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Old 11-28-2008, 02:22 AM   #31
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When a pole transformer goes bad


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Originally Posted by InPhase277 View Post
He heard the conductors jumping around in the conduits. During high current shorts, the magnetic field is powerful enough to move the conductors. Inside large switch gear it is a good idea to wrap the incoming feed with a rope and secure them in place to prevent them from ripping the gear apart.

He is correct on describing on this.,

Yeah belive or not there is documeted case some conduit actally ripped or bent heckva out of line with very hevey short circuit { this more common happend with very large system and with multi parallel conductors }

If you have free hanging conductors you will see them jump up and down or both sideway as well.

The last one I did see it was three freehaning MV conductors { 4160 volts } going to the large trailer mounted air compressor it did make weird shape almost like triangle format with jumping around { that time the motor actally have a serious arc fault ( not bolted fault )

Any one who been doing jump start the cars you may notice the jumper cable do move when you crank over the engine it will move due so much current running thru the cable by magatinc foruce.

Merci,Marc
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Old 11-28-2008, 08:23 AM   #32
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When a pole transformer goes bad


For a good example, check out this test on You Tube. All three phases are grounded during the fault and you can see the violent reaction to the intense magnetic field created by the fault current.

http://www.youtube.com/watch?v=DCyMS...eature=related
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Old 12-02-2008, 05:59 PM   #33
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When a pole transformer goes bad


When I was a kid, I recall watching Sci-Fi movies and wondering whether the wires really danced like they do in the movies.
Then, I finally got to see one first hand after a storm, when there were lines down. I was in my car, and the POCO and cops were on the scene. I saw a wire dancing on the wet pavement as I drove past.

As for my "pot", I got a response from POCO whom I e-mailed before T-giving. She told me that they did send someone to check it out, and the report came back that all is well.
She told me the transformer is from the 60's (not much older, as I had thought), and that I am probably better off with that one than with a new one owing to their superior construction and ability to withstand overloads than today's models.
She told me also that there are no PCB's in the transformer. It uses mineral oil, and the older ones used linseed oil.
Further, she told me that the open-air wiring is fine. As long as it doesn't get broken, they don't replace it. Being open air, it doesn't pose the overheating problem that twisted would.

I don't know if there really is a fuse on this pot. I'll have to take a look at it.
So, in spite of my worrying, and some of us ranking on POCO, they do respond to customers inquiries and concerns. Perhaps what got me the call-back is that I mentioned PVC's in my e-mail. They want to make sure I don't take this thing one step further and call the EPA or someone.

So, I am satisfied and content. Now I can sleep without worrying about the pot. All I have to do now is talk to someone at NASA to confirm that there are no Earth crossing asteroids that may threaten to put a hole in my head someday<g>
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Old 12-02-2008, 06:20 PM   #34
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When a pole transformer goes bad


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Originally Posted by KE2KB View Post
When I was a kid, I recall watching Sci-Fi movies and wondering whether the wires really danced like they do in the movies.
Then, I finally got to see one first hand after a storm, when there were lines down. I was in my car, and the POCO and cops were on the scene. I saw a wire dancing on the wet pavement as I drove past.

As for my "pot", I got a response from POCO whom I e-mailed before T-giving. She told me that they did send someone to check it out, and the report came back that all is well.
She told me the transformer is from the 60's (not much older, as I had thought), and that I am probably better off with that one than with a new one owing to their superior construction and ability to withstand overloads than today's models.
She told me also that there are no PCB's in the transformer. It uses mineral oil, and the older ones used linseed oil.
Further, she told me that the open-air wiring is fine. As long as it doesn't get broken, they don't replace it. Being open air, it doesn't pose the overheating problem that twisted would.

I don't know if there really is a fuse on this pot. I'll have to take a look at it.
So, in spite of my worrying, and some of us ranking on POCO, they do respond to customers inquiries and concerns. Perhaps what got me the call-back is that I mentioned PVC's in my e-mail. They want to make sure I don't take this thing one step further and call the EPA or someone.

So, I am satisfied and content. Now I can sleep without worrying about the pot. All I have to do now is talk to someone at NASA to confirm that there are no Earth crossing asteroids that may threaten to put a hole in my head someday<g>
I have 2 200 amp square d breaker panels, when ever the 40 amp ac turns on, you can feel the 2/0 service conductors buzzing.

This is the fuse
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Old 12-02-2008, 11:22 PM   #35
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When a pole transformer goes bad


This is an older style installation.

Around here, the 3 phase 7.2 to 15 kv is one of two systems. The first is 7200 volts to neutral (ground), and 12,470 volts phase to phase. The other is 14,400 volts to ground, and 24,940 volts phase to phase. Commonly called 12.5KV and 25 KV. The phase (hot) wires are above the pots (transformers), and the neutral is below. On all POCO (power company) installations, neutral and ground are the same.

These are called 'one bushing pots'. On a transformer, any terminal is called a bushing. The top one is the high voltage bushing, and the ones out the side are low voltage bushings. On these pots, the high voltage is connected to the top bushing, and the case is the other side of the high voltage winding. You can see the 3 small wires connected to the neutral wire below the pots. Most modern pots have two high voltage bushings.

The fuse/disconnect things are called cut-outs. The smaller part on the left is actually the fuse. It is a ceramic tube that holds a fuse element. You make up the element on the ground, and raise it up to the cut-out with a hot-stick (a long telescoping fiberglass pole with a hook on the end). The rings at the top and bottom are where the hot-stick hook goes. As you can imagine, this gets tricky in the wind! While this can all be done from the ground, most of us prefer to go up in a bucket truck, and use an 8' hot-stick.

If a cut-out is closed in to a short, or a short develops, the fuse element will explode. I mean EXPLODE!! It sounds just like a shotgun blast. Maybe a bit louder. The bottom end of the ceramic tube is open, and fire, sparks, etc., fly out about 10' long. It's quite a sight, you'll never forget it.

We've all heard how the POCO gets away with grossly overloading wire, and the secondary (low voltage) side of this installation is a good example. The secondary connection here is called a 120/208 3 phase 4 wire wye. Each pot is internally connected to deliver 120 volts. The number 50 on the pots is the KVA rating. 50,000 divided by 120 equals 416 amps. Those wires look like about 2/0 or so. And they're still there after all these years!

Rob
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Old 12-03-2008, 07:30 AM   #36
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When a pole transformer goes bad


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Originally Posted by micromind View Post
This is an older style installation.

Around here, the 3 phase 7.2 to 15 kv is one of two systems. The first is 7200 volts to neutral (ground), and 12,470 volts phase to phase. The other is 14,400 volts to ground, and 24,940 volts phase to phase. Commonly called 12.5KV and 25 KV. The phase (hot) wires are above the pots (transformers), and the neutral is below. On all POCO (power company) installations, neutral and ground are the same.

These are called 'one bushing pots'. On a transformer, any terminal is called a bushing. The top one is the high voltage bushing, and the ones out the side are low voltage bushings. On these pots, the high voltage is connected to the top bushing, and the case is the other side of the high voltage winding. You can see the 3 small wires connected to the neutral wire below the pots. Most modern pots have two high voltage bushings.

The fuse/disconnect things are called cut-outs. The smaller part on the left is actually the fuse. It is a ceramic tube that holds a fuse element. You make up the element on the ground, and raise it up to the cut-out with a hot-stick (a long telescoping fiberglass pole with a hook on the end). The rings at the top and bottom are where the hot-stick hook goes. As you can imagine, this gets tricky in the wind! While this can all be done from the ground, most of us prefer to go up in a bucket truck, and use an 8' hot-stick.

If a cut-out is closed in to a short, or a short develops, the fuse element will explode. I mean EXPLODE!! It sounds just like a shotgun blast. Maybe a bit louder. The bottom end of the ceramic tube is open, and fire, sparks, etc., fly out about 10' long. It's quite a sight, you'll never forget it.

We've all heard how the POCO gets away with grossly overloading wire, and the secondary (low voltage) side of this installation is a good example. The secondary connection here is called a 120/208 3 phase 4 wire wye. Each pot is internally connected to deliver 120 volts. The number 50 on the pots is the KVA rating. 50,000 divided by 120 equals 416 amps. Those wires look like about 2/0 or so. And they're still there after all these years!

Rob

Reminds me of when a tree fell on a 3 phases outside of my home. Long story short, the wires melted, split apart, touched each other. A shower of sparks fell from the lines and there was a large explosion sound (no, it wasn't the short), I knew exactly what that was. Has happened about three times in the last 6 months around here.
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Old 12-03-2008, 05:00 PM   #37
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When a pole transformer goes bad


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I have 2 200 amp square d breaker panels, when ever the 40 amp ac turns on, you can feel the 2/0 service conductors buzzing.

This is the fuse
Why have three xformers on a pole? Why not just one big one? Isn't there an Economy of Scale?
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Old 12-03-2008, 05:53 PM   #38
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When a pole transformer goes bad


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Why have three xformers on a pole? Why not just one big one? Isn't there an Economy of Scale?

Not sure why they do that...I'm not a lineman. Im guessing its cheaper and lighter.
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Old 12-03-2008, 06:10 PM   #39
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When a pole transformer goes bad


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Why have three xformers on a pole? Why not just one big one? Isn't there an Economy of Scale?
Three single phase transformers are cheaper than one three phase. And if one goes bad, you only have to replace that one.
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Old 12-03-2008, 09:44 PM   #40
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When a pole transformer goes bad


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The number 50 on the pots is the KVA rating. 50,000 divided by 120 equals 416 amps. Those wires look like about 2/0 or so. And they're still there after all these years!
416 amps is really not too much for 2/0 AL under those conditions.

A few back of the envelope calculations for an uninsulated wire suspended in free air gives me an upper bound of 550 amps for a 40C temperature rise in 2/0.

The NEC is pretty conservative, which is good, you should have a pretty big safety margin for things that are in places where people live and work.
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