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Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
wrote ...
You need to be brought up to potential though, and with the vast amount of current available, I would imagine that could happen rather violently.
I don't think it would be a big deal. For example, when helicopters clip on to high voltage lines, the resulting arc between the helicopter and the line is NOT due to the helicopter charging up to the potential of the line, but rather due to the static discharge from the helicopter to the line. Take a large VDG, and stand on an insulated box a good 5-10 feet away with a pole. Now, touch the pole to the large sphere topload of the VDG. This large sphere can produce a high peak current discharge, but you'll find out nothing really happens.
Plus, if you are jumping through air to a line, assuming you are far away from nearby objects, once you hit the line, other than the stray capacitance from yourself to ground (which is very minimum), there is no current path for current to flow.
Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
EastVoltResearc wrote ...
wrote ...
You need to be brought up to potential though, and with the vast amount of current available, I would imagine that could happen rather violently.
I don't think it would be a big deal.
I would have thought it WAS a big deal. For example, many of us have been zapped by a single lead from a PF cap, and I'd argue that a person standing indoors with rubber shoes is not a very tempting ground path.
In fact, couldn't you estimate the capacitance of the human body with an approximation of the topload formulae you guys use for TCs? And then estimate the current over time required to bring you to potential? It seems to me that the current you feel is limited only by the resistance of the source.......
Registered Member #188
Joined: Thu Feb 16 2006, 05:18PM
Location:
Posts: 67
Andrew wrote ...
EastVoltResearc wrote ...
wrote ...
You need to be brought up to potential though, and with the vast amount of current available, I would imagine that could happen rather violently.
I don't think it would be a big deal.
I would have thought it WAS a big deal. For example, many of us have been zapped by a single lead from a PF cap, and I'd argue that a person standing indoors with rubber shoes is not a very tempting ground path.
In fact, couldn't you estimate the capacitance of the human body with an approximation of the topload formulae you guys use for TCs? And then estimate the current over time required to bring you to potential? It seems to me that the current you feel is limited only by the resistance of the source.......
I would expect it to be a more or less big deal while standing on the ground, yes. I measured myself about 80pf (although the RLC meter could be confused by the high ESR (wwith a wet copper pipe held in hand firmly as connection around 1kOhm) and read wrongly but 80pf agrees well with the values stated in many ESD related publications) while standing on a 5cm thick HDPE sheet, (some isolation that wont break down at 15kV for sure) and at least 2 meters away from any wall. I didnt try but i would expect to feel a zap if id touch one output from a 5kv supply (other output grounded) but nothing overly painful or even dangerous.
Now if i was far from any grounded object my capacitance to ground would be more or less the free space capacitance, im estimating it to be around 10-20pf. Probably wouldnt really hurt to touch a 5kv supply then.
So id say it would be "safe" to climb to the wire using a huge DRY styrofoam block to stand on.
Corona discharge is another thing but i dont expect it to be a issiue at 5kv.
Now all this is assuming DC!
For AC, a current determined by the reactive resistance of your capacitance and voltage would flow, possibly having bad effects.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
wrote ...
In fact, couldn't you estimate the capacitance of the human body with an approximation of the topload formulae you guys use for TCs? And then estimate the current over time required to bring you to potential? It seems to me that the current you feel is limited only by the resistance of the source.......
Assuming you are well grounded. If you were simply floating in the air (i.e. jumping to a wire well away from ground), then you really wouldn't feel anything when you contacted the wire.
Try this experiment.
Take any capacitor, connect it to a voltage source, leaving the other lead just floating in the air, and you will not charge that capacitor.
wrote ...
For example, many of us have been zapped by a single lead from a PF cap
Uh yeah . . . Thats because the case of a PF capacitor is electrically hot, and the plastic covering of these caps is usually quite poor, especially at higher voltages. So, just holding the cap in your hand, and touching one terminal will usually get you a shock, especially if you hold your hand around it (the plastic will actually act like a dielectric and create another small capacitance)
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
wrote ...
EVR, when you attach voltage to a single lead of a cap, indeed current does flow, does it not?
How can it? Remember, looking at the source (the voltage potential you are speaking of hooking up to one terminal of the capacitor), the sum of currents into and out of the source must equal zero. If you are putting current into one lead of the capacitor from the source, then current must be returning back to the source.
Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
I see what youre saying... however!
Consider two capacitors, both charged. If one was to connect the positively charged plate on one cap to the negatively charged plate on the other cap while leaving the two other leads floating, the two connected plates would neutralize by distributing the excess charges. there WOULD be current in that junction then, without any current returning to the source... this was what I was imagining... flawed?
Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
Could you enlighten me as to why nothing happens?
If the inner plates become electrically connected, they become a single conductor; I thought there was something that said only insulators can maintain charge gradients, or some such.
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
potential is relative! you are doing the same thing as putting two batteries/caps in series? without a discharge path? so yea, you are just stacking voltage. caps are all insulated and voltage is floating.
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Moron Alert
Thats because the case of a PF capacitor is electrically hot, and the plastic covering of these caps is usually quite poor, especially at higher voltages. So, just holding the cap in your hand, and touching one terminal will usually get you a shock, especially if you hold your hand around it (the plastic will actually act like a dielectric and create another small capacitance)
