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Author Topic: Utility HV Grounding Question  (Read 532 times)

Bill Koonce

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Utility HV Grounding Question
« on: July 28, 2017, 07:30:20 pm »

I was our for a walk in my neighborhood, and spotted something that bothered me. On every power pole that I passed, there was a ground wire that I would expect to be there, one that should complete the circuit for the 7kV (I was told that by the utility electrician when I upgraded my service drop) high voltage lines that feed the transformers that output 120/240V split phase AC that goes into our homes. That wire is connected to the transformers on poles that have them, to the insulated side of the disconnect switches on poles that have them, and to the neutral conductor of the split phase 2-wire circuit that runs below the HV wire. So that's one high voltage wire on top, 3 medium voltage wires below (counting the neutral) and vertical ground wires running down each pole. So far, so good. In theory, those vertical wires should complete the HV circuit using the earth ground.

The problem is that every single ground wire that I saw had been cut off about 6 feet above the ground. Same height on every pole, except one that had lost its insulation higher up, and was unraveled and very corroded. All the rest had been cut neatly just above 6 feet and an inch above ground. I almost reached up to examine one of the cut wires, but then it dawned on me that my 6 foot body was now the most conductive thing in that space, and that it was monsoon season--lots of lightning in the area, and the ground was still moist from the last rainfall.

Absent any earth ground, the neutral wire in the 120/240V circuit that was still connected to the upper part of the ground wires would have to be what completed the 7kV circuit. My question is "is that safe?" Is this normal? What happens if lightning strikes? Or if the HV wire breaks and shorts against the neutral? It looks to me that the nice new copper clad ground stakes that I put in just last year is the path of least resistance to any earth ground...perhaps for the whole neighborhood...eek!

If anyone can shed some light on this unsafe-looking, but mostly trouble-free utility wiring, I'd be most grateful. To answer the obvious question, yes I did call PNM, my electric utility. But as expected, the call center operator had no comment other than to ask if the power was out.
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Tim McCulloch

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Re: Utility HV Grounding Question
« Reply #1 on: July 28, 2017, 08:41:36 pm »

I was our for a walk in my neighborhood, and spotted something that bothered me. On every power pole that I passed, there was a ground wire that I would expect to be there, one that should complete the circuit for the 7kV (I was told that by the utility electrician when I upgraded my service drop) high voltage lines that feed the transformers that output 120/240V split phase AC that goes into our homes. That wire is connected to the transformers on poles that have them, to the insulated side of the disconnect switches on poles that have them, and to the neutral conductor of the split phase 2-wire circuit that runs below the HV wire. So that's one high voltage wire on top, 3 medium voltage wires below (counting the neutral) and vertical ground wires running down each pole. So far, so good. In theory, those vertical wires should complete the HV circuit using the earth ground.

The problem is that every single ground wire that I saw had been cut off about 6 feet above the ground. Same height on every pole, except one that had lost its insulation higher up, and was unraveled and very corroded. All the rest had been cut neatly just above 6 feet and an inch above ground. I almost reached up to examine one of the cut wires, but then it dawned on me that my 6 foot body was now the most conductive thing in that space, and that it was monsoon season--lots of lightning in the area, and the ground was still moist from the last rainfall.

Absent any earth ground, the neutral wire in the 120/240V circuit that was still connected to the upper part of the ground wires would have to be what completed the 7kV circuit. My question is "is that safe?" Is this normal? What happens if lightning strikes? Or if the HV wire breaks and shorts against the neutral? It looks to me that the nice new copper clad ground stakes that I put in just last year is the path of least resistance to any earth ground...perhaps for the whole neighborhood...eek!

If anyone can shed some light on this unsafe-looking, but mostly trouble-free utility wiring, I'd be most grateful. To answer the obvious question, yes I did call PNM, my electric utility. But as expected, the call center operator had no comment other than to ask if the power was out.

Meth heads stealing copper for "recycling", i.e. drug money.  It's a huge problem and I bet if you asked around the neighborhood you'd find folks that have had their air conditioner condenser coils and tubing stolen, too.

My understanding is the grounding wires on the poles are for lightning dissipation and are not part of the normal circuit.
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Stephen Swaffer

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Re: Utility HV Grounding Question
« Reply #2 on: July 28, 2017, 10:27:40 pm »

As far as I know-and have observed-the earth is never used as part of the power circuit.  The resistance of the earth is relatively high and varies quite a bit-even if you could use it it would introduce a lot of variables.

That said, it is certainly a safety issue that really ought to be addressed.  If you know an electrician, sometimes they work enough with the POCO people that they can get things addressed.  My experience with our local utility is that if I can get past the bureacracy, the people that actually do the work want to do it right-it affects their personal safety-but when they are talking to an electrician they naturally give a little more weight to the concern.
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Steve Swaffer

Bill Koonce

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Re: Utility HV Grounding Question
« Reply #3 on: July 28, 2017, 11:04:24 pm »

Meth heads stealing copper for "recycling", i.e. drug money.  It's a huge problem and I bet if you asked around the neighborhood you'd find folks that have had their air conditioner condenser coils and tubing stolen, too.

My understanding is the grounding wires on the poles are for lightning dissipation and are not part of the normal circuit.
Good point, Tim. Although copper theft is in my city, I hadn't even considered it here in part because the cuts are so neat and uniform. Seems a lot of work for very little wire; there's only a dozen or so poles on this overhead circuit, and they could have taken more, unless it was a very short person.  ;D Then again, I shouldn't expect drug addicts to be rational. That would explain why aluminum is the standard metal for both mains wiring and A/C condensers here though!
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Bill Koonce

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Re: Utility HV Grounding Question
« Reply #4 on: July 29, 2017, 12:05:12 am »

As far as I know-and have observed-the earth is never used as part of the power circuit.  The resistance of the earth is relatively high and varies quite a bit-even if you could use it it would introduce a lot of variables.

That said, it is certainly a safety issue that really ought to be addressed.  If you know an electrician, sometimes they work enough with the POCO people that they can get things addressed.  My experience with our local utility is that if I can get past the bureacracy, the people that actually do the work want to do it right-it affects their personal safety-but when they are talking to an electrician they naturally give a little more weight to the concern.
Thanks Stephen, knowing this I think I will try to escalate the issue with PNM via my electrician. I've seen some pretty oddball power distribution systems, e.g. a 19th century 100VDC buried coaxial cable for the street lights in my home town, and single-wire earth return (SWER) is a real thing in some places. I didn't think it likely in such an arid area, but where I live used to be ranch land not that long ago, so it seemed plausible. Either it's SWER or sharing a neutral between the 7kV and the 120/240V sections is intentional, legal and I hope, safe.

Either way, as Tim noted, the ground leads should be there for lightning protection at least. I guess the good news is that since lightning can travel for miles through the air, a 6 foot gap isn't likely to change the path of a strike much.
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John Roberts {JR}

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Re: Utility HV Grounding Question
« Reply #5 on: July 29, 2017, 10:53:17 am »



My understanding is the grounding wires on the poles are for lightning dissipation and are not part of the normal circuit.
This

JR
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Bill Koonce

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Re: Utility HV Grounding Question
« Reply #6 on: July 29, 2017, 05:57:22 pm »

This

JR
Yep, no doubt that that's what it was originally designed to do. But with all paths to ground disconnected, what's left is a shunt between all the HV mounting hardware and the low voltage neutral. Now, if an insulator should break or the wind blows the 7kV wire off, that's 7kV that goes right into my house, with no breaker or fuse to stop it. Ditto if lightning strikes.
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Stephen Swaffer

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Re: Utility HV Grounding Question
« Reply #7 on: July 29, 2017, 09:47:02 pm »

Actually, if the 7 KV shorts, it should trip some sort if over current device-how quickly who knows-but that is why the ground rods and/or grounding electrodes at your home are critical-they are your last line of defense.

Incidently, I had a customer experience a primary/neutral short a couple of years ago due to a vehicle hitting their transformer pole.  Surprisingly, the only casualty was a cheap surge protector power strip that smoked the MOVs and melted the plastic causing plenty of smoke.
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Steve Swaffer

David Buckley

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Re: Utility HV Grounding Question
« Reply #8 on: July 30, 2017, 05:18:23 pm »

The resistance of the earth is relatively high and varies quite a bit-even if you could use it it would introduce a lot of variables.

Strangely, that's not the case.

If you can make a decent connect to earth, the resistance between any two points on the planet is a few hundred ohms.  A "decent connection" is just an electrode building problem.

This low resistance was important in the early days of telegraph and submarine telegraph cables, which used just one signal wire and an earth return.  It's still important for submarine communications cables, as the return path for power for the repeaters for the fibre optic link;  Typically each end of the cable has plus or minus several KV applied to it, and all the repeaters are in series along the cable, with the planet being the return path.

Bill mentioned SWER a/k/a Mandeno's Clothesline, still used, and in the right circumstances works very well.  It is more reliable than standard multi-wire distribution.  I look after a community radio station, and our transmitter on the top of a mountain is fed by a 6.6KV SWER overhead line.

Finally, HVDC power transmission can put thousands of amps of current through the earth over hundreds or thousands of miles.  These systems use very impressive electrodes!
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