Fuse and Feeder Settings

[mbrooke]

Single feeder substation, 69kv 5/6.25MVA transformer. Outgoing 13.8kv feeder loaded at 303 amps.

Setting the feeder pickup at 200% as done normally results on a grossly oversized primary fuse.

 

[wbdvt]

Is there a question here or is this just for general information?

 

[mbrooke]

Is there a question here or is this just for general information?

The existence of a paradox in the listed information would hint at a question. I guess I'm asking if its ok to have a grossly over sized primary fuse or if I can see the feeder pickup near 303 amps.

 

[ron]

MV often gets most of its protection for short circuits. Overload protection is tough unless you really know the load profile intimately, so it is mostly set high, and the code allows for it.

 

[mbrooke]

MV often gets most of its protection for short circuits. Overload protection is tough unless you really know the load profile intimately, so it is mostly set high, and the code allows for it.

What are distribution feeders typically set at? 200% of the contingency load is what I hear often.

 

[ron]

For digital protective relays, I set 51 settings tighter due to their good accuracy if I know the profile. Closer to 125%.
For older analog protective relays, they are often at 200%.
For fuses protecting feeders and a variety of transformers, E rated fuses are traditionally pigs in the long time (51) region.

 

[mbrooke]

For digital protective relays, I set 51 settings tighter due to their good accuracy if I know the profile. Closer to 125%.
For older analog protective relays, they are often at 200%.
For fuses protecting feeders and a variety of transformers, E rated fuses are traditionally pigs in the long time (51) region.

125% works for me.

E rated fuses can be sized to 100% in that they already start at 220-240%?

 

[mivey]

normal load at 303 amps? you are overloading the transformer. no room for additonal overloading at 200%

 

[mbrooke]

normal load at 303 amps? you are overloading the transformer. no room for additonal overloading at 200%

Briefly. They can handle short time overloads provided enough time to cool down in between. 200% short time is not a need here.