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Posted By: Trumpy Faults Calls - 06/14/03 06:20 AM
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?.
Posted By: pauluk Re: Faults Calls - 06/14/03 10:15 AM
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.... [Linked Image]
Posted By: Bjarney Re: Faults Calls - 06/14/03 07:04 PM
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.
Posted By: Dapo Re: Faults Calls - 06/14/03 10:55 PM
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.
Posted By: Trumpy Re: Faults Calls - 06/15/03 02:45 AM
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.
Posted By: Trumpy Re: Faults Calls - 06/21/03 02:46 AM
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?. [Linked Image]
Posted By: Trumpy Re: Faults Calls - 06/28/03 06:54 AM
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?. [Linked Image]
Posted By: pauluk Re: Faults Calls - 06/28/03 09:14 AM
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. [Linked Image]

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.
Posted By: Trumpy Re: Faults Calls - 06/28/03 11:01 AM
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!. [Linked Image]
Posted By: Bjarney Re: Faults Calls - 06/28/03 07:34 PM
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.

[Linked Image from 64.146.180.232]

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).]
Posted By: pauluk Re: Faults Calls - 06/29/03 01:15 AM
Is there an explanation to go with that picture? Any idea where this was, or what level of HV was involved?
Posted By: Bjarney Re: Faults Calls - 06/29/03 02:53 AM
I don't remember the details. It was posted at powerlineman.com a couple of years ago. I believe it was at a US utility—maybe 34.5kV [19.9kv to ground.]

The webmaster there mentions that due to disk-space limits he only keeps images for about a year.
Posted By: Trumpy Re: Faults Calls - 06/30/03 04:03 AM
Bjarney,
Thank you very much!. [Linked Image]
Quote
where in medium-voltage [>2kV] faults it is intended to be sensed by and operate overcurrent-protective devices
I take it by this you mean, a system like Merz-Price(?) HV protection?.
When I was being trained up for Faults work, I was warned to keep my feet together, if I suspected that there may be a HV ground Fault, nearby, this was brought back to me recently, when I was training up a new Liney as a faultsman and he was leaping around(with his feet together) at a callout we were doing one night, I asked him why and he said, in case there was an Earth Fault!.
(Better to be safe than sorry, I suppose) [Linked Image]
Posted By: Trumpy Re: Faults Calls - 10/25/03 08:48 AM
I posted a topic related to this a while back, but I went to a Faults callout this afternoon, a broken pole.
Of all the poles on the road in question, they had to hit the one with the Xformer on it.
33kV lines and a 50kVA tranny on it.
Luke(new Faults guy) and I got there and the Fire Crews were at work, as were the Ambulance people.
Went w/ Luke to pull the ABS at each end of the ring.
Oddly enough, none of the DDO fuses dropped, even though the pole top was lying on the ground.
First time I ever saw this as a FF, it scared the Hell out of me, you don't have street-lights out in the country.
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