Question about equipotential grids.

[Fjag62]

Just got done watching Mike's video on stray voltage on youtube and have a couple of questions.

I think I understood why you need a gradient ramp to get cows to walk from the earth into the steel shed with the 3 volt energized slab. But later in the video, when he talked about the solution for the energized water by energizing the deck to equal the pool water , I was wondering if a wet person or dog could get a shock when stepping on to the energized deck.

Am I not understanding something correctly to ask that question? Any help would be greatly appreciated. Thanks.

I am not an electrician and never plan to any work that's required to be done by a licenced electrician. I am a GC that does pour concrete and would like to get a good understanding of the subject.

 

[kwired]

The pool water is required to be bonded to the equipotential bonding as well as all conductive objects within a certain zone around the pool, such as diving platforms, ladders, underwater lighting shells, lifts for ADA purposes...

This bond may be via other bonded items in contact with the water including plumbing for circulation equipment, if there is no bonded items in contact with the water then NEC tells us a minimum size of metallic component must be used to make an electrode in that water.

If the pool water, the concrete around it, and all conductive items within a users reach are all bonded together then there is no potential (at least of enough significance to matter) to expose the user to a shock hazard, even though all of those items may actually be at a voltage above true earth. The same reason why a bird sitting on a high voltage power line feels nothing - because it can not touch another object of different potential at the same time. Now if you have a hole in your equipotential bonding system, say you have a ladder or diving platform that is not bonded to the system, that unbonded object may be at or at least closer to true earth potential and if the equipotential bonding system is operating at a voltage above true earth, then that voltage will be between said ladder or platform and all other objects that are bonded - including the water that is supposed to be bonded.

 

[don_resqcapt19]

... I was wondering if a wet person or dog could get a shock when stepping on to the energized deck.
....

Yes, that will happen. The deck will be energized and the earth around the deck will not, however we are talking a very small voltage, under normal conditions, a voltage that is most often too low to be felt unless your body is immersed in water.

 

[Fjag62]

Thanks. I probably should have specified stepping on to the deck from the earth around it,but you understood what I meant.


But this gets me thinking how much concrete decks get energized by differently designed grids- or maybe I should say how the voltage gets distibuted through the deck.


If you look at the one example from Mike's video where the voltage solution could be the 8 AWG single wire placed 18 to 24 inches from inside of pool, I'd guess -but not sure- that the voltage at the end of the 3 ft deck would be less than a deck with a rebar mat placed throughout the slab. And then also how much a difference the spacing of the rebar makes.

I've worked with some lazy masonry crews that that aren't careful how they support the rebar to stay in center of slab and won't bother lifting the rebar if they drop to ground during pour. Is there a known maximum amount of concrete you can have between the rebar and top of concrete pad. I'm sure there are a lot of factors to consider (like thickness of concrete and placement of rebar) and really no way to get a precise calcultion of how the voltage gets distributed. Just wondering if there are some rules of thumbs I should be aware of.

 

[kwired]

Thanks. I probably should have specified stepping on to the deck from the earth around it,but you understood what I meant.


But this gets me thinking how much concrete decks get energized by differently designed grids- or maybe I should say how the voltage gets distibuted through the deck.


If you look at the one example from Mike's video where the voltage solution could be the 8 AWG single wire placed 18 to 24 inches from inside of pool, I'd guess -but not sure- that the voltage at the end of the 3 ft deck would be less than a deck with a rebar mat placed throughout the slab. And then also how much a difference the spacing of the rebar makes.

I've worked with some lazy masonry crews that that aren't careful how they support the rebar to stay in center of slab and won't bother lifting the rebar if they drop to ground during pour. Is there a known maximum amount of concrete you can have between the rebar and top of concrete pad. I'm sure there are a lot of factors to consider (like thickness of concrete and placement of rebar) and really no way to get a precise calcultion of how the voltage gets distributed. Just wondering if there are some rules of thumbs I should be aware of.

I wasn't thinking of stepping onto the deck from earth or other surfaces when I made my earlier reply, but yes there would be greater risk of voltage at that point, but people are not submerged in the pool at that point either and the exposure to that voltage is more like it is with any grounded object at other locations and the reference to true ground, and contact points are likely just any voltage drop between the two points in contact with a person's feet, and if you have shoes on probably is totally a non issue.

As far as rebar in the deck, I think the reinforcing value for the concrete is increased if it is near center of the thickness, but equipotential grounding values likely don't change all that much if relocated. Now if it is laying on the bottom of the pour and not completely encased in the concrete it may deteriorate (rust) at a faster rate, and of course will provide little if any concrete reinforcement.

 

[augie47]

keeps the cows out of the pool