edit: I didn't see iwires post 10 before I posted this one. But this answer still fits.
gmtt said:
...My question is, if there is a short circuit between the transformer secondary point to the disconnect, how it will be protected? Shouldn't a fused disconnect was warranted within close proximity to the transformer secondary?
A lot of the stuff I work with has the xfm secondary protection away from the xfm - maybe even 500 (+) feet.
Regularly someone will question what protects the secondry conductors between the xfm and the secondary OCP.
The Answerer: "Protect from what?"
Questioner: Well, overload.
A: The secondary OCP. What ever current is going in to the xfm end on the secondary conductors is coming out the OCP end.
Q: What protects the conductors from a heavy current draw between the xfm and the secondary OCP? (
Yep - someone actually asked that)
A; Ahhh - like a backhoe attack? Well, the secondary conductors are mostly protected by concrete and steel. After the backhoe attack there aren't any conductors to protect - they are toast. The issue now is to put out the fire with minimal damage. So the next protective device in line is the xfm primary protection. That should open to prevent the xfm from turning into a fireball.
Q: But the utility can set the primary way high.
A: Yes they sure can. And if the primary OCP doesn't open on a xfm secondary conductor fault, it generally will open when the xfm internally faults. And if the primary OCP is still set too high, then eventually the distribution line will fault and go down.
cf