Why ground [earth]

[tryinghard]

I have redrawn this scenario with the utility grounding jumper.http://home.comcast.net/~ronaldrc/wsb/High_voltage_illistration.htm

Chances are of this ever happening are very slim.
But never the less it could happen...

Stuff does happen

Does the utility company ground their primary side? I know we ground their secondary side on pad mounts, or at least I assume so because the electrode conductor enters in through their secondary hole.

 

[ronaldrc]

Yes they do on single phase 240/120 volt homes and Farms.

Thats what the Worse case High voltage scenario is about. They drive a ground at the pole and connect the high and low voltage together at the transformer.

 

[tryinghard]

Yes they do on single phase 240/120 volt homes and Farms.

Thats what the Worse case High voltage scenario is about. They drive a ground at the pole and connect the high and low voltage together at the transformer.

So these

http://home.comcast.net/~ronaldrc/wsb/High_voltage_illistration.htm

Chances are of this ever happening are very slim.
...

are premises wiring systems that are NOT separately derived systems. Wow I did not know they did this!

One thing for sure the secondary voltage is going to drop substantially by losing the primary neutral, this will have a 'brown out' low voltage result which should blow transformer fuses.

I notice your scenario has lost the premises ground as well but if it didn't I suppose this premises ground would try to take the whole unbalanced load and it would also cause a brown out.

The utilities really do create a parallel return path between their neutral and earth, yet in our world of premises wiring we have to bond either at the transformer or the first disconnect but not both.

 

[don_resqcapt19]

Trying,

One thing for sure the secondary voltage is going to drop substantially by losing the primary neutral, this will have a 'brown out' low voltage result which should blow transformer fuses.

It is very unlikely that a brown out would cause the transformer primary fuses to blow. The utility does not protect the transformers like that, the real purpose of the fuses on the primary side of their transformers is to prevent a transformer fault from taking out the primary distribution circuit. They are not sized to protect the transformer or the secondary conductors.
Don

 

[tbro723]

Refer to IEEE Std 142 "Grounding of Industrial and Commercial Power Systems"

The main reason we ground systems is because in ungrounded systems there is the possibility of huge transient overvoltages in the fault is an arcing fault. Overvoltages are dangerous because they can cause insulation failure which can lead to exposed conductors but can also lead to excessive equipment damage and other bad situations. There is much to be said on this topic and I would recommend reading the standard above or even this article from EC&M.

http://ecmweb.com/power_quality/electric_ground/

 

[72.5kv]

Grounding also help deals with capacitive coupling between conductor and the earth.

Unless it is a substation transformer, utilities sometime fuse distribution transfomers at 300% full load current. So overload protection is out of the question only short circuit.