Grounding and electrical wires outside of home question. - Page 1
Forum section: electronics / general

author: Mint.
author_rank: Frequent Contributor
timestamp: November 19, 2011, 04:47:38 am
content: 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?

author: Psi
author_rank: Super Contributor
timestamp: November 19, 2011, 05:56:37 am
content: 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?

author: IanB
author_rank: Super Contributor
timestamp: November 19, 2011, 07:22:32 am
content: I already answered this question for you in the other thread. Remember that voltage is the potential difference betweentwopoints. 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.

author: Mint.
author_rank: Frequent Contributor
timestamp: November 19, 2011, 10:01:16 am
content: Perfect thanks guys, was just double checking if my understanding is correct.

author: IanB
author_rank: Super Contributor
timestamp: November 19, 2011, 10:09:43 am
content: 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?

author: Psi
author_rank: Super Contributor
timestamp: November 19, 2011, 10:58:27 am
content: hehe 500kV is a little different to 110/230v mains

author: Zero999
author_rank: Super Contributor
timestamp: November 19, 2011, 11:05:28 am
content: Quote from: IanB on November 19, 2011, 10:09:43 amAgain, the answer has already been hinted on in the other thread.

author: Mint.
author_rank: Frequent Contributor
timestamp: November 19, 2011, 08:11:34 pm
content: Quote from: IanB on November 19, 2011, 10:09:43 amPerhaps 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?

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 19, 2011, 09:17:18 pm
content: Quote from: Psi on November 19, 2011, 05:56:37 amdidnt 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 50-60Hzthe 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.

author: IanB
author_rank: Super Contributor
timestamp: November 19, 2011, 10:39:23 pm
content: Quote from: MintyCondition on November 19, 2011, 08:11:34 pmThat'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.)

author: JonnyBoats
author_rank: Regular Contributor
timestamp: November 20, 2011, 06:02:13 am
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: November 20, 2011, 01:33:16 pm
content: Quote from: IanB on November 19, 2011, 10:39:23 pmThe 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.

author: Mint.
author_rank: Frequent Contributor
timestamp: November 20, 2011, 08:04:51 pm
content: Another question. The wires seem to be insulated to me, but why do sparks fly from them if they are insulated?

author: IanB
author_rank: Super Contributor
timestamp: November 20, 2011, 08:41:50 pm
content: 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.

author: Mint.
author_rank: Frequent Contributor
timestamp: November 20, 2011, 08:57:01 pm
content: Really are they bare metal? Because the wire going into my house is insulated, however I am not sure about the power lines...

author: IanB
author_rank: Super Contributor
timestamp: November 20, 2011, 09:22:38 pm
content: 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.

author: Mint.
author_rank: Frequent Contributor
timestamp: November 20, 2011, 09:35:18 pm
content: Wow alright, thanks fro pointing that out to me.