How unsafe electricity can be.

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Cariboocoot
Cariboocoot Banned Posts: 17,615 ✭✭✭
in General Solar Power Topics #1
This evening's news:
An electric line in Mission, B.C. broke (probably due to the storms), fell, and energized a stop sign it hit. That sign's post was deep in the ground, and close to a gas main. The pipe ruptured from the underground current and ignited.

Right now it is burning because the gas company truck is stuck in traffic; it hasn't got there to shut off the feed.

Now think about that: an energized line breaks and hits a metal sign post in the ground. That should ground it and pop the line breaker (yes, they have them) and make it safe. Instead there was enough energy for long enough, underground, to zap and ignite a gas main.

Do you wonder why we have so many discussions about how best to ground systems for safety and lightning protection? Even when you take precautions it doesn't always work and some very strange and dangerous things can happen.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #2
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    Re: How unsafe electricity can be.

    My boat dock has 600 amp main fuses, 480 v, 3 phase, the under water cable has broken several times and those fuses never blow. You can see this glow under water and bubbling and smoke rises up to the surface of the water and the cable moves around like a snake. It will rise up to the surface and then fall back into the water, kind of scarey. The power company has to come cut it off.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #3
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    Re: How unsafe electricity can be.
    Cariboocoot wrote: »
    Now think about that: an energized line breaks and hits a metal sign post in the ground. That should ground it and pop the line breaker (yes, they have them) and make it safe.

    Why should that pop the line breaker? The path to ground is not low enough resistance to pop the breaker. This is why ground and arc fault breakers exist, although I don't know how practical these are for high voltage main lines.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #4
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    Re: How unsafe electricity can be.
    vtmaps wrote: »
    Why should that pop the line breaker? The path to ground is not low enough resistance to pop the breaker. This is why ground and arc fault breakers exist, although I don't know how practical these are for high voltage main lines.

    --vtMaps
    Then perhaps it's time for utilities to install GFbreakers on their HV main lines :D
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #5
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    Re: How unsafe electricity can be.
    waynefromnscanada wrote: »
    Then perhaps it's time for utilities to install GFbreakers on their HV main lines :D
    Maybe so, maybe not. It is impossible to protect against every freak accident that could possibly occur.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
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    Re: How unsafe electricity can be.
    ggunn wrote: »
    Maybe so, maybe not. It is impossible to protect against every freak accident that could possibly occur.

    Which is the point here.
    The HV line shorted to ground should have pulled enough current to pop the line breaker; that's why they put those breakers in.

    It's the sort of thing that makes me question NEC's fuse/breaker sizing formula (not that it is used on HV lines).
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #7
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    Re: How unsafe electricity can be.

    Falling lines/crosses with tree limbs are very common events. And the amount of current/power flowing in shorted lines would seem to be a non-trivial amount of current--Would need to be install on each distribution transformer.

    But, as I under stand, the present utility distribution protection scheme is designed to automate resting of the grid anyway... If there is enough current flow to cause a fault, the breakers will trip, pause a bit, then switch back on--and repeat a time or several. The idea is if this is a "temporary" fault (squarell across power line, tree branch or bark on line, the arcing can clear the short.

    Which brings up the question that an arc can weaken the power line and eventually be a point of failure (line separation) months/years down the road.... But so far, folks are just warned to stay away from downed line. One of those "acceptable risk" questions.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
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    Re: How unsafe electricity can be.

    The arcs are usually the worst problem for fallen lines; rag ends of wires loose and sparking but never enough contact to draw current and trip the CP. In this particular instance there appears to have been enough contact to push enough power through an underground gas main and rupture it and ignite the gas. You'd hardly think that possible.

    Of course the report could have been erroneous too, as what we hear first thing through the media is not always exactly what happened as revealed in the investigation.

    Arc fault protection on HV lines would seem like a good idea. Although I know one power company that would have real trouble with them as their lines were always in such bad shape that the arcs were necessary to get the electric to flow at all. :p
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #9
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    Re: How unsafe electricity can be.

    Gas distribution mains in our area run at something near 400 PSI -- These is not insignificant pressure and if anything weakens the gas main steel piping (such as heating above ~400F), all bets are off... I used to be sort of worried about a 50 gallons air compressor tank at 140 PSI--Not so much when I think of 3 foot gas mains with 350+ psi in them buried since the 1950's with miles of stored natural gas under pressure.

    One of the many little issues that came up during the PG&E investigation after the major blowout just south of San Francisco was that the leak detection unit got bonuses if they found fewer/smaller/lower merit leaks (heaven forbid that they actually got bonuses for finding more verified leaks)... And surprise! They found less leaks after the bonus plan went into affect...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
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    Re: How unsafe electricity can be.
    BB. wrote: »
    One of the many little issues that came up during the PG&E investigation after the major blowout just south of San Francisco was that the leak detection unit got bonuses if they found fewer/smaller/lower merit leaks (heaven forbid that they actually got bonuses for finding more verified leaks)... And surprise! They found less leaks after the bonus plan went into affect...

    -Bill

    To the logical mind that is entirely backward as finding (and correcting) the major leaks would be more beneficial to the company. If they don't believe this, they should take another look at the result of that major blow-out. Especially the liability issues. :roll:
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #11
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    Re: How unsafe electricity can be.

    I think the premise was that finding less leaks indicated the company's pipeline maintenance was "improving" so bonuses for all (at least all management). :p

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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