SDS Ground Fault

[A/A Fuel GTX]

Since the primary and secondary of a transformer have no physical contact, would a ground fault or a phase to phase short in the secondary before the secondary OCPD still cause the primary OCPD to trip?

 

[zog]

Since the primary and secondary of a transformer have no physical contact, would a ground fault or a phase to phase short in the secondary before the secondary OCPD still cause the primary OCPD to trip?

What else would? The primary sees that fault as a big load and the OCPD is sized to protect the transformer from a fault like that, the OCPD on the secondary wont see anything.

 

[A/A Fuel GTX]

I agree but sometimes I need affirmation to keep myself in check. Now, could the same thing be said for service entrance conductors before the first disconnect? Would a fault there take out the POCO's OCPD????

 

[peter d]

I agree but sometimes I need affirmation to keep myself in check. Now, could the same thing be said for service entrance conductors before the first disconnect? Would a fault there take out the POCO's OCPD????

Not very likely. The poco primary fuse is not likely to open at all with a secondary fault, though I have heard of it happening. I don't think they size their fuses to open at the slightest amount of fault current to prevent interupptions of service.

 

[zog]

I agree but sometimes I need affirmation to keep myself in check. Now, could the same thing be said for service entrance conductors before the first disconnect? Would a fault there take out the POCO's OCPD????

Maybe the fuse on the pole for the transformer, maybe, and if it does than it wont be quick.

 

[A/A Fuel GTX]

Maybe the fuse on the pole for the transformer, maybe, and if it does than it wont be quick.

So on a fault involving service entrance conductors, it's hot molten CU or AL spewing all over the place.

 

[don_resqcapt19]

So on a fault involving service entrance conductors, it's hot molten CU or AL spewing all over the place.

Yes, those conductors are only protected from overloads, not from short circuits or ground faults. This is one of the reasons why the code requires that the service disconnect, if inside the building, be located "nearest the point of entrance of the service conductors". The utility primary fuses are most often sized to protect the distribution system, not the transformer or its secondary conductors.

 

[A/A Fuel GTX]

Yes, those conductors are only protected from overloads, not from short circuits or ground faults. This is one of the reasons why the code requires that the service disconnect, if inside the building, be located "nearest the point of entrance of the service conductors". The utility primary fuses are most often sized to protect the distribution system, not the transformer or its secondary conductors.

Don......How are they protected from overloads?

 

[don_resqcapt19]

Don......How are they protected from overloads?

By the OCPD in the service equipment....you can protect conductors from overloads by installing the OCPD at the load end of the conductors.

 

[A/A Fuel GTX]

By the OCPD in the service equipment....you can protect conductors from overloads by installing the OCPD at the load end of the conductors.

Ok but I was referring to the supply side of the service entrance conductors before any OCPD intervention on the load side.

 

[mikeames]

There not protected from overload. They are in free air, or underground. If they burn they burn. The utility sizes them and are most always undersized with respect to the service. They are generally sized based on what the utility believes will be the load not what is calculated for the service.

 

[don_resqcapt19]

Ok but I was referring to the supply side of the service entrance conductors before any OCPD intervention on the load side.

The supply side service conductors are protected from over load by the service OCPD at the load end of the conductors. All of the loads are on the load side of the OCPD so the service OCPD does protect the line side conductors from overload.

 

[brian john]

And this all depends on the coordination of the OCPs. I have seen faults take out the main, utility on the pole and at a substation up the street.

 

[rabtrfld]

The power company is more willing to let things burn than to interrupt service! On the other hand NFPA would have a cow if your toaster burned a pop-tart without blowing some safety device.

 

[davidr43229]

SDS Ground Fault

The power company is more willing to let things burn than to interrupt service! On the other hand NFPA would have a cow if your toaster burned a pop-tart without blowing some safety device.

Several cities in the US have a Vault, which consists of 15 + Transformers underground to supply power to the buildings in that town, and capable of developing 200,000 amps + of Available Fault Current. No building in my city has it's own transformer. It simply takes up too much Sq Ft.
To that point, every once in awhile a fault or overload develops. When this event happends, it propells a 200lbs manhole cover upwards in the air. The record for height here is 25 stories. Now there is a weld-spec for those manhole covers to prevent damage to personal or property (cars).
Just my $.02

 

[A/A Fuel GTX]

The supply side service conductors are protected from over load by the service OCPD at the load end of the conductors. All of the loads are on the load side of the OCPD so the service OCPD does protect the line side conductors from overload.

Thanks Don, I get it. I just had a brain freeze trying to rationalize an OCPD protecting the line side conductors coming into it......