Power Distribution Design -- why did the person get shocked?

[Derill03]

Hello I am an electrical engineer freshly out of school and i was wondering if anyone could help me reach an answer to this design problem. I am attempting this problem solely for personal knowledge because in school I never took a course on distribution design. I found this problem looking around on google but honestly did not find much more information that seemed relevant. Any help would be appreciated.
You are sent to investigate an incident where someone received a shock from a construction pedestal in

a nearby new underground residential construction area. The area is served at 12.47/7.2 kV primary

underground. The location you are asked to look at has a 50 kVA padmount transformer supplying a

hand hole that has three - three connector bus bars. The transformer is a standard 120/240 low side

center tapped (grounded) connection.

Given:

There is only a temporary metered construction pedestal connected to the hand hole and has three

outlets (GFCI protected) to allow for both 120 volt and 240 volt hand tools and construction power to

the residential home being built there. Two outlets are 120 volts and one is 240 volts. The pedestal has

an installed 5/8” x 8’ ground rod (copper weld) to ground the metal pedestal enclosure.

The shock happened when the person touched the open metal lid on the pedestal after switching the

hand tool plug locations. An offhand comment by someone noted that in using hand tools on the job

site, one outlet seemed to make the tools run much faster.

To Do:

1. Clearly show the electrical connection diagram of what you think happened at the pedestal to

provide the voltage needed for an electric shock? Please draw on a separate piece of paper.

2. What voltage to ground would be present?

3. If the electrical resistivity of the soil at a nearby substation location was measured at 50 ohm-
meters what is the calculated ground rod resistance at the pedestal?

 

[berkeman]

Hello I am an electrical engineer freshly out of school and i was wondering if anyone could help me reach an answer to this design problem. I am attempting this problem solely for personal knowledge because in school I never took a course on distribution design. I found this problem looking around on google but honestly did not find much more information that seemed relevant. Any help would be appreciated.
You are sent to investigate an incident where someone received a shock from a construction pedestal in

a nearby new underground residential construction area. The area is served at 12.47/7.2 kV primary

underground. The location you are asked to look at has a 50 kVA padmount transformer supplying a

hand hole that has three - three connector bus bars. The transformer is a standard 120/240 low side

center tapped (grounded) connection.

Given:

There is only a temporary metered construction pedestal connected to the hand hole and has three

outlets (GFCI protected) to allow for both 120 volt and 240 volt hand tools and construction power to

the residential home being built there. Two outlets are 120 volts and one is 240 volts. The pedestal has

an installed 5/8” x 8’ ground rod (copper weld) to ground the metal pedestal enclosure.

The shock happened when the person touched the open metal lid on the pedestal after switching the

hand tool plug locations. An offhand comment by someone noted that in using hand tools on the job

site, one outlet seemed to make the tools run much faster.

To Do:

1. Clearly show the electrical connection diagram of what you think happened at the pedestal to

provide the voltage needed for an electric shock? Please draw on a separate piece of paper.

2. What voltage to ground would be present?

3. If the electrical resistivity of the soil at a nearby substation location was measured at 50 ohm-
meters what is the calculated ground rod resistance at the pedestal?

Please show us your work on this problem so far...

 

[Derill03]

So far i have deduced that the voltage is somehow higher than 120 in one of the 2 outlets or higher than 240 in the single 240 outlet, it does not specify which outlet the person switched to when he was shocked. So my thoughts so far lead me to believe one of the outlets is wired wrong and giving a higher voltage (hence the clue of the tools running faster).

 

[Derill03]

I think part 3 should be about 19.94 ohm using formula R= Ro/6.283L*(ln(8L/d)-1).

Unsure how to calculate part 2 at this point still.

 

[donpacino]

http://www.ranprieur.com/house/house-wiring.bmp

take a look at this... you say one of the outlets could be wired wrong or something is wrong... how so

hint, sometimes higher voltage will make things run faster

 

[Derill03]

Well so let's say the 120v outlets are wired as 220v ok, that would cause the hand tool to run faster i agree but how could that cause a potential to build-up on the chasis and cause a shock?

Another possibility I've thought of is what if the transformer has a bad ground connection, then the 120v outlets would be at some other potential probably lower than 120v since they are not referenced to 0v any longer, but again how would that cause a shock to occur?

Maybe I am missing something because I am not familiar with the pedestal box they are referring to, i assume its wired just like a house would be.

 

[Derill03]

Could it be that the ground clamp has came loose or not hooked up? This could pose a shock hazard, and it would cause the transformer to have no center tap.

 

[donpacino]

Could it be that the ground clamp has came loose or not hooked up? This could pose a shock hazard, and it would cause the transformer to have no center tap.

yes that's possible. why don't you draw a schematic to prove it?!

 

[Derill03]

Ok so if this fault condition were true, it could explain the shock that occured, but it seems if you measured across the 120V outlets you would see about 120V-60V = 60V? Hmm I'm not sure that's correct, i know if the transformer is grounded to CT then you get a +120 and a -120 at 180 deg out of phase from one another across transformer coil but if the CT is not grounded would it show 60V or 120V at its center point? I think it has to be 60V at its center with no CT ground

See attached schematics

 

[chandan219]

Ok so if this fault condition were true, it could explain the shock that occured, but it seems if you measured across the 120V outlets you would see about 120V-60V = 60V? Hmm I'm not sure that's correct, i know if the transformer is grounded to CT then you get a +120 and a -120 at 180 deg out of phase from one another across transformer coil but if the CT is not grounded would it show 60V or 120V at its center point? I think it has to be 60V at its center with no CT ground

See attached schematics

Hi, I am thinking the neutral and one of the hot line were interchanged.

 

[rcgldr]

In the case of USA distribution, missing from the diagrams shown here is second ground at the meter / panel at a customer site. There's a ground for the center tap of the step down transformer, and there is a second ground at the meter / panel. Three prong electrical outlets within a customer site also have a ground / Earth connection in addition to the neutral wire, and there can be a slight voltage difference between the neutral and ground wire.