Can someone explain this?

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hillbilly

Senior Member
Re: Can someone explain this?

Just a shot in the dark without having the facts. If the box was laying on the ground and it had a exposed "hot" non grounded part, it could cause this. As she stepped on the box with her other foot on the ground she made a path to ground through her legs and was shocked or she could have been touching a grounded object with her hand and had current flow through her upper body. The story didn't give any indication of voltage at the box, but 120V can cause instant death.
 
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jackmcg

Member
Re: Can someone explain this?

I recall there were similar stories to this in New York city involving the death of some pet dogs.... you can easily have a difference of potential sufficient for fatality between 2 points as described in the story... this is why it is required to "bond" all metal parts in swimming pool construction... in the case of this woman.. the traffic light box was probably energized and a path was created from it through her foot to her body and out her other foot to ground.. the story stated that she was "barefoot"....it is also possible the junction box was plastic, thereby isolating it from ground but the lid may have been metal and energized... of course .. all one can do is speculate...
 
K

karl riley

Senior Member
Re: Can someone explain this?

This is similar to the deaths at lampposts which have a ground rod but no EGC. A fault to the metal post is not affected by the rod and a person has the full voltage across them when they touch it.

So does this mean traffic signals are also powered by circuits without a grounding conductor?

Karl
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Can someone explain this?

I've never seen a metal cover on an underground junction box with a bonding jumper. Many cases the box itself is concrete or other poor conductor. If a wire is touching the underside of the metallic cover, over time the conductor insullation can be worn away by vibration and the cover can become energized. The cover should be connected to the equipment grounding conductor.
As far as traffic signals, almost none of them in Illinois that were installed over 2 years ago had an equipment grounding conductor. They used ground rods only. This changed after Mike taught a grounding class for the IDOT engineers.
Don
 
C

charlie tuna

Senior Member
Location
Florida
Re: Can someone explain this?

the metal box covers of newly constructed concrete boxes have grounding lugs on the inside. many older boxes have no grounding means to the metal cover. if the energized conductors contact this cover it becomes energized with line voltage. if a person steps on this cover and at the same time somehow contacts a ground path back to the power source they will get shocked. we had a fatality occur here where a teenager stepped on the box cover while leaning against a metal bus bench support. if the box cover was grounded this would not have happened! you also have the problem where the original box covers were made of concrete and where replaced with cast metal covers when the concrete covers failed. you don't have to look into too many of these underground boxes to find poor insulation conditions from wheather related conditions. how many of these boxes are "maintained"? i get a little worried whenever i look inside some of these pole light "hand holes" - not large enough and poorly spliced wires with wire insulation so bad that colors cannot be determined!
 
T

torint

Member
Re: Can someone explain this?

I've been thinking about this for 2-3 day's. I'm not the greatest person on grounding. The only thing that I feel is safe is that one can never go wrong by grounding a ckt. That being said. Why do you have to pull a grounding cdr inside the pipe? If you had a ground rod (grding electrode) at the pole with a grounding cdr from the rod to the pole, why would you have to pull a grd cdr in the pipe? :confused: I've alway's been taught to pull a grd cdr in the pipe but now I want to know why. :confused:
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Can someone explain this?

torint,
The connection to earth (grounding electrode) will not normally clear a fault on a low voltage system. The impedance of the earth is too high to permit enough current to flow so that the overcurrent protective device will open. The equipment grounding conductor is (should be) a low impedance path back to the current source and will flow enough current to make the OCPD open the circuit. The connection to earth provides almost no protection from shock. The EGC provides some protection by causing the OCPD to open, but it will be at an elevated voltage from the time that the fault occurs until the OCPD opens.
Don
 
tom baker

tom baker

First Chief Moderator & NEC Expert
Staff member
Location
Bremerton, Washington
Occupation
Master Electrician
Re: Can someone explain this?

Torrint-here is a link to an article I wrote in a traffic signal magazine that will explain our question.
http://www.imsasafety.org/journal/mayjun/mayjun8.htm scroll down until you find this:
Authors Note: The following is reprinted with permission from Mike Holts 2002 Grounding and Bonding Text.


Some to the material I reprinted with Mikes permission.

Dons answer is what I would of said, as well.
I recommend you contact order from Mike Holt his grounding and bonding text and video.
A
 
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