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Post by Lokifan on Dec 11, 2014 20:03:31 GMT
I heard a new myth from my wife, and it reminded me of an old one... Myth 1: Psychic Phone Dog I first heard this myth thirty years ago while still in school. The story goes that a person claimed his dog was precognitive--he could predict the future. The owner said that whenever the phone was about to ring, the dog began barking and howling. A phone repair person was called to analyze the situation. It turned out that the phone system was grounded to a metal stake to which the dog was also chained. The ground around the stake was dry, usually, and a bad conductor. So, when the 48V ring signal came into the phone, the dog would feel a shock and begin to bark or howl, and would also act as a dummy load that would prevent the phone from ringing. After several jolts, the dog would urinate on the stake. The urine improved the conductivity for the ground, and essentially "shorted out" the dog's load effects, letting the phone ring. Myth 2: Killer Crossing light This one my wife told me. She said it was in the news but I didn't see it. The story is that a woman was walking her very small dog on a metal leash. She went to cross a street, pressing the crossing signal light button. As soon as she did, her dog dropped dead. Supposedly, the light button was miswired in some manner, and allowed a slight charge to be present on the metal button. When pressed, although the current was too low to hurt the person, it was more than enough to kill the dog. I know electrical myths aren't usually photogenic, but fake dogs are popular!  |
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Post by GTCGreg on Dec 11, 2014 21:09:47 GMT
I've heard of the first one but never actually saw anything published on it. The second one I did see in the news but the story was a little different. The lady went to cross the street as the dog stepped on a manhole cover that was electrified by a short circuit in the electrical wiring underneath it. The dog was electrocuted but it had nothing to do with the woman pressing the walk button. In fact, there have been a number of news reports of people and/or animals being shocked by stepping on electrified manhole lids. www.nytimes.com/2006/03/04/nyregion/04voltage.html?_r=0 |
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Post by the light works on Dec 12, 2014 3:52:04 GMT
The first one is extremely improbable, and this is why: a telephone service is a complete circuit and the (earth) ground is only to prevent transient voltages. In fact, on a modern telephone service, the earth ground is connected only to the shield of the cable, and not to any of the circuit conductors. therefore, the ring signal would not be forced to ground, nor would the conductivity of the dog effectively short out the ring signal. also, if the stake was not making sufficient connection to ground, neither would the dog's feet. finally, if the dog was "sinking" the ring signal to ground, then by urinating on the stake and increasing its conductivity, it would continue to interfere with the ringing of the phone.
the second one is somewhat improbable. there would have to be several circumstances in place for such an electrocution to happen.
1: there would have to be a fault in the crossing signal system that was isolated from (earth) ground enough not to interfere with the operation of the signal or trip the circuit breaker.
2: the leash would have to be conductive from the collar to the part the lady was holding in her hand.
3: the dog's pads would have to be in contact with a surface that was an effective earth ground.
4: the lady's heart would have to be more resilient to electrocution than the dog's.
now, 2 and 4 could be at least partially bypassed by having the dog in a metal choke chain and having the chain leash contact the energized pole.
so all of the requirements are possible, but in combination, I would say the odds are against it.
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Post by Cybermortis on Dec 12, 2014 16:44:39 GMT
A similar myth was posted on the old MBFC about dogs getting fatal electric shocks when stepping onto metal hatch covers in the street. Turns out that this could well be true. It is documented that static electricity can build up on metal objects in the street, from fire hydrents to shop doors, and cause a shock.
A woman in New York was also electrocuted by stepping on a metal hatch cover - this was the official cause of death.
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Post by Lokifan on Dec 12, 2014 19:21:31 GMT
Well, at least that proves that just because a chunk of metal is on the ground doesn't make it electrically grounded.
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Post by the light works on Dec 13, 2014 3:13:01 GMT
Well, at least that proves that just because a chunk of metal is on the ground doesn't make it electrically grounded. the catch is there needs to be a dielectric between the object and the ground. that essentially creates a crude capacitor. it is also possible to electrically charge something and not have the ground effectively carry the voltage away - the shock victim essentially becomes a better conductor than the earth - this is sometimes called a voltage gradient - but usually happens at high voltages. |
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Post by Cybermortis on Dec 13, 2014 14:44:47 GMT
In the New York case I think the cause was bad wiring - as I said this was discussed in detail on the MBFC and I can still remember some of the discussions that went on. In other cases it is static electricity, which is well documented and a result of passing traffic etc helping build up a charge in some conditions.
The original 'Salt Shocker', which I seem to recall was posted by MBFC member Draco, was about winter conditions. The general throught was that either the snow was getting around the edges of the cover and insulating it, or that salt water (from gritting the roads) was dripping down and creating an electrical circuit between old and badly maintained electrical pannels under the cover and the cover itself. When someone stepped on the cover while the other foot (or paw) was on the sidewalk it completed the circuit in both cases.
The problem is, as always, exactly how to do electrical myths in a visualy interesting way. Viewers tend to expect electrical effects of the 'six foot wall of sparks' variety, and as this doesn't usually happen in real life get disappointed when it seems nothing is or has happened.
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Post by the light works on Dec 13, 2014 14:46:30 GMT
In the New York case I think the cause was bad wiring - as I said this was discussed in detail on the MBFC and I can still remember some of the discussions that went on. In other cases it is static electricity, which is well documented and a result of passing traffic etc helping build up a charge in some conditions. The original 'Salt Shocker', which I seem to recall was posted by MBFC member Draco, was about winter conditions. The general throught was that either the snow was getting around the edges of the cover and insulating it, or that salt water (from gritting the roads) was dripping down and creating an electrical circuit between old and badly maintained electrical pannels under the cover and the cover itself. When someone stepped on the cover while the other foot (or paw) was on the sidewalk it completed the circuit in both cases. The problem is, as always, exactly how to do electrical myths in a visualy interesting way. Viewers tend to expect electrical effects of the 'six foot wall of sparks' variety, and as this doesn't usually happen in real life get disappointed when it seems nothing is or has happened. you're right with that. |
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Post by Cybermortis on Dec 13, 2014 15:01:16 GMT
Hence why I keep asking for ideas as to how to show such electrical myths in a safe, impressive, cheap and easy to repeat way  |
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Post by the light works on Dec 13, 2014 15:29:18 GMT
Hence why I keep asking for ideas as to how to show such electrical myths in a safe, impressive, cheap and easy to repeat way unfortunately, the challenge is in doing it in such a way as to not perpetuate their own myth. |
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Post by ironhold on Dec 13, 2014 23:26:23 GMT
Hence why I keep asking for ideas as to how to show such electrical myths in a safe, impressive, cheap and easy to repeat way There's always the old hot dog trick. Here in the US, it's a fairly common high school-level experiment for kids to cook hot dogs using electricity. Kids are given a hot dog, shiny clean nails, wiring, and a power supply. If they hooked up the hot dog as a part of the circuit and set up the circuit itself properly, then as soon as the power supply was activated the hot dog would start cooking. |
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Post by tom1b on Dec 14, 2014 0:35:47 GMT
It's not static electricity, but stray electricity. Dog walkers warned of electrocution risk from stray voltage as snow meltsN.Y. Utility Looks for 'Stray Voltages' After Woman's Deathwiki: stray electricityAfter Jodi Lane was electrocuted in 2004 (2nd link), they made a stray voltage website just for ConEd. They seemed to stop updating in 2009. They found 2,384 energized objects in 2004 and 11,125 in 2009. Stray voltage is also a cause of corrosion. And testing is very safe and easy but not interesting. They have electrical hazard boots for people working with electricity. They are rated, according to ASTM F1117-03, "to withstand the application of 14,400 volts at 60 hertz for one minute with no current flow or leakage current in excess of 3.0 milliamperes, under dry conditions." They can add another insulating layer if concerned. Then, they switch out the sensors they used for walking over coals. Remove temperature sensors and put in voltage/current sensors. |
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Post by the light works on Dec 14, 2014 1:05:04 GMT
It's not static electricity, but stray electricity. Dog walkers warned of electrocution risk from stray voltage as snow meltsN.Y. Utility Looks for 'Stray Voltages' After Woman's Deathwiki: stray electricityAfter Jodi Lane was electrocuted in 2004 (2nd link), they made a stray voltage website just for ConEd. They seemed to stop updating in 2009. They found 2,384 energized objects in 2004 and 11,125 in 2009. Stray voltage is also a cause of corrosion. And testing is very safe and easy but not interesting. They have electrical hazard boots for people working with electricity. They are rated, according to ASTM F1117-03, "to withstand the application of 14,400 volts at 60 hertz for one minute with no current flow or leakage current in excess of 3.0 milliamperes, under dry conditions." They can add another insulating layer if concerned. Then, they switch out the sensors they used for walking over coals. Remove temperature sensors and put in voltage/current sensors. right. they already established how hard it is to build enough static electricity to become hazardous to a healthy person. |
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Post by silverdragon on Dec 14, 2014 12:17:22 GMT
You can walk under power lines holding a Neon tube light, and it will light up.
Way cool if you ask me...
But it begs the question...
WTF is going on?....
Is it "Safe"?...
I drive under power lines many times in one day.
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Post by the light works on Dec 14, 2014 15:49:40 GMT
You can walk under power lines holding a Neon tube light, and it will light up. Way cool if you ask me... But it begs the question... WTF is going on?.... Is it "Safe"?... I drive under power lines many times in one day. my non-contact tracer is driven more buggy by neon signs than it is by overhead power lines. |
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Post by OziRiS on Dec 16, 2014 11:23:32 GMT
Hence why I keep asking for ideas as to how to show such electrical myths in a safe, impressive, cheap and easy to repeat way I still think this is the best visual they've done on electrocution: The flash powder gives an instant idea of the result and it's easy to reset. What's more, it's a proven concept, so there's no need to re-invent the wheel. |
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