isolation transformer

[spd]

Customer is experiencing electric shock while touching pool and concrete pad simultaneously. Equipotential grid will help but how far do you go???
Looks like you could move the potential difference closer to the source by installing an isolation transformer. Does the code allow installation of an isolation transformer to separate the incoming utility ground from the transformer secondary grounding system?

 

[tom baker]

Thats not whats causing the shocks. Its utility primary neutral in parallel with the earth. There is a small amount of current there all the time.
You may have a utility with a bad underground neutral somewhere nearby.
The utility will say its your problem.

 

[spd]

thanks, but I think my problem is different than you're thinking...

I know for a fact that the 14.4kV system neutral is directly connected through grounding conductors to the water. Since current is flowing back to the source on the system neutral (through resistance of neutral wire) there is a voltage on the neutral to earth (measured 3V). The water is in a fiberglass molded pool that is insulated from the concrete pad. The issue is that the voltage on the neutral is connected to the water......The neutral energizes the water....

When a person touches the water, he in reality touches the voltage on the neutral (3v). If he's standing on the concrete at zero volts, then he has 3V accross his body. If you divide 3v by 1000ohms of the human body, you get 3 mA....that gives the tingle.

If the 14.4kV system had a short circuit to neutral/ground, then it would kill the guy at the pool.

Question is...how do you fix it? Can you install an isolation transformer and separate the primary grounding system from the secondary system?

 

[don_resqcapt19]

Can you install an isolation transformer and separate the primary grounding system from the secondary system?

Not if you comply with the rules in the NEC.
Don

 

[spd]

I've noticed that an isolation transformer is frequently recommended as a solution to the stray voltage issue...even some of the technical resources available through this website make that recommendation.

Looks like it is the best solution available, even though it is not allowed by code. It appears to move the stray voltage problem from the edge of the equipotential grid to a safer, known location which is between the low side and high side grounding conductors on the isolation transformer.

Am I thinking right on this???

 

[don_resqcapt19]

When a person touches the water, he in reality touches the voltage on the neutral (3v). If he's standing on the concrete at zero volts, then he has 3V accross his body.

If the metal in the concrete is also bonded, then the problem goes away. Both the water and the concrete will be a 3 volts above remote earth.
Don

 

[Pierre C Belarge]

As Don is saying, you want to eliminate the "potential difference" thereby eliminating current flow. With a nonmetallic pool, this is more difficult, but I am sure that a code compliant creative solution can be found.

 

[don_resqcapt19]

With a nonmetallic pool, this is more difficult, but I am sure that a code compliant creative solution can be found.

There was a proposal last code cycle that would have required that the water in pools be bonded....it was rejected.
Don

 

[don_resqcapt19]

The code rules cause part of this problem. Even where a double insulated pool pump is used, the code rules require us to make a connection that energizes the pool bonding grid. If the pool pump is double insulated and there are no lights or other eletrical items in the pool there is no technical reason why the pool bonding grid has to be connected to the electrical grounding system. This code required connection actually creates the problem and makes the installation less safe.
Don

 

[M. D.]

It sounds to me that there is a violation already. Is steel in the concrete bonded?

If it were, I don't see why there would a potential difference.

Where is there a connection between the neutral and the water??

I agree with Don, If it were bonded , there would have been no problem.

 

[spd]

When the ladder, handrail, and light trim ring is bonded, then the water is energized to system neutral voltage level. When an equipotential grid is in the concrete, then the concrete is also at system neutral voltage.

If a fault occurs and the neutral becomes hot...then the water and the concrete are elevated to that voltage.

What happens when a person steps off the concrete onto the ground with wet feet during the fault condition?

 

[M. D.]

When the ladder, handrail, and light trim ring is bonded, then the water is energized to system neutral voltage level. When an equipotential grid is in the concrete, then the concrete is also at system neutral voltage.

If a fault occurs and the neutral becomes hot...then the water and the concrete are elevated to that voltage.

What happens when a person steps off the concrete onto the ground with wet feet during the fault condition?

Tell him to fill it in I guess, remove the bonding wires completely , or try bonding the concrete.
Do you have an equipotential bonding grid at all?

680.26(c)(3)(b) "Grid structure . The equipotential bonding grid shall cover the contour of the pool and the pool deck extending 3 ft horizontallay from the inside walls of the pool...."

If not may be that is the problem.