I know a power transmission line carries usually two sets of 3 phase conductors. But I have noticed a relatively thin wire at the center-top of the power transmission line. What is the purpose of this thin wire?
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1\$\begingroup\$ Do you have a picture? \$\endgroup\$– jippieCommented Jun 22, 2013 at 13:29
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3 Answers
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Wikipedia says they are ground wires to protect the live wires from lightning.
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\$\begingroup\$ So is this ground wire grounded at each tower structure? \$\endgroup\$– KawaiKxCommented Jun 22, 2013 at 11:14
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\$\begingroup\$ @Saurabh I don't know - I had assumed it was a steel wire for structural purposes, but checked before posting, and this was not the case, so don't really know more than the link says. \$\endgroup\$ Commented Jun 22, 2013 at 11:15
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\$\begingroup\$ found another link that confirms that the ground wire is grounded at each tower "Towers do not transmit electricity themselves unless lightning strikes the ground wire strung along the top of the structure. This cable is designed to protect conductors by allowing lightning discharges to reach the ground through the tower." hydroquebec.com/learning/transport/types-pylones.html \$\endgroup\$– KawaiKxCommented Jun 22, 2013 at 11:20
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\$\begingroup\$ Some utilities will also have fibre or pilot wires up top too for comms and/or protection signalling, they can be incorporated into the overhead earth wire. \$\endgroup\$– RagglesCommented Jun 23, 2013 at 8:27
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\$\begingroup\$ From memory, the ground wire is not necessarily grounded at each and every tower. When using wooden poles, a cable has to be run from the poletop to an earth stake, which is expensive; and there's no step/touch voltage hazard (wood isn't conductive); so they don't bother earthing every wooden pole. \$\endgroup\$ Commented Apr 30, 2015 at 14:15
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Top of the cable is ground wire rest and other (1 or 3 lines) are phase cables. It is just for the protection from lighting strikes. They serve to shield the line and intercept lighting stroke before it hits the current carrying conductors below. That's why they are often made of steel.
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As stated when they use the ground wire for communication it have the signal inside its. And yes, the ground wire is grounded on each tower (one of the best transmission type for protection of electric systems is the fiber optical inside a grounded wire). In some arrangements, there are more than one guard cable. It can be up to 3, depending mostly of the size of tower and placement of the phase cables. The worst case I've ever seen is below
answered Mar 20, 2015 at 10:34
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1\$\begingroup\$ Why is it the worst case? I can't see the bottom wire protecting against lightning, but maybe it shields against electrical noise or capacitive link to lossy ground below the power line? What is worst about that? \$\endgroup\$– user56384Commented Sep 2, 2015 at 23:15
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\$\begingroup\$ @nocomprende you have to realize lightning goes both ways... we think of it from the sky down, but there is also a counter strike from the ground up. \$\endgroup\$– Trevor_GCommented Jun 5, 2017 at 15:59
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\$\begingroup\$ @Trevor I used to think that burying a cable would protect it from lightning, but I read that if lightning strikes nearby, it will be drawn to the cable as a conductive object and can even burn through any protective outer shielding. This is probably why there hasn't been a bigger push to buried electrical service: it doesn't give that much more protection. \$\endgroup\$– user56384Commented Jun 5, 2017 at 16:11
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\$\begingroup\$ @nocomprende yes indeed, plus buried cables are harder to cool. \$\endgroup\$– Trevor_GCommented Jun 5, 2017 at 16:16
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\$\begingroup\$ I should've used "the most pessimistic", maybe \$\endgroup\$ Commented Aug 29, 2017 at 21:22