Electrical Potential and Current Flow

Brandon Lange

Member
Location
Iowa
Occupation
Seeking Electrical Apprenticeship
I have a theory question that came from Mike’s bonding and grounding course. For sake of theory...I have a pool where all parts are metal (including where we walk) and all are bonded together to have the same potential. Unknowingly a fault occurred while I was standing by the edge, grabbing the ladder with one hand and touching the water with my other hand. Since all materials are the same potential and I am not touching any ground reference point, I shouldn’t get shocked. Right? I also had a guy saying that (using the bird on the wire example) current flows through the bird even though it has the same potential as the line. That seems contrary to the theory I’ve learned (law of attraction). Thank you in advance for the help thinking through this! God bless!
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
If _everything_ is at the same potential, then there is no potential difference to cause a shock.

The bird on a wire is an apt analogy, because there _is_ some current flowing through the bird, and birds won't stand on wires where that current is too great.

1) When current flows in the wire there is always some voltage drop along the length of the wire ('step potential'), so some tiny amount of current will flow up one bird leg and down the other. This is probably too small for the bird to notice in an situation.

2) The wire is at elevated potential relative to surrounding materials, so you actually get a small amount of capacative current flow through the bird, as a field through the surrounding space, to ground, as well as a small amount of ion current flow through the air.

My understanding is that birds will stand on the local distribution wires (2300-7200V) but not on the high voltage long distance wires, although I can't find a reference for this.

Back to your pool example. If the fault were large enough, you _would_ feel it. But if _everything_ is metal and bonded (the deck, the pool wall, etc) and the fault were something normal in a residential or light commercial setting, and to that metal, I bet you wouldn't notice it. On the other hand, if a wire were to fall into the middle of this pool, so that current was flowing through the water seeking all that nice bonded metal, and you were in the water intercepting this current flow, you would notice it big time.

Jon
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
I would like to expand on Jon's point 1. If the fault occurs (let us say) north of the pool, and if its attempt to return to its own source takes it past the pool from north to south, and if all the metal and bonding and whatever you described does not have exactly zero resistance (which it won't), then the current passing through that metal will cause a (small) voltage drop along its path. So if you are standing on the metal deck facing west, with your feet a foot or so apart, the potential where your right foot touches the metal will be higher than the potential where your left foot touches the metal. Yes indeed, that will cause current to flow up your right leg, through your torso, and down your left leg. As Jon pointed out, the current will be so small that you probably won't feel it, and you will say with confidence that you did not receive a shock.
 

mivey

Senior Member
To get shocked requires energy transfer. Energy transfer requires both current and voltage. No voltage gradient = no shock.

Current in a wire or other material does not require a voltage gradient along the material. A super conductor segment between two normal conductors is a perfect example. Keep in mind we are talking about many orders of magnitude difference, not perfection.

Current may pass through the pool and the voltage gradient in the water or across you be so small you feel nothing significant even for a large current. Eventually the current will be large enough that your body resistance will cause a voltage gradient. However, if the body resistance is much higher than the water/metal resistance (which it should be) most of the current will travel around you.

Air is a good insulator. Closing the air gap between water and a ladder would normally be risky but if that gap is closed by a low resistance bond, the risk to you is drastically reduced.

Similarly for closing the gap between water at elevated potential and water at the ladder with a lower potential. Equipotential bonding tries to get all the water and stuff around the pool tied together with a bond that has a much lower resistance than your body.

Like winnie and charlie b said.
 

mivey

Senior Member
Also, look at Mike's info on electroshock drowning. The people were in water with electrical current. They were hurt even when in the water touching nothing else. They were experiencing a voltage gradient across their body. Fresh water alone is not a good conductor. Salt water is better.

We are mostly water so being in water with a current is not so good for us. If we can shunt the current around the water with a low resistance bond then we have reduced our risk when in the water.
 
1) When current flows in the wire there is always some voltage drop along the length of the wire ('step potential'), so some tiny amount of current will flow up one bird leg and down the other. This is probably too small for the bird to notice in an situation.

2) The wire is at elevated potential relative to surrounding materials, so you actually get a small amount of capacative current flow through the bird, as a field through the surrounding space, to ground, as well as a small amount of ion current flow through the air.

My understanding is that birds will stand on the local distribution wires (2300-7200V) but not on the high voltage long distance wires, although I can't find a reference for this.

So to expand on this a bit, and something I have always been unclear on: Those guys who climb on the high voltage lines for inspection and maintenance....What exactly is the farady cage suite they wear for? Is that to reduce the step potential due to voltage drop on the line between different parts of their body? What might be a plausible value for the voltage drop across 5 feet of such a line?
 

mivey

Senior Member
So to expand on this a bit, and something I have always been unclear on: Those guys who climb on the high voltage lines for inspection and maintenance....What exactly is the farady cage suite they wear for? Is that to reduce the step potential due to voltage drop on the line between different parts of their body? What might be a plausible value for the voltage drop across 5 feet of such a line?
To shunt around them. The static is tremendous.

I have a field calculator around here somewhere that I use when we get close to federal or restricted lands but haven't looked at 500 kV stuff in about 40 years. There are probably readily available resources online.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I think it is to deal with the electric field around them, otherwise on the high voltage lines they would get a certain amount of air ionization and sparking at the pointy bits (elbows, hands, etc.)

Take a look at this classic at about 1:04
The guy extends a rod to the line to start the bonding process. That is a single point connection to the line, so voltage drop along the line isn't a factor. Instead you are looking at the capacitive charging of the helicopter and any ion currents from the helicopter through the air. At these voltages that causes enough current to make some nice sustained sparks.

My recollection is that for lower voltage 'hot hand bonding on work' you just have workers standing in insulated bucket trucks. I don't know if they wear Farady suits.

-Jon
 
I think it is to deal with the electric field around them, otherwise on the high voltage lines they would get a certain amount of air ionization and sparking at the pointy bits (elbows, hands, etc.)

Take a look at this classic at about 1:04
The guy extends a rod to the line to start the bonding process. That is a single point connection to the line, so voltage drop along the line isn't a factor. Instead you are looking at the capacitive charging of the helicopter and any ion currents from the helicopter through the air. At these voltages that causes enough current to make some nice sustained sparks.

My recollection is that for lower voltage 'hot hand bonding on work' you just have workers standing in insulated bucket trucks. I don't know if they wear Farady suits.

-Jon
That is exactly the video I was thinking of. Kudos to the guy who tried that the first time, after the engineers told him it should work :oops:

Makes sense. So the suit is not for step potential.
 

jimport

Senior Member
Location
Maryland
There is a video that Mike shows where he walks in the pool at his house charged to 120 volts. Interesting to see watch. He also tests out the perimeter bond and step potential.
 
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