Touching a HT Line

You are right protection should have operated.But it is possible th line was close to ground and not actually touching .In all probability it should have been a 11kv line when they say HT.

Since for most journalists "High Tension" ("High Voltage") might be anything starting at 110V, I checked the news. Well, it was 11 kV, as Nesubra correctly guesses. But it turns out that it wasn't a live wire lying on the floor. The wire, which ran "across a roof", was touched by a boy with a piece of metal wire:

http://timesofindia.indiatimes.com/Cities/Bangalore/Boy_suffers_90_burns/articleshow/3480970.cms

So CBs are not to blame. The power company's carelessness is.

Perhaps the following is off topic, but I think it's somewhat related. I've been living in Florida for a full year, and had been here a few times before. Since then I've noticed that after a few months with little or no rain there are always blackouts when the first rain happens, even if it is light and with no signs of thunderstorms.

My conjecture (which is just that, a guess) is that these first coming rains make some tree branches heavier, and then they bend over the wires. The power company has a poor inspection/maintenance record, so it's easy to suspect that they wouldn't care much about lopping potentially dangerous branches.

Can anyone clear this up?

Surely are they. But personal safety shouldn't rely just on parental guidance. Please, spend a minute reading The Times of India, especially the final paragraph:

"Hanging high-tension wires are a menace. Climb up any terrace, ans you will see a clutch of wires tangled together, all perhaps within arm's reach".

Oh, but you are ever so English sometimes

Although trees may account for some of the momentary outages (or sustained outages if the resulting fault cannot be automatically cleared) another more likely cause is contamination flashover. Contamination flashover of insulators typically occurs when heavily contaminated insulator surfaces are lightly wetted by fog, dew, light rain, etc.

As long as the contamination coating remains dry, there is no danger of flashover but when the contaminant includes water soluble salts and is lightly wetted, leakage currents begin to flow in the interface between the contamination coating and the insulator surface. Flashover occurs if those currents reach 100 milliamps or so.

The most severe contamination is hard, crusty buildup from fertilizers, salt, cement dust, discharge stacks, bird droppings, etc. that is not removed by natural heavy rain washing. In those cases, utilities must remove the contamination by washing with high pressure water, abrasion blasting with soda, crushed walnut shells or some other media while the line is energized or chipping and hand washing if the line can be deenergized for cleaning.

I remember reading several stories of people in india stealing power from the lines by attaching hooked bare wires to the lines. This may also had been a factor. They would pay children to shimmy up poles and clip on the wires. Maybe he attached to the high tension by accident. It's a shame it happened to anyone, never mind a young boy.

Never trust a circuit breaker or automatic disconnect. These protections are expected to work but if they are not maintained properly they can fail. I always expect that a circuit is still live (even with the CB open) until I test it with a meter.

I saw a video several days ago with a guy on top of an electric train in either India, Bangladesh, or some other country nearby, who started walking down the train car and casually reached up and grabbed the HV wire above it. Needless to say, he's no longer with us.

I don't know how to upload the video, and it may be a good thing, as it's rather gruesome!

hey bill. i think those were just low-lying power lines...don't ask me why they were that low, but here's the link to the video you might be referring to.

Yep, that's it. If the train isn't electric, I wonder why anyone would run a power line down the tracks above the train??? Most are off to one side . . .

" . . . then no matter how high the line is run someone will make a stick long enough to reach it."

Yep, you're right! I gave a deposition to a lawyer for an insurance company (as a so-called expert as well as a professional engineer) saying that the 7.5 kV line running across the plaintiff's property was above the minimum height requirement, and when asked, "What do you think happened?," I replied, "I think the guy and his wife lost control of his 80 foot CB antenna pole when they were trying to move it to a new location, and they allowed it to fall onto the power line."

Luckily for them, the pole was so flimsy that it bowed enough to contact the ground line running beneath the three hot legs just after it first contacted the lowest hot leg, blowing the fuse, and they escaped with their lives, although the arc burned them both and actually blew the shoes off the woman and set them on fire.

The case never went to court.

I hads to search first to find it, here is the link I believe:-

http://www.youtube.com/watch?v=ausFe4svcLs

Traction current is often 6000volts I believe on many railways.......

I think I saw the video you refer to. Traction voltages often used on overhead traction lines are 25kV AC, 3kV DC, 1,5kV DC, 50kV AC, and some non-standard frequencies AC in Europe. Lower voltages are often fed by a third-rail method, mostly in urban areas. The line seen in the picture is often called the contact wire, as it is the wire that the pantograph contacts to draw current, and is always "live". It is suspended from the wire above it (often called the catenary wire, due to the catenary curve of such free-hanging wires) with dropper wires or jumpers to keep the contact wire parallel with the track.

In South Africa we also have the so-called train surfers, that stand on top of moving suburban coaches as the train moves through the section on overhead line-fed sections, swerving to duck the steel structures and registration arms that register the contact wire. Needless to say many of them don't make it, touching 3,000V DC is rather bad for your heart. Remember that DC traction systems as a rule cannot discriminate between a fault current, overload or earth faults - it just measures the current flowing through the CB and it trips at a given calibrated value (between 3000A-5000A in our case). The fact that the breakers sometimes do trip when these events take place just goes to show how unpredictable electrical parameters can be!

BTW, from the actions of the bystanders, I'm of the opinion that the chap in the video was committing suicide, or was doped/drunk.

Andy

They change it again. the new on is here

http://vinothkumarm.blogspot.com/2009/05/electric-train-high-voltage-wire-is.html

Well done.

Good Point J-o-a-T

The OP originally asked about a line that falls to the ground. Everyone is trying to explain what happened based on an altogether different situation.

GA from me.

I agree with your explanation. When we were in the 6th semester of engineering, we studied, the following types of faults:

line to ground,

line to line,

double line to ground,

all 3 lines to ground and

Series fault.

What about the last type of fault. I think, the moment the HT line got disconnected from the pole ( a case of open circuit), some CB should trip, indicating unbalance.

what are your views?

Regards,

PG

What about the last type of fault. I think, the moment the HT line got disconnected from the pole ( a case of open circuit), some CB should trip, indicating unbalance.

Like a loss of load trip. I am not sure as I have never come across this specific form of protection. It may just be implemented at the SCADA level as a warning to the control room operators that the load to a feeder has suddenly been lost, as the common problems are momentary or permanent short circuits due to trees touching lines or lines knocked down by cars hitting power poles. Normally the fallen power line has to fall on something and create a fault path thru ground back to the protection (in the case of an earthed electrical system).

I agree in principle, but do they do that on the HV side? It would be interesting to find out!!!

All the protection monitoring I deal with is on the HV side, as is most of the lines company SCADA operator control room monitoring. LV transformers branching off the HV lines are protected by their own fuses so in the event of a problem on the LV the fuse opens and isolates a few (or one) customers, and generally the fault will occur inside a specific home and as such will be disconnected by the customers protection inside their home before the LV transformer fuse operates (hooray for circuit protection discrimination).

So you're saying that not all earth connections should be considered a good ground. Interesting.

And fully correct.

It also depends upon when it last rained as to how conductive or not, the ground is at any one point.....what the composition of the ground is, how far away it is from the sunstation etc etc etc etc.....

Sorry, "Substation" was meant, it was dark and the characters are next to each other on the keyboard....

So you're saying that not all earth connections should be considered a good ground

Or at the same potential.

We have one site example where the earth resistance between a substation and remote piece of concrete pad-mounted metal switch gear was large enough to create a potential difference between the switchgear metal case and the ground next to switchgear, during a permanent low current phase to earth fault (to low to be detected by the MCB protection). Some time later a child standing in the grass touched the metal switchgear and recieved a shock, fortunately the child was not seriously injured.

An earth is an earth is an earth? Yeah right, its a lot more complicated than most people think (that's why we Engineers get paid the big bucks end up trying to sort out all the real world problems).

Apparently we must have overloaded YouTube. The link given in post #9 has been taken off line. I saw it earlier but it is no longer available.

Many years ago I was staying at a farm in Maine. A hurricane knocked over a high voltage pole in a field, and a cow that was near the live wire was electrocuted. When the workers came to fix the power line, they wore rubber boots and took very small steps as they approached the live wire. Upon inquiry, they told me that the surface of the earth is not always at ground potential. Voltage gradients from the wire potential to ground can easily be lethal and chances of electrocution increase with the distance between ones feet. The poor cow didn't know this and even if she did, there was probably little she could do about it.

So, if you are playing golf and a thunderstorm comes up, stand with both feet firmly on the ground, and your hands gripping a putter likewise held to ground.This will form a 3 legged voltage divider so it wont hurt so bad when you get struck by lightning (:;).

welderman brought up an important point and I think someone with the knowledge should clarify it. I took an OSHA class and one of the discussions was on how to dis-mount from an accidently energized vehicle. Now my memory is fuzzy on it, but it seemed to involve jumping clear and shuffling feet, not lifting feet above surface to exit danger zone. Anyone qualified to discuss this. maybe prevent a tragedy in the future?

And those rubber tires, definitely NOT INSULATED!

If the vehicle is not burning or arcing and there is no danger of explosion, it's best to remain in the vehicle, without touching door handles, frame or any other metal object until the voltage source has been disconnected or removed and the vehicle deenergized.

Here are another couple of You Tube videos of power lines discharging directly to earth without causing immediate circuit breaker interruptions.

http://www.youtube.com/watch?v=p3T9qF0nMxY&feature=related

The above video shows a live cable lying on the ground. The constant quiet growl in the audio background implies to me that this is just 120/240VAC house current.

http://www.youtube.com/watch?v=zcW3dbyRLxY&feature=related

The above video shows a power line that has fallen into a creek. The arc is very bright.

People wrongly think that circuit breakers are to protect people, they are not. They protect wiring from overheating from excessive current. As somebody earlier stated, Ground Fault Circuit Interrupt (GFCI) protect people. This is why in the US they are mandatory for outlets installed in close proximity to plumbing and outside services.

I had a 25 kva sub 12.46 kv where a phase fell into a ditch with water and did not trip the breaker. I stopped and watch the water boilling. The sub was fairly close by and the ground fault relay was set to pickup at 250 amps but never triped the breaker..

You are absolutely right. This has been a common problem in india, especially during rains seasons and is story of many movies.

Regards

PG