you considered an electronic circuit breaker that you can adjust the instantaneous trip on?
 
X

xguard

Senior Member
Location
Baton Rouge, LA
petersonra said:
I don't know where you came up with the 1.33 number. The code would allow you to go up to 250% if you have secondary protection.

Personally I think you might well end up with nuisance tripping. Have you considered an electronic circuit breaker that you can adjust the instantaneous trip on?
Click to expand...
I was trying to avoid going up to larger equipment. The loads aren't that high but it seems the transformer size is dictating everything.

1.33 was just the 800 amp fuse divided by the primary current. I wasn't trying to say 1.33 was good or anything, just included for convenience.
 
X

xguard

Senior Member
Location
Baton Rouge, LA
petersonra said:
I don't know where you came up with the 1.33 number. The code would allow you to go up to 250% if you have secondary protection.

Personally I think you might well end up with nuisance tripping. Have you considered an electronic circuit breaker that you can adjust the instantaneous trip on?
Click to expand...
Yeah looks like I need a big one of those if I go that route as well. Is a 1200 amp electronic circuit breaker in an outdoor enclosure a thing?
 
X

xguard

Senior Member
Location
Baton Rouge, LA
I came across this, a lot lower than the other method. Of course they don't list inrush for their 500 kva transformers.

Max inrush for a transformer.

trcsupport.eaton.com trcsupport.eaton.com

Max inrush for a transformer.​

Absolute Max is the inrush value expected if you energize the Transformer when the Voltage sine wave is equal to zero, inrush will occur in the first ½ cycle so there is a very slim chance customers will hit this value.

Practical Max: This is most likely the maximum Inrush current the customers will see when they energize the transformer .

If you compare the current values at time equal to 0.0083 sec. (1/2 cycle) you will conclude that the inrush event will not be seen by the breaker.



For a listing of Eaton's transformers with listed inrush values please see our Catalog Volume 2 Tab 2
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The problem is, you won’t know until you get there, meaning you could install the 800A fuses and discover that the transformer magnetic inrush is too much for them, then you have to replace them with something else.

Magnetic inrush on a transformer by the way has nothing to do with the load on the secondary. It’s only related to the size of the transformer.
 
Status
Not open for further replies.
Top