Supply side terminology

[don_resqcapt19]

Another question. How does the first ocpd after a transformer protect its supply conductors. Wouldn’t it just protect the conductors downstream of itself?

Just like the service OCPD, it only provides overload protection. It does not provide ground fault and short circuit protection. That is why the length of the transformer secondary conductors is limited and why the service OCPD, if inside, must be "nearest the point of entrance" of the service conductors.
Overload protection can be provided at any point on the circuit, but short circuit and ground fault protection can only be provided at the supply end of the conductors.

 

[Pinnie]

Just like the service OCPD, it only provides overload protection.

That makes sense.

 

[don_resqcapt19]

What happens in the primary if say there’s a bolted short on the secondary?

Either the primary device opens, or the short burns itself clear. With a service, it is rare that the utility primary device opens, and the fault on service conductor typically burns itself clear.
I watched about 25' of four 500 kcmil service conductors in 4" rigid conduit pretty much vaporize over about 15 minutes and the utility primary protection for their 750 kVA transformer never opened.

 

[Pinnie]

Either the primary device opens, or the short burns itself clear.

That seems unsafe

With a service, it is rare that the utility primary device opens, and the fault on service conductor typically burns itself clear.
I watched about 25' of four 500 kcmil service conductors in 4" rigid conduit pretty much vaporize over about 15 minutes and the utility primary protection for their 750 kVA transformer never opened.

in the instance it opens what happens in primary windings to trip their ocpd. Im assuming the current induced trips it. Does a certain system (wye,delta,etc) trip better than others?

 

[kwired]

What happens in the primary if say there’s a bolted short on the secondary?

Impedance of the transformer limits the current compared to what the primary current would be if the fault were on primary conductors.

Is still going to be pretty high current relative to the transformer rating but could be low enough the primary overcurrent device trips on thermal rather than magnetic function which will mean longer duration of the fault event.

 

[Pinnie]

Impedance of the transformer limits the current compared to what the primary current would be if the fault were on primary conductors.

Is still going to be pretty high current relative to the transformer rating but could be low enough the primary overcurrent device trips on thermal rather than magnetic function which will mean longer duration of the fault event.

Gotcha thank you

 

[don_resqcapt19]

That seems unsafe


in the instance it opens what happens in primary windings to trip their ocpd. Im assuming the current induced trips it. Does a certain system (wye,delta,etc) trip better than others?

It is simply the setting of the primary OCPD that the utility selects. Often they select it not to protect the transformer, but just to make sure a transformer fault does not take out the next upstream OCPD in the distribution system.

There was no damage to the transformer. This occurred at a building fire and the conduit and conductors were the service conductors to the building. The transformer was in an underground vault. The arcing and sparking was still occurring when the utility guy got to the scene. He popped the manhole cover, and took one look and saw the secondary conductors were smoking and said I will be back. He did not feel safe entering the underground vault. He went to a main substation and opened the primary circuit that fed that transformer, then came back and cut the cables for the fire building loose from the transformer. He then went back to the substation and re-energized the primary circuit, and that restored power to the other buildings in the area that were supplied by that transformer. That was about 20 years ago, and as far as I know that transformer is still in service.

 

[Pinnie]

It is simply the setting of the primary OCPD that the utility selects. Often they select it not to protect the transformer, but just to make sure a transformer fault does not take out the next upstream OCPD in the distribution system.

There was no damage to the transformer. This occurred at a building fire and the conduit and conductors were the service conductors to the building. The transformer was in an underground vault. The arcing and sparking was still occurring when the utility guy got to the scene. He popped the manhole cover, and took one look and saw the secondary conductors were smoking and said I will be back. He did not feel safe entering the underground vault. He went to a main substation and opened the primary circuit that fed that transformer, then came back and cut the cables for the fire building loose from the transformer. He then went back to the substation and re-energized the primary circuit, and that restored power to the other buildings in the area that were supplied by that transformer. That was about 20 years ago, and as far as I know that transformer is still in service.

That’s a really cool story!