ECN Forum Forums Technical Discussion Electrical Theory and Applications crazy 3 phasy

Redsy;
no one-liner yet.....no accident either

O.K. I stopped the truck and took a good look. One of the 3 wire on the crossarm is tapped, and right before it connects to the transformer, it seems to split into two and connects to the primary. Is it shielded cable with a grounded shield providing the 2nd connection? I will have to get pictures.

>Is it shielded cable...?
Now I think you've hit on something!

man oh man... ive never ever heard of a single primary with no neutral feeding a tx... now what you bay be thinking of here is a can grounded TX. theres only one insulator on the top of the transformer, so one leg of the primary coil is tied to the primary, the other leg is tied directally to the transformer tank, and that its bonded to the pole ground, and then the system ground. if what your saying is true, and they do use the ground as a return path, imaginw what would happen if the #6 bare copper line running down the pole in to the ground was CUT! primary voltage at your finger tips.. i DOUBT this is likley.

ever hear of 40.1 miles of underground primary run? i found out that there is such an insulation in teh florida everglades... they said it faulted about once a week...

-m

>there's only one insulator on the top of the transformer
Yes, that is common.

>imagine what would happen if the #6 bare copper line running down the pole in to the ground was CUT! primary voltage at your finger tips.. i DOUBT this is likley.

The situation you describe is not as bad as if the poco grounded conductor were broken for a transformer primary wired to the poco grounded conductor.

I guess you are saying that the center tap (X0) of the secondary would be in series with the now otherwise ungrounded end of the primary (P2).

Cut the ground wire on the pole and it is still grounded at the service.

But for a system relying on the poco grounded conductor, the ground on the pole and at the service are presumed neglible.

The real answer is that current can't flow in two directions at the same time through the transformer.

The is no complete circuit.

First let's consider the set up with no magnetic field. In this case, X0, X1, and X2 all have the same potential. Even if that is primary voltage, it obviously has nowhere to go. So the measured voltage to ground or anything else is zero.

If it is not, then a magnetic field will arise in the primary and secondary coils. Say that the tranformer is a 16 to 1 step down.

Now look at the circuit with the magnetic fields standing. In order for energy to flow from P1 to P2, it must pass by X0. To the extent that it goes into the earth, 16 times as much current can flow between X1 and X2 at 1/16th the voltage -- as expected.

However, to get primary voltage to your finger tips, it must somehow travel from X0 to X1 or X2. Let's suppose that somehow it manages to get to X1. Where can it go next? It is already the same potential as X0, so it can't flow there. If it tries to flow to X2, it has to flow from X2 to X0. But it can't flow from and to X0 simultaneously. So the path is shaped like a "6". It's a dead end. There is no way to get primary voltage into the house through an undamaged transformer. Aren't we lucky?

>they said it faulted about once a week...
What kind of fault? A lightning strike?
That seems likely. But the kind of fault that requires dispatching a repair crew and digging up the wire seems unlikely.
Do you know who "they" are?


[This message has been edited by Dspark (edited 10-05-2001).]

This message is being posted for Allan Hildenbrand.

---Quote---


[- Word Wrap was driving me crazy]

[This message has been edited by Webmaster (edited 10-10-2001).]

Thanks, Scott. I knew I wasn't crazy. (am i?)

It gets even better!!

After I sent my comment on this thread to Scott35 yesterday, I took a walk down the alley to the pole I describe above. And what to my wondering eyes should appear but the absence of the bottom five feet of the ground wire!!!

The end of the ground wire is oxidized black, which, to me, means the cut end has had a LONG time to weather.

The xformer is a 50 KVA unit which, with an 8 KV primary voltage and 150 percent overload means the primary current is about 9.5 amps.

Al Hildenbrand

[This message has been edited by ElectricAL (edited 10-10-2001).]

hey dspark,

a guy i work with, he does primary fault locating, hes worked with FPL for like 40 years.. he said they ran 1ø aluminum 40.1 miles from a riser to a few pad mounts.. radial feed... the guy i work with said it took something like a 45 amp fuse just to energize the cable.. anything lower and it would overload due to resistance or something.. the fuse ended up being around 100a with the pots at the end... he said there was a splice pit every 500 or so feet... thats like over 400 segments of cable... this is in the everglades too, so the cable is constantly surrounded by water... he said a few years after it was installed, it started faulting regularly once every week or 2.. and yes, a crew had to some out dig and repair...

-m