Power Transformer Low Voltage Protection

Quote "Most of the system have been installed by contractors We are finding many Power Transformers that do not have Low Voltage Fuses installed."

Do the conductors not terminate in a panel? Is the not protection in the panel? How long are the secondary conductors?

Quote "Example the low voltage out (220-380) 3 phase of a 250 kva Transformer the secondary is fused with 400 amp block fuses, at least that is the way the power company and we do it."

The full load on the 250 kva is 380 amps. In my opinion the fusing is too close. You could go to 125% of the FLA. Of course it depends on the conductor size.

If you are useing the NEC

I can address the last one...yes. Bonded at the serving transformer (assuming delta/wye, may be assuming too much) and the first disconnect...in this case the MDB (main distribution board?).

Let me ask this.

Is protection required for the secondary side of a power transformer.

If a short happens between the transformer secondary wires and the MDB main breaker, what protects the transformer from damage?

Thanks

Never mind code, er, mind it, but go beyond it, into the area of common sense. EVERY power circuit needs a protection against short. It is a rare, purpose built circuit, that does not require protection against short. So, normally all do. Even at very low voltages, automobile type, very cheap fuses will protect.

Going down on these logical steps, I fail to grasp, why the original questioner even asks the question. Slavishly sticking to what was done before? I would review all, and do the right thing, erring on the safe side, code or not.

Some utilities install secondary fuses on overhead transformer secondaries, but most do not. My utility does not use them. We've never found enough of a benefit to justify that cost. Over the last 20+ years I can't remember a single time we replaced secondary wiring due to overcurrent.

You should note that if the transformer and service drop are owned by the electric utility (power company), the NEC does not apply - see article 90-2 (b)(5). If the equipment is not owned by the utility, fusing at the transformer secondary is still not required if you meet the requirements of article 240.21(C) or 240.21(D).

And as cuba_pete has already stated, the neutral and ground must be bonded at both the transformer secondary and the service entrance equipment (MDB).

Quote "Do the conductors not terminate in a panel? Is the not protection in the panel? How long are the secondary conductors?

Quote "Example the low voltage out (220-380) 3 phase of a 250 kva Transformer the secondary is fused with 400 amp block fuses, at least that is the way the power company and we do it."

The full load on the 250 kva is 380 amps. In my opinion the fusing is too close. You could go to 125% of the FLA. Of course it depends on the conductor size.

If you are using the NEC" go to 240.21. I left this off my post. It covers transformer secondary overcurrent protection.