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#27229 - 07/04/03 03:46 PM An High Voltage question ?
wocolt Offline
Member

Registered: 12/06/02
Posts: 117
Was down in the Richmond/Petersburg area and saw something I thought peculiar.
Looked like a 7200-8400 volt line on a pole.
one leg fed a single phase transformer.
From there the three lines went into conduit, (not the new part) insulated of course, but what was strange there were three taps from the neutral (bare wire) fed back up to the insulated 3-conductors, NOt on the phases themselves but just down onto the insulation, wrapped around it.
the question is why is that done.??

Saw this in a few places not just at the motel.
Wm.Colt

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#27230 - 07/04/03 05:51 PM Re: An High Voltage question ?
Ichabod Offline
Member

Registered: 05/24/03
Posts: 26
Loc: Statesboro, GA, USA
I think you are talking about underground cables tapped off an overhead line. The insulated cable requires some sort of stress relief, which would require a ground on the cable shield. Take a look here

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#27231 - 07/04/03 06:43 PM Re: An High Voltage question ?
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Is it possible that the medium-voltage distribution circuit had a multigrounded neutral? In some cases, utilities connect primary and secondary transformer terminals together with a primary bare span, a jumper to pole-base ground rod, and an insulated secondary service-lateral conductor.



If that doesn’t sound like it describes the situation you saw, can you give more details?

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#27232 - 07/05/03 06:15 AM Re: An High Voltage question ?
wocolt Offline
Member

Registered: 12/06/02
Posts: 117
Ok, as best as I can describe it.
Three HV lines going laterally overhead, each was tapped with an insulated conductor and the 3 were fed into a piece of conduit, the conduit was strapped to the pole.
There was a single-phase distribution tranny there and just below it was the neutral, there were 5 taps on the neutral, one to the tranny secondary, one to the tranny primary and 3 more, these last three were run individually up to the insulated conductor (not to the conductor material itself but to the insulation and wrapped and secured there.
It didnt appear to be for 'strain-relief', because they( the neutrals came up from the bottom where the neutral was. My first thought was some sort of 'field-rigged' capacitance, but each would probably be different.
Hope this explanation helps clarify what was there.
Wm.

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#27233 - 07/05/03 11:34 AM Re: An High Voltage question ?
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Some kind of surge protector?

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#27234 - 07/05/03 04:18 PM Re: An High Voltage question ?
Ichabod Offline
Member

Registered: 05/24/03
Posts: 26
Loc: Statesboro, GA, USA
I think you misunderstood "stress". Where the oh line makes the transition to cable and goes into the conduit there must be some sort of "electrical stress" relief. The link I posted should explain how the stress cone works. Briefly, the cable is a center conductor surrounded by a dielectric, typically cross link polyethelene about quarter inch or one cm thick. This is surrounded by a shield which is grounded. To make the transition the shield is cut away from the dielectric. This would result in a very high level of "electrical stress" at the edge of the cut shield without the stress cone. and would cause the cable to fail at that point. Looks like you understood that to mean "strain", as in physical load or weight. The weight of the pothead was likely carried by the oh line, while the weight of the cable in the conduit was held by a cable grip that works like a Chinese handcuff. The number of taps to the neutral that you explained would be right, two for the tranny and one for each of the three stress cones or potheads. Do a google search for "stress cone" and "pothead" and you should find more info. The Asahi Cable is the best picture that I found. After you click on the link the picture on the left explains what you were seeing. Except they call it an "earthing" wire.

[This message has been edited by Ichabod (edited 07-05-2003).]

[This message has been edited by Ichabod (edited 07-05-2003).]

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#27235 - 07/05/03 05:57 PM Re: An High Voltage question ?
wocolt Offline
Member

Registered: 12/06/02
Posts: 117
Icabod
Thanks for the explanation, makes sense now, I didnt realize it was a shielded cable.
I did save the site you gave but did not read it yet.
I have always been fascinated by HV, and Yes am aware that that is really not HV, (but that is HV as far as I am concerned)after seeing that video on 'real high-voltage' I have a new found respect for it.
Anything greater than 480 I really dont want to get anywhere near it. The bare-handing and pulling themselves along on that trolley while on 500kv lines, ....I dont think so.
Any way thank you everyone for the replies, and the effort.
Wm.Colt

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#27236 - 07/06/03 03:53 AM Re: An High Voltage question ?
Ichabod Offline
Member

Registered: 05/24/03
Posts: 26
Loc: Statesboro, GA, USA
Since you brought up the hv thing, here's a picture of me sitting on a live 500 kv line. You wear a conductive suit, including socks and hood. Sounds like your head is in a beehive.


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