Connections to dirt do little below 600 volts

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iwire

Moderator
Staff member
Location
Massachusetts
I happened across this video of a power line down, it involves a swing set and a long length of metal fence.

I think it is a great example of how connections to the earth will do little for safety when dealing with low voltages.


 

kwired

Electron manager
Location
NE Nebraska
I agree that was a line operating over 600 volts. Resistance still wasn't low enough to allow enough current to operate whatever overcurrent protection was ahead of that line.

Firemen were in a danger area near that fence, after it became obvious that fence was energized they moved a little further away but were still within possible danger zone for step potential hazards.

I am pretty certain this same video has been linked to before on this site but has been quite a while ago.
 

kwired

Electron manager
Location
NE Nebraska
Though I don't do MV/HV, what I have seen for failures at 34.5 kV - those firemen near the fence are likely now dead if that was a 34.5 line. They are lucky as is if it is a 7.2kV line. I don't know if the typical fireman's boots have any electrical insulation rating - but those boots probably did protect those pictured in the foreground of that video compared to someone wearing casual footwear.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Though I don't do MV/HV, what I have seen for failures at 34.5 kV - those firemen near the fence are likely now dead if that was a 34.5 line. They are lucky as is if it is a 7.2kV line. I don't know if the typical fireman's boots have any electrical insulation rating - but those boots probably did protect those pictured in the foreground of that video compared to someone wearing casual footwear.
They do not.
 
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Tony S

Senior Member
Though I don't do MV/HV, what I have seen for failures at 34.5 kV - those firemen near the fence are likely now dead if that was a 34.5 line. They are lucky as is if it is a 7.2kV line. I don't know if the typical fireman's boots have any electrical insulation rating - but those boots probably did protect those pictured in the foreground of that video compared to someone wearing casual footwear.

One of the things I was taught about OH MV switching was the “fireman’s shuffle”, never put one foot in front of the other. More than once I’ve had a freezer full of lamb or beef following a fault.
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
How come nobody says to hop on one foot? Seems like a safe, quick way to get out of the danger zone. Just don't fall down!

I think that there is your answer. I'd much rather do the Tim Conway old man shuffle, thank you very much.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
How come nobody says to hop on one foot? Seems like a safe, quick way to get out of the danger zone. Just don't fall down!
They don't tell you to hop on one foot for that very reason. They do tell you to hop on both feet keeping them close together.
 

Tony S

Senior Member
At my age?

61551-283x424-Pogo-stick_zps3akqmh41.jpg
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Registered Professional Engineer
They do not.
Some of them do.

http://workingperson.com/viking-mens-black-vw91-snug-fit-firefighter-chainsaw-boots.html
For additional protection, this pair of EH firefighter boots also meets CSA Omega safety standards. As a result, they'll provide a secondary source of electric shock-resistance protection against incidental contact with electrical circuits of 18,000 volts or less. EH-rated footwear is manufactured with non-conductive electrical shock-resistant soles and heels to offer protection against accidental contact with live electrical circuits, electrically-energized conductors, parts or apparatus.


http://safemanitoba.com/sites/default/files/uploads/bulletins/standardcsa_foot_195_02.pdf
Electric shock resistant footwear shall withstand a test potential of 18kV 60 Hz for 1 minute, with no discharge to the ground, and current leakage shall not exceed 1 mA ...
 

user 100

Senior Member
Location
texas
High voltage fuse did not operate? Protective relaying did not work? :?

I think what kwired said earlier in the thread about there not being a low enough resistance in the path back to the source was likely accurate- even with the downed line touching/very near all of that metal that was in good contact with the soil, just too much impedance.

And even though it has been pointed out about the voltage difference in video versus topic posting and iwire has said it was a mistake , I think it's still a good thread and still gives food for thought for the "earth" crowd- if soil as a fault clearing often doesn't work for higher voltages ( it for sure didn't work for the high voltage in the video), why would they believe it would be effective for the simple lower voltages used everywhere else?:D
 
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