Grounding and electrical wires outside of home question.

[Mint.]

Hey guys,
I got a question here. Taking a look at this picture, will the jumping kid on the trampoline get electrocuted if he hits the wire that is going into a house mid air? Am I correct that the answer is no because he is not grounded electricity will not pass through him?

[Psi]

Yeah, he would be safe if he only touched one of the wires while the rest of his body was in midair.

However if he grabbed hold of the wire, got scared and started to urinate then things might not go so well.

Do i get points for doing your homework? 

[IanB]

I already answered this question for you in the other thread. Remember that voltage is the potential difference between two points. When the kid hits the wire the wire becomes one point. Where is the other point? If you don't have two points with different potentials connected by a conductor (the kid is the conductor in this case) then no current can flow. It's like connecting a bulb to a battery with only one wire. The bulb will never light up in that case.

[Mint.]

Perfect thanks guys, was just double checking if my understanding is correct. 

[IanB]

And now a homework question to follow. Look at all the sparks flying in this video--where do they come from, given that our flying man is floating in the air and not in contact with the ground?

[Psi]

hehe 500kV is a little different to 110/230v mains

[Zero999]

And now a homework question to follow. Look at all the sparks flying in this video--where do they come from, given that our flying man is floating in the air and not in contact with the ground?

Again, the answer has already been hinted on in the other thread.

[Mint.]

And now a homework question to follow. Look at all the sparks flying in this video--where do they come from, given that our flying man is floating in the air and not in contact with the ground?

Perhaps like said in the video it had different voltage potential, so it was like static electricity, when he brings the rod to the wire the electrons equalize between the wire and the helicopter and don't move anymore therefore not creating sparks?

[Mechatrommer]

However if he grabbed hold of the wire, got scared and started to urinate then things might not go so well.

didnt electricity contracts muscle? when the circuit is grounded, his bladder might contract and stop urinating, hence opening the circuit again. scared again and urinating again. might be a funny thing looking at that guy pulsing urine at 50-60Hz

the guy in the helicopter still need faraday cage. a short burst of high V (and A) through his body to the helicopter (capacitor) still can kill him instantaneously i think. its like semiconductor got damaged by HV electrostatics.

[IanB]

Perhaps like said in the video it had different voltage potential, so it was like static electricity, when he brings the rod to the wire the electrons equalize between the wire and the helicopter and don't move anymore therefore not creating sparks?

That's basically it, although the system is not quite static. If the wires are carrying (say) 250 kV AC, then the potential of the wire is changing from +354 kV to -354 kV, 60 times a second. (354 kV is the peak voltage of a 250 kV RMS sine wave.) When the helicopter is touching the wire, then in order to equalize the potentials the helicopter has to change from +354 kV to -354 kV in step with the wire, and to achieve this current has to flow back and forth between the wire and the helicopter. The helicopter is acting as a capacitor, or storage container for charge, and current has to flow to make a capacitor change its voltage.

Since the potential on the wire is constantly changing with the AC voltage the sparks would fly forever and never stop unless they bonded the helicopter to the wire with the metal clamp in the way they showed.

(You may ask, how does the wire "know" the helicopter is at a different potential when it is isolated in mid-air like that? The answer is that the high voltage wire has previously had it's voltage referenced to ground at the transformer station by an earth bond on the neutral conductor. Tracing back in time the helicopter also was touching the ground before it took off, so the helicopter started out at ground potential with reference to the high voltage wire. Since the helicopter is acting like a capacitor, it has remembered it's potential in the same way that capacitors hold on to voltage.)

[JonnyBoats]

Lets not forget that the rotation of the blades on the helicopter is generating static electricity. Even if the transmission wire were at zero volts (grounded) sparks would still fly.

Just like when you walk across a carpet on a dry day.

Whenever a rescue helicopter is going to lift someone off a ship or boat  using a cable it is important that nobody touch the cable that is being lowered until it has been grounded, either in the water or on the metal deck. Motor lifeboats carry special insulated poles just for the purpose of snagging the cable being lowered.

[Zero999]

(You may ask, how does the wire "know" the helicopter is at a different potential when it is isolated in mid-air like that? The answer is that the high voltage wire has previously had it's voltage referenced to ground at the transformer station by an earth bond on the neutral conductor. Tracing back in time the helicopter also was touching the ground before it took off, so the helicopter started out at ground potential with reference to the high voltage wire. Since the helicopter is acting like a capacitor, it has remembered it's potential in the same way that capacitors hold on to voltage.)

The high voltage system wouldn't even need to be earth bonded in order for sparks to fly, even if it floated all the wires would still have enough capacitance to allow a current to flow.

[Mint.]

Another question. The wires seem to be insulated to me, but why do sparks fly from them if they are insulated? 

[IanB]

Which wires seem to be insulated?

The electric power transmission cables are bare metal--they are insulated by the air surrounding them, and by the big glass or ceramic insulators they are suspended from.

Although air is an insulator, when the electric field is strong enough the air breaks down and lets the electricity pass through--that's when you see the sparks.

[Mint.]

Really are they bare metal? Because the wire going into my house is insulated, however I am not sure about the power lines... 

[IanB]

Yes, absolutely they are bare metal. Wires only need to have insulation around them when they might touch something. If they are suspended in the air then there is no need.

[Mint.]

Wow alright, thanks fro pointing that out to me.