Paper Mill Fault Current Expectations

Status
Not open for further replies.
Tip DS

Tip DS

I'm here.
Location
The Great Meme State
Occupation
Electrical Engineer
Trying to spec out the OCPD for the PRIMARY side of a transformer in a paper mill. It's a small 30kVA 575-480 3-phase transformer. My understanding is that garden variety paper mills have pretty substantial short circuit ratings at some points in their system. Anybody have a rule of thumb regarding what sort of fault current should be planed for at a 575VAC feed in a paper mill? I'm sure it's >22kA, but... 250kA, 65kA, what? I've asked for the fault current at the installation location, but I'm getting my information from a salesman who's dealing with a supply chain manager, who eventually gets their wish list from the plant engineer or electrician. That makes the supply of solid information pretty weak. I'd like to adequately protect the equipment, but I don't want to unnecessarily give anyone sticker shock, either. Any help will be much appreciated.
 
30kva ??
typo ? Thats a small transformer
For 30kva I would be surprised if it was even close to 10k....
 
Start with full load current. Take 30,000 VA, divide by the rated voltage of 575 volts, then divide again by the square root of 3 (about 1.732). That gives you 30.1 amps. Next divide by the transformer's percent impedance. I generally use 5%, if I don't have the actual value. 30.1 ÷ 0.05 = 602.4 amps. As Gus predicted, it is well below 10KA.

Using the same steps on the secondary side gives a max fault current of 721.7 amps.
 
charlie b said:
Start with full load current. Take 30,000 VA, divide by the rated voltage of 575 volts, then divide again by the square root of 3 (about 1.732). That gives you 30.1 amps.
Click to expand...
This is all you can do on the primary side. The impedance of the transformer does not come into play when determining the primary side available fault current.
charlie b said:
Using the same steps on the secondary side gives a max fault current of 721.7 amps.
Click to expand...
This is the highest possible fault current that transformer would be able to supply, regardless of the amount available on the primary.
 
Tip DS said:
Trying to spec out the OCPD for the PRIMARY side of a transformer in a paper mill. It's a small 30kVA 575-480 3-phase transformer. My understanding is that garden variety paper mills have pretty substantial short circuit ratings at some points in their system. Anybody have a rule of thumb regarding what sort of fault current should be planed for at a 575VAC feed in a paper mill? I'm sure it's >22kA, but... 250kA, 65kA, what? I've asked for the fault current at the installation location, but I'm getting my information from a salesman who's dealing with a supply chain manager, who eventually gets their wish list from the plant engineer or electrician. That makes the supply of solid information pretty weak. I'd like to adequately protect the equipment, but I don't want to unnecessarily give anyone sticker shock, either. Any help will be much appreciated.
Click to expand...
Basically there are no valid rules of thumb, you would have to do some investigating. Anything else is just a guess.
 
electrofelon said:
Basically there are no valid rules of thumb, you would have to do some investigating. Anything else is just a guess.
Click to expand...
Yeah, that's what I was afraid of. I was HOPING there.might be some 'typical' range, the way residential is typically covered by a 22kAic rating. Oh well, I guess I will just have to be persistent in asking.for the value from the mill.
 
Tip DS said:
Yeah, that's what I was afraid of. I was HOPING there.might be some 'typical' range, the way residential is typically covered by a 22kAic rating. Oh well, I guess I will just have to be persistent in asking.for the value from the mill.
Click to expand...
Yep, if you want primary side available fault values, they have to come from the paper mill. The typical range is <100kA.
Honestly it all depends on the source and the feeding conductors impedance, both of which can be high or low. Imagine a source of 100kA and a conductor of #8 and 800'.
 
electrofelon said:
What about just using a fused safety switch which is going to have a really high AIC rating anyway, and just skipping the investigations? Even your typical rk5 fuse has an AIC of like 200k IIRC.
Click to expand...
Yeah, that's what I ended up specifying; some class J fuses @ 125% rated current for the primary side only. Not my favorite configuration by far, but... it's a safe bet & it meets code. 🤷‍♂️
 
Status
Not open for further replies.
Top