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#137230 - 06/13/03 11:20 PM Faults Calls
Trumpy Offline


Member

Registered: 07/05/02
Posts: 8552
Loc: SI,New Zealand
I've been a Faultsman for a few years now and one thing about them all is this:
  • There is always a guy(Homeowner) at the fault, that knows heaps more about it than what you do!.("I think you'll find" is a phrase I have become used to)
  • Everyone hates the power being turned off.
  • People want to know why 11kV+, makes such a loud BANG when the wires clash.
  • Why do people that have struck a power pole complain so much about the cost of the pole they have snapped off, after they have got out of Hospital?, are they not allowed to have Telephones in the wards?.
  • Why is that everyone wants to help you to replace a DDO fuse, on a 33kV line, even though they have no insulated gloves, or any idea of the consequences if there is a ground fault through the Hot-Stick.
    I mean, having six people on a Hotstick, is just.......

What are your ideas on this?.
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#137231 - 06/14/03 03:15 AM Re: Faults Calls
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Well, my HV line experience is limited to a few operations on the 11kV distribution on a telecoms site, but the loud BANG! certainly reminds me of the first time I heard one of the 11kV breakers open. It was one of those "trolley" types, and boy did it make me jump when it released!

As for the guy who know more than you, I've had that all the time on normal domestic wiring. You end up wondering why he called you if he knows so much about it himself....

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#137232 - 06/14/03 12:04 PM Re: Faults Calls
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Trumpy, maybe if more people witnessed a close-by "…loud BANG when the wires clash," their ‘enthusiasm’ in the other four scenarios would be a bit more reserved.

Another aspect that few bystanders comprehend is the almost unbelievable touch-and-step potentials {voltage gradients} that can occur during a ground fault.

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#137233 - 06/14/03 03:55 PM Re: Faults Calls
Dapo Offline
Member

Registered: 03/04/03
Posts: 52
Loc: Australia
I think it is the same in all industries, many years ago in the mining game, I was a shift electrician, and you knew every time you were called to a machine, the operator would be telling you constantly how to fix it, or that the last electrician fixed it in this panel here!!!
The best I had was when I was called to fix a Tamrock an underground electric/hydraulic drilling machine. When I arrived and repaired the machine, the operator said that his mates machine, in the next drive was also broken. I thought this was a bit strange, when I arrived at the second machine and asked what was wrong. The operator said that his machine was working ok, but he opened one of the control panels to show his mate how to fix his machine, and when he closed it again his machine wouldn’t work. Well it was a simple fix, the panel door limit switch needed a little adjustment. I think that would have been the last time this bloke went in messing with something he shouldn't have, especially since they were paid contract on the footage they drilled per shift.

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#137234 - 06/14/03 07:45 PM Re: Faults Calls
Trumpy Offline


Member

Registered: 07/05/02
Posts: 8552
Loc: SI,New Zealand
Guys,
One of the scariest calls I've been to involved a house out in the country that had no power, one night.
I had the flu at the time(middle of winter) and I was in no mood at all to play silly b$#*%rs.
When I got there, everyone(most of thier extended family was there, I think)was looking up at the pole.
As I thought it was just a bad set of DDO fuses(these do wear out), I went about replacing them.
First one went in OK, no sparks, there was a small spark with the second, I then told everyone to stand away from under the pole,
but I might as well have been talking to myself, as no-one moved.
I just touched the top of the DDO on the clip and all hell broke loose, all three DDO's went off, people were running everywhere, one just about knocked me over, it was raining sparks and flames.
The transformer had a dead short in the Y and B windings, so I found out from the line crew that replaced it the next day.
I suppose that these people will be staying INSIDE the house the next time the power goes off.
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Let's face it, these days if you're not young, you're old - Red Green grin

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#137235 - 06/20/03 07:46 PM Re: Faults Calls
Trumpy Offline


Member

Registered: 07/05/02
Posts: 8552
Loc: SI,New Zealand
Bjarney,
I've never seen or heard a plausible reason why Voltage Gradients occur, after all the ground is a reference point which the system is based on.
Could you please explain how Step and Touch Potentials work?.
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Let's face it, these days if you're not young, you're old - Red Green grin

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#137236 - 06/27/03 11:54 PM Re: Faults Calls
Trumpy Offline


Member

Registered: 07/05/02
Posts: 8552
Loc: SI,New Zealand
I am led to believe that if a wire touches the ground, there will be a difference in potential all around the point of contact of the wire.
From there on, until the Circuit protection operates, if you happen to stand inside the area of the Electricity radiating out from the point of contact, there will be a potential difference between each of your feet, in a HV system, this could quite concievably kill you.
Bjarney, what are your thoughts on this interpretation of Voltage gradients, during Earth Faults?.
What does anyone else think?.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#137237 - 06/28/03 02:14 AM Re: Faults Calls
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Imagine the area immediately surrounding the point of contact as a resistance extending for several yards and the earth beyond that point as a solid conductor.

As you know, if you pass a current through a resistance, you get a voltage drop across it. I don't pretend that these figures are accurate or representative, but just for the sake of argument let's say that a line at about 20kV (e.g. one leg of a 33kV delta) touches the ground and winds up with a resistance to earth of 2000 ohms. That'll result in about 10 amps flowing to ground (assuming a solidly grounded neutral).

Now suppose that the resistance area around that ground connection extends to 20 yards. That will result in a voltage gradient over which the 20,000V is distributed. If the gradient were linear across that 20 yd. distance, then somebody standing with his feet 1 yard apart would experience a potential difference of (2000 ohms / 20 yd.) * 10A = 1000V.

In practice you'll find that the voltage gradient is much steeper closer to the point of contact and tapers off rapidly at the outer edges.

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#137238 - 06/28/03 04:01 AM Re: Faults Calls
Trumpy Offline


Member

Registered: 07/05/02
Posts: 8552
Loc: SI,New Zealand
Thanks for that explanation Paul,
being on the opposite end of the scale (as in replacing fuselinks after a fault has occured).
Without a proper test of the windings being made, there and then on the spot, it really gets me worried about replacing any links on 3.3kV to 33kV lines, even with a Hot-Stick.
I tried meggering a Tranny one night and got hit by an 11kV feeder that was just "sitting" in the insulator bushing.
Mind you, I wear boots, that would put KISS to shame, really thick!.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#137239 - 06/28/03 12:34 PM Re: Faults Calls
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
My apologies, Trumpy. I completely overlooked your post from last week.

Paul explained it very well.

As I understand it, 'touch' (hand-to-foot) and 'step' (foot-to-foot) voltage comes about during a medium or high-voltage fault where earth return and ground currents are involved. Ohm's law applies here. When the situation is modeled or described, [too often after an accident] the path is depicted as a number of series/parallel resistors.

So, for instance, for an 11kV circuit {11kV/sqrt3 or about 6.4kV to ground} where the downed line touches soil, with return current back to the substation ground mat [usually solidly connected to the serving {~66-11kV} transformer's secondary-neutral terminal.] The across-the-earth voltage at the point of soil contact forms 'gradient rings,' where the voltage from one foot to the other can be of deadly proportions. Unless specially constructed and so rated, shoe soles are effectively very poor insulators. It can be a staggering, confusing, almost unbelievable experience.

Note that for low voltage, 'earth return' current should not be expected to reliably clear overcurrent devices, where in medium-voltage [>2kV] faults it is intended to be sensed by and operate overcurrent-protective devices.

Conductor movement from an unsecured (fallen) span contacting the earth is subject to wild gyrations from arcing and magnetic forces. It can look very much like the exaggerated scenes you find in a Disney television cartoon. Vitrification can occur, where sand turns into glass from very high heat.



Remembering that the safest thing to do is "never turn the switch on in the first place," utility and industrial electrical engineers spend significant resources in trying to best decide what makes a reasonably safe system when someone finally does "turn the switch on." A modern term for this process is risk management, but for electricity it's been extensively practiced for well beyond the last century.




[This message has been edited by Bjarney (edited 06-28-2003).]

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