Electrical delivery question
 

Is there a lineman here? In our neighborhood the electricity is carried on poles. 3 wires and up on top of the poles now and again are three big fuses. There's an insulator and about a 14" fuse that carries power around it. Periodically, squirrels will short out the wires at a transformer or something and one of these fuses pops. It makes a big bang like a shotgun blast. You can tell the problem because the fuse hangs down. On my block, when one fuse pops, everyone loses all their electricity. I have a rental property 4 blocks away. When one of the fuses pops on that block, only about half the electricity goes out in the houses. It's kind of a PIA because tenants (and homeowners) then think it's an internal problem to the house since some electricity works. Why the difference between the two blocks?

The three wires at the top of the pole carry 7200V three phase. One of those phases is tapped and provides power to the transformer. If the animal takes out that line, all power to the transformer and all power to the street is lost.

Once the transformer has power (7200V) it reduces the 7200V to the split single phase common to most residences (240V). This setup is three wires, two with voltage and a common neutral. The two with voltage are 120V, 180 degree out of phase to produce 240V when measured against each other. Any one of the two 120V lines will read 120V when measured against the common neutral.

If one of those 120V legs is lost at the transformer, the residences will lose about one-half of the circuits in the house (as one leg is now zero). Also, none of the 240V appliances will function as they can only receive 120V from the single working leg (and might receive nothing if the failed leg is floating without a ground).

So there are three fuses at the top of the pole. Why does losing just one put out all the power at some houses but not others?

Any of the three phases might be tapped for use as line voltage. The times you've seen it blow the fuse that kills power to those houses, the squirrel just happened to go across the particular line being tapped for those houses. All three phases only go to commercial users that require three phase power. Most all residences only use single phase.

There should be one fuse on one of the 7200V lines leading to the transformer.

and so.....
 

If your renter loses power in PART of the house, but not all from a power company issue; then that means the main fuse (7200v) supplying power to the transformer did not blow....but the transformer lost one of the 120volt feeds into the house.

So last night when the power went out at my rental property, there were three transformers on the pole and 3 fuses. One of the fuses was blown and half the electricity was out at the property. I'm guessing that what this means is that while the house could have been receiving all its voltage from the transformer or line where the fuse blew, it wasn't. It was getting part of its voltage thru another transformer on the pole. Whereas on our block all the houses are fed off one transformer. Is there a reason to feed houses off one transformer or is it normal to feed houses of more than one transformer?

Those fuses at the top of the pole are very big and make a loud sound when they trip. I don't think they are 120 volt fuses. I think they must be 7200v.

It appears that they have a single transformer for each phase. The 7200V lines each feed a single transformer and each transformer provides 120V to the street.

What I don't quite grasp in that setup is how they achieve the 180 degree split single phase when they do that. The three phase lines are 120 degree out of phase from each other and the split single phase lines are 180 degree out of phase from each other.

I just went and checked on our block. There is only one transformer on the poles from which 3 wires exit. On the pole at the rental property, there were 3 transformers.

at my house....

there is one transformer that one high voltage (9kv around here) input and 3 120v outputs - the outputs are wound/wired so that they are out of phase with
each other (how far - not sure, the 180* number seems most correct but flies in the face of what I see behind my house)

Each house only get 2 of these 3 phases. (incl mine)
Now sure how this makes my dryer and A/C work, but they do.

let me look outside and be sure...

Yep, 1 xfmr, 1 lead in, 3 leads out. the 3 leads run the length of the road and each house gets 2 of the 3 leads.

I can see that making 3-phase lets them power a 3 phase business with it somewhere.

This also lets your transformer "lose a phase" which causes some houses to lose power and some not.

The only memory of failure I have is when a tree fell on the 9kv line and we lost the whole block.

not sure.

-John

Is it possible that the 220v is an RMS thing ? IIRC when you put an oscilloscope on an outlet you get a voltage of 180v peak to peak.
If its and RMS difference, maybe the phase shift doesnt make that large a difference.

(sorry, I do more PLC's than T&D projects)

-John

You are confusing three phase with split single phase.

The transformer receives one phase from the three at the top of the pole. It takes the 7200V and provides 120V on two lines and a neutral on the third line. This is referred to as "split single phase". It is absolutely not three phase.

No, the two lines to the house are both 120V. The 240V is generated because the 120V lines are 180 degrees out of phase.

240 degrees out of phase. Depending on which of the 2 legs you are getting. A,B,and C.

A-B=240V(120 Degrees apart)

A-C=240V(240 Dgrees apart)

B-C=240V(120 Degrees apart)

Seeing 99% of houses use single phase capacitor start motors the degrees between the phases really dont matter.