uninsulated overhead grounded conductor

[bgisborn]

Why does the NEC allow an uninsulated grounded overhead service for a service less than 300 volts? Running an insulated grounded conductor adds just a little bit to the cost of an installation but is far less likely to be a hazard to property or of a person-unless you think a kid's life is not a worth a couple of bucks.

Please don't tell me this wiring is under the jurisdiction of the utility and they get to write their own rules.

Bob

Edited to remove your e-mail address. Please use the PM (private message) system and then trade e-mail addresses.

[ December 09, 2004, 09:25 PM: Message edited by: charlie ]

 

[iwire]

Re: uninsulated overhead grounded conductor

It is very likely under the utility's jurisdiction.

I think a few here may have an issue with suggesting the power company makes up rules. I would bet the NESC allows a bare grounded conductor.

NEC 230.41 allows bare grounded conductors in some locations.

 

[jimwalker]

Re: uninsulated overhead grounded conductor

ok we wont tell you.
And just what danger do you find

 

[jimwalker]

Re: uninsulated overhead grounded conductor

We have just as leathal electric all thru most houses within about 1/4 of an inch and nobody says nothing

 

[charlie]

Re: uninsulated overhead grounded conductor

For what it is worth, you are permitted to run insulated wires on a bare, current carrying messenger the same as we do.

 

[rattus]

Re: uninsulated overhead grounded conductor

You are talking of the neutral in a 240/120 residential service right? Even if it is broken like two of my clumsy neighbors, one an engineer, have done, it should be grounded on both ends. In this case, there would be no potential to ground. If all wires break, then the frayed ends of the live wires would be dangerous. And too, I would think these would short against the neutral causing the xfmr overload to pop. I presume that there is some manner of protection there. Tell me if I am wrong.

 

[charlie]

Re: uninsulated overhead grounded conductor

Rattus, it depends on the electric utility and the size of the transformer. The smallest fuse we use is a 15T and that will be used on a 15 kVA or a 25 kVA transformer. Our ? to neutral voltage is 7.62 kV, do the math and tell me how much fault current has to flow on the secondary side to take out the primary fuse and how much kVA this it takes. By the way, a medium voltage T rated fuse will carry 150% for an indefinite period of time so that needs to be factored into your calculations.

 

[don_resqcapt19]

Re: uninsulated overhead grounded conductor

rattus,

Even if it is broken like two of my clumsy neighbors, one an engineer, have done, it should be grounded on both ends. In this case, there would be no potential to ground.

If only the grounded conductor is broken there could be a potential(if the the grounding electrode at the service is not a common metal water pipe, there will be a potential) from the load side grounded conductor to earth. (if there is any load on the panel)
Don

[ December 10, 2004, 11:20 AM: Message edited by: don_resqcapt19 ]

 

[rattus]

Re: uninsulated overhead grounded conductor

Don, yes, if the grounds are not done properly there could be a problem. Saw this happen once, and I was afraid the unbalanced loads would zap all sorts of things in the house, but the service ground took care of it. Apparently this sort of thing happens quite often and the power company fixed it for free.

I understand also that in some third world countries, they use the earth as a neutral return. Now that scares me.

 

[don_resqcapt19]

Re: uninsulated overhead grounded conductor

Ratus,

Don, yes, if the grounds are not done properly there could be a problem.

It doesn't matter how the grounds are installed. If the building is served by a nonmetallic water pipe, there will be hazardous voltage on the broken grounded conductor on the section that is connected to the building. If the only return path is the earth, and that will be the case anytime the building does not have a metal water service, there will be hazardous voltage on the open grounded conductor. The connection to earth does not make it safe.
Don

 

[don_resqcapt19]

Re: uninsulated overhead grounded conductor

Bob,
Why do you think that the bare grounded conductor is a safety hazard?
Don

 

[dereckbc]

Re: uninsulated overhead grounded conductor

Originally posted by rattus:
I understand also that in some third world countries, they use the earth as a neutral return. Now that scares me.

It is called a SWER system, and is used in the US and Australia to name a few. Earth is used as the return on the high voltage side, not low voltage. It is not that different from from what is used by any MGN system in use today. High voltage is completely different than low voltage used in your home. You could make an argument about EMF, but it no more dangerous than an MGN system.

 

[highkvoltage]

Re: uninsulated overhead grounded conductor

bgishorn. How is this a hazard? You really don't understand a utility system? The 3 main reasons utilities use bare conductors:
1.The conductors are remain cooler in the open air so they factor that in when designing the line.
2. Bare conductors are alot easier to work with when doing hot work or any other type line work.
3. Utilies use to use insulated conductors for the primary and after some time (10 to 20 years) the insulation would start splitting and peeling.

 

[charlie]

Re: uninsulated overhead grounded conductor

Festooning insulation was never a problem on medium voltage lines but it has been a problem on open wire secondary. The insulation that we used was called weatherproof and when the conductors were overloaded (heated up) the insulation would dry out and start festooning.

We still have a lot of the old copper weatherproof wire in the air and we replace the #4 WP with 2/0 Al. parallel cable (bare neutral and two phases run parallel and lashed together).

 

[bgisborn]

Re: uninsulated overhead grounded conductor

The safety problem with an uninsulated grounded service conductor occurs when one or both ungrounded conductors are cut and the uninsulated grounded conductor is also cut. The problem is on the transformer side but first let me point out that earth grounds have resistance and this electrical resistance can vary quite a bit, depending on if the earth is wet-or other natural factors.
Let's take a realistic example: The insulated conductor falls in a puddle and the resistance back to the pole earth ground is 2000 ohms through/over the earth. We have potential difference at the puddle of 220 volts at point of contact of the conductor. Clearly, we're not going to cause any overamp protection to open at the transformer nor is there any dramatic arcing as we see when a power line hits the earth. The conductors just sit there. A child, seeing no danger, steps in the puddle and grabs the uninsulated grounded conductor and puts himself and his wet little hands in parallel with the earth current and across considerable potential.
The issue with uninsulated power lines is different because these are usually higher-less likely to be damaged- and carry much higher voltage. It's this high voltage that allow them to expose themselves as dangerous when they contact the earth. They spark and smoke and when approached give a warning stepping voltage shock-voltage difference between the two feet-depending on earth resistance.

 

[bgisborn]

Re: uninsulated overhead grounded conductor

Oops, I made a misake in my last post. That part where I wrote 220 volt potential should really read 120 volts- for this example. Here the potential difference between the two ungrounded conductors does not apply, but the difference between a single ungrounded conductor and it's grounded partner.

 

[charlie]

Re: uninsulated overhead grounded conductor

I fail to see where an insulated neutral would lessen the danger.

 

[rattus]

Re: uninsulated overhead grounded conductor

Don, when I say "grounded", I mean properly grounded. And, unless all three wires are broken, they should be out of reach of humans. If they all break, there would be no load, and the problem would be with the frayed ends of the hot wires. Most likely, the hot wires would short against the neutral and burn off the ends, and maybe trip the xfrmr breaker. Charlie can compute this current for us.

It speaks well for the designers and installers of these systems that there are so few accidents involving fallen power lines.

[ December 13, 2004, 12:58 AM: Message edited by: rattus ]

 

[bgisborn]

Re: uninsulated overhead grounded conductor

The scenario I described is one of many possible with an uninsulated grounded conductor. Albeit, the chances of any of these actually happening is in the order of millions to one. That is, there is only a very tiny chance this would happen if there were only one installation of this type. But there are millions of such installations. With any safety issue we're dealing with probabilities. A single house wired internally with a bare gounded conductor might never have any problem, but in millions of such houses there is likely there will be problems.
We've all seen the "Red neck circuit": a load circuit completed through the grounded frame of a building-a money saver-like that uninsulated grounded conductor.

 

[iwire]

Re: uninsulated overhead grounded conductor

Originally posted by bgisborn:
Let's take a realistic example: The insulated conductor falls in a puddle and the resistance back to the pole earth ground is 2000 ohms through/over the earth. We have potential difference at the puddle of 220 volts at point of contact of the conductor. Clearly, we're not going to cause any overamp protection to open at the transformer nor is there any dramatic arcing as we see when a power line hits the earth. The conductors just sit there. A child, seeing no danger, steps in the puddle and grabs the uninsulated grounded conductor and puts himself and his wet little hands in parallel with the earth current and across considerable potential.

Now I saw you correction from 220 to 120 and even taking that into consideration I am really not following you.

Any person grabbing a live 120 volt line while standing in a puddle has a great chance of getting blasted with or without the bare grounded conductor in the area.

I agree with Charlie, how would an insulated grounded conductor reduce hazards?

As we bond most all metallic building components (building steel, metal water piping, etc.) to the grounded conductor would you also have us insulate all theses surfaces as well?

By the way what is a "General Electrical Administrator" ?

220 volts is not a standard voltage, which suggests to me that you do not work directly with electrical systems.

Thats not a problem but how about telling us how this became something you where curious about?


Bob

[ December 13, 2004, 04:49 AM: Message edited by: iwire ]