Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Overhead electrical lines and vehicles

  1. Dec 8, 2014 #1
    Hi all... practical applications here...

    I have questions about RV travel, or travel trailer/ 5th wheel and electrical safety.

    With taller RV's or trailers, there is a danger of striking low power lines in the more rural areas.

    My question is specifically and only about what happens *when* the power line is *not* noticed... and there *will* be a collision between the vehicle and the power line.

    First... what are the specifics of what might happen when the strike occurs? I assume occupant electrocution, fire danger, damage to electrical systems on the vehicle. Others??? Are any of these unlikely?

    I suppose it depends somewhat on what power the line is carrying...

    For low rural wires, I tend to want to assume lower voltages. 240? Might there be others?

    Then finally... what can be done to an RV ahead of time, to protected *in the event* of a strike, to protect against these kinds of damage? (electrocution, fire, electrical system).

    I suppose there is the physical damage as well (a cable that catches a vehicle and doesn't break might do a hell of a lot of physical damage).

    Thank you for your thoughts on this...

  2. jcsd
  3. Dec 10, 2014 #2


    Staff: Mentor

    The consequences range from nothing to death.

    This YouTube video shows a truck, not an RV, that a line. Rescuers were unable to get the driver out.

    Firemen are forbidden to approach the truck or the downed line because they too might get electrocuted.

    [Moderator's note: removed a comment quoting a deleted post]
    Last edited by a moderator: Dec 10, 2014
  4. Dec 10, 2014 #3


    User Avatar

    Staff: Mentor

    Yes, and unfortunately the higher voltage is more likely in rural areas. The voltage in the power lines is typically 2 KV or more; it's stepped down to 240/120 by a transformer very near the building(s) being served. In a city, a single transformer may feed multiple buildings so some of the lines on the poles are 120/240 from the transformer to nearby houses. In a rural area, chances are that the line along the road is the line to a transformer, so carrying distribution voltages.
  5. Dec 10, 2014 #4

    Doug Huffman

    User Avatar
    Gold Member

    The particular voltages don't matter, 120 VAC is just as fatal as distribution voltages. The zero current/power trip on the circuit is the significant protection. The vehicle tires are significant.

    My father was a Cupertino PG&E distribution engineer and gave me a 25 cubic inch chunk of pavement boiled and blackened by a transmission line which trips failed.
  6. Dec 10, 2014 #5
    Hi Doug...

    I'm pretty sure that voltages have to matter... that's why distribution lines are low and easily accessible... while HT lines are suspended hundreds of feet into the air.

    For any given system (r), the sole determining factor of current is voltage. That which determines the lethal component of discharge through a given system has to matter.

    For example... and this is slightly off the question being posed... but in a *near miss* of a tractor trailer to a power line... a 1 inch gap will not likely be bridged by a 120vac source.... however, a 30,000 volt line... passing 1" above a tall truck let's say... a discharge is far more likely, is it not?

  7. Dec 10, 2014 #6
    Thanks, anorlunda... I did happen to see such videos (including this one) in my search for information. But of course in physics... the consequences ranging from nothing to death (and fire, etc), are where physics comes in.

    You can work with and even "touch" very high voltage systems... and you don't have to die (no consequences). These aren't random. Not acts of god. So understanding/dealing with the actual physics so that the former instead of the latter consequence occurs is what science/engineering is all about.

    What I notice about the events with serious consequences... is that they seem to correlate with being "stuck" on a line, or otherwise in continual contact... leading me to wonder if brief/controlled contact is a far safer situation.

    For example, helicopters face a similar danger with respect to the physical damage of a cable strike. To combat this, they have devices that, along with the shape of the airframe, cause the cable to ride up or down to a cable cutter as the aircraft moves forward into the obstruction. Causing the cable to part allows the contact with the cable to be brief, and therefore more survivable. For electrical hazard, clearly the specifics are different... any instantaneous discharge through a body is a big issue... in a practical sense, there is no such thing as a "brief" discharge through your body from a high voltage/high current source.
  8. Dec 10, 2014 #7


    User Avatar

    Staff: Mentor

    If Doug is trying to say that you shouldn't be complacent just because it's "only" a 240V line, he's right - 240V is lethally dangerous.
    And you are right that higher voltages are even more lethally dangerous.

    You don't want to drive your RV into any power line carrying any voltage for the same reasons that you don't want to to drive your RV off a cliff.
  9. Dec 10, 2014 #8
    Well, that's a useful thing to know... (higher voltages in rural areas).

    But in this case, I have to wonder about only one kind of line... and that would be the low lines that directly cross a roadway.

    There has to be some confusion about what is exactly crossing a road. A wire is could be phone or cable service, I suppose.

    But it's best to assume that it's going to be power. Perhaps state or county regs have a strong influence on this. These visually appear to be the smallest lines in the system. Thicker lines run along a road, and tend to cross high at intersections... but the thinner, low lines that cross at unpredictable locations, bringing power in lot by lot or house by house, are quite probably stepped down, yes? I guess these are questions for the power company...

    But for purposes here, let us assume somewhat higher voltages.

    For an RV striking a 2-3KV line, and a range of damage possible (fire, electrocution, etc)... the issue of survivability comes down to physics.

    I notice a cursory correlation between a strike that is sustained (the vehicle hangs up on the line), versus a strike that happens briefly, then is broken.

    This seems to point to preventing a snag, first and foremost... this would seem to accomplish three things: little chance of downing the wire, little chance of downing a pole, a higher likelihood that the vehicle will be stopped after the incident, and no longer be subjected to the voltage (making it safe to exit the vehicle, or be tended to by EMS).

    But this brings into question some ancillary issues:

    What happens to the engine during the strike?
    What happens to the breaking system during the strike?
    What happens to the driver during the strike?

    If current passes into the metal skin of the vehicle... then through the tires to ground... then is there a high likelihood that the driver will not be especially affected during the initial strike? If so, then this would allow the driver to control the vehicle... (like steering and application of brakes).

    (I notice that drivers tend to die when they hop OUT of the vehicle, while still holding onto it: Path = vehicle -> person -> ground).

    If we have vehicular control... we now have to ask what is the condition of the vehicle? Is it likely that the engine is still running for several seconds? Is it likely that the brake system is still working?

    In the case of electrical brakes (which many/most trailers have), will these probably be disabled? Or the converse... might they be fully actuated during the discharge ? (after all, they are electrically actuated!) If the latter, this might be a problem... rapidly stopping a trailer while in contact with the line, might cause the vehicle to essentially park itself while in contact...

  10. Dec 10, 2014 #9

    Interestingly, I have driven a VW bug off a cliff... with a drop of only three feet. Apart from embarrassment... no real damage. I think the old bugs were tougher than the new ones.

    But this does bring up an interesting distinction... and that is the case of *not* driving into the line, but merely passing a fraction of an inch beneath it. I think that the potential (no pun intended) for an incident in these "near miss" situations might be important to understand...

    Do you think that such a thing might also be a dangerous situation? Or do you think that only in the cases of a strike?
    Last edited: Dec 10, 2014
  11. Dec 10, 2014 #10

    Doug Huffman

    User Avatar
    Gold Member

    The vehicle is insulated by the tires. The cab/body is a Faraday cage of some effect.
  12. Dec 10, 2014 #11
    That's right. Step and touch voltage outside the vehicle is higher, higher is the power line voltage. Just one more reason HV line is more dangerous than LV line.

    Ask yourself why HV power lines pylons have a long insulators (higher the voltage, longer the insulator required).
    Are you sure?*

    *rethorical question
  13. Dec 10, 2014 #12

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Typical distribution is in the 4 to 32 kv range,
    A clean dry tire ought to withstand 16KV.
    We had a sailboat on a trailer being pulled out of the water , the guy got his mast into a medium voltage distribution line and the wet salty trailer tires burst into flame. Luckily the line relayed out but his fiberglass hull caught fire too..

    Woe to the guy who steps out placing one foot on the ground....... his left leg just completed the circuit, same as would a wet tire.

    If you get into that situation, be aware the car body will make a Faraday cage around you. Sit still until you figure out what to do.
    Foremost, dont complete a path from vehicle to earth with your own body parts.
    Myself I'd try to extricate the vehicle, maybe backing out from under the wire. If exit were a must because of fire i'd leap as far as i could and hop away on one foot.

    old jim
  14. Dec 10, 2014 #13
    In many cases contact of road vehicles with 10kV+ power lines results in arcing. This means tyres are not clean and dry enough.

  15. Dec 11, 2014 #14


    Staff: Mentor

    I agree when you know who your audience is. But this is a public forum. Not only that, but it is archived and searchable meaning that it persists indefinitely with the whole world as a potential audience.

    Common sense should make us fear that non science/engineering people might read portions of this thread in the future, and be led to behavior that causes serious harm.

    Just as doctors should not joke on a public forum about fun ways to make psychedelic drugs in the kitchen, neither should we shop talk on public forums about subjects that can be hazardous to laymen readers in the future.

    The self-censoring rule I strive to follow before posting anything online is, "Would I be upset if what I write ends up on the front page of tomorrow's newspaper?" If the answer is yes, then do not press SEND.

    I'm distressed that this whole thread is part of the record. I hope that the mentor will delete it.
  16. Dec 11, 2014 #15


    User Avatar

    Staff: Mentor

    Pending moderation
  17. Dec 11, 2014 #16


    User Avatar

    Staff: Mentor

    Anorlunda has explained the reasoning behind the PF policy on dangerous activities well. There's also this recent thread.

    I thought about it, and I've already taken down one post that was egregiously wrong and irresponsible. I've closed the thread, but I'm going to leave it up because I think on balance the discussion is now making the right points. But lest there be any misunderstanding about what we're saying here:

    Line voltage can kill you or people around you. Some ways of mishandling it are more likely to kill than others, but that does not make any mishandling acceptable under any circumstances.
    Playing russian roulette with one chamber loaded is a far safer situation than with two chambers loaded. But the Darwin award was earned when you decided to play at all, not when you started thinking about how many chambers were loaded.
    Last edited: Oct 22, 2015
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook