Pool Bonding

[bennie]

Re: Pool Bonding

Don: I can't think of an appliance that will be used near the pool metal parts. So lets say the lady has her washing machine under the diving board

The washer draws 10 amps. The No.#12 run is 200 feet in length. Voltage drop is 12.6 volts.

This appears as 12.6 volts common mode voltage at the appliance.

Now consider the pool metal. This metal is connected to the earth and the MGN of the distribution system. Current will be flowing in this conductor. This current can be 180?out of sync with the appliance. The voltage difference can be elevated according to the amount of transient current in the pool system.

Now feature a flashover at the high voltage bushing on the distribution transformer. This arc will raise the voltage at the pool equipment more than at the neutral bus of the service.

I maintain the pool metal should not connect to the electrical system.

I am open to other opinions. This is only my perception.

 

[bennie]

Re: Pool Bonding

Another point to consider when dealing with open circuit common mode voltage.

Medical diagnostic equipment sensors, placed on the patients skin, are coated with a solution that penetrates the skin to the low impedance parts inside the body.

Clorine and other chemicals in pool water tend to lower the skin resistance considerably. This makes a person more susceptable to detecting low voltage, when closing the common mode open circuit.

Dampness at dairy farms, is a contributing factor in the problem with cows getting zapped.

 

[don_resqcapt19]

Re: Pool Bonding

Bennie,
I would agree if you make contact between the grounded conductor near its load end and the pool bonding system that there will be voltage. I do not agree that there would normally be voltage between the pool bonding system and the EGC where the two systems are bonded.

Now feature a flashover at the high voltage bushing on the distribution transformer. This arc will raise the voltage at the pool equipment more than at the neutral bus of the service.

Why? If the pool bonding system is connected to the electrial grounding system, how can the voltage be greater on the pool equipment than on the grounded conductor?
Don

 

[bennie]

Re: Pool Bonding

It all comes down to...There is no voltage difference across a bond conductor. There is a voltage difference across an (inductor) equipment ground conductor.

 

[don_resqcapt19]

Re: Pool Bonding

Bennie,

There is a voltage difference across an (inductor) equipment ground conductor.

I don't understand. Show me the math. At what length does a wire change from a short to an inductor? Doesn't the frequency play a big part in this? Is this really a problem at 60 hz?
Don

 

[bennie]

Re: Pool Bonding

There was a fatal accident involving a child in a swimming pool. The victim was standing on the concrete bottom of the pool, and touched the metal ladder.

The cause of the lethal voltage was the next door underground service feeder faulting to the earth.

This voltage appeared on the bottom surface of the pool, the ladder was connected to the MGN of the utility.

Had the ladder only been bonded to the rebar, there would be a lot more impedance back to the transformer. The lethal voltage may not have appeared.

 

[don_resqcapt19]

Re: Pool Bonding

Bennie,
That tragic case sounds like a lack of bonding between the re-bar in the concrete and the ladder.
Don

 

[bennie]

Re: Pool Bonding

Don: Rebar oxidizes (rust) this is an insulator. The rebar is insulated from the concrete. The voltage from the faulted line energized the concrete. Perfect conditions to kill someone.

 

[bennie]

Re: Pool Bonding

Most in ground pools, with rebar, are finished with sprayed on gunite. This material has a fairly high resistance. There will always be a high impedance connection to the rebar.

 

[T-Wragg]

Re: Pool Bonding

I am working on a 30 year old pool which appears to not have been properly bonded when built, or repairs over the years have disrupted the bonding grid. There is not a # 8 solid copper wire connecting the pool moters to anything and the circulation pipes coming from the pool are plastic. I have explained to the customer that these moters (both 240v, not double insulated) have to be connected to the pool bonding grid (i.e. connecting a #8 solid copper wire to the rebar of the pool and then to the moters). In her search for a quicker cheaper solution others ( some electricians and pool people ) have told her that if an 8' ground rod is driven into the ground and attached to the moters then everything will be fine. I told her I wont do this as a solution. Does this suggestion do anything to make her situation safer at all? Thanks

 

[charlie]

Re: Pool Bonding

There was a fatal accident involving a child in a swimming pool. The victim was standing on the concrete bottom of the pool, and touched the metal ladder.

Bennie, this was reported in the Nov/Dec 2002 edition of the IAEI News (unless I have the wrong incident). A lifeguard stand at a large community pool in the Kansas City area caused a near electrocution when a 12.47 kV line was contacted with a track hoe. 7.6 kV was conducted to earth which caused a voltage gradient and involved the pool. The gradient froze the lifeguard to the lifeguard stand since only one side was bonded to the pool's ground grid. The unbonded side picked up the gradient and passed the current through the lifeguard's body to the pool's ground grid. This incident also involved a worker that was in the hole the track hoe was excavating.

 

[bennie]

Re: Pool Bonding

I can understand...the 7.6 KV was heading to the nearest MGN to get home. Anyone completing the circuit will have a bad day.

The pool rebar is a good Ufer ground electrode. It is connected to the neutral bus, of the service. It is not bonded (shorted) to the service ground electrode system. Physically it constitutes multi-grounding of the premises system.

 

[paul renshaw]

Re: Pool Bonding

Hi Bennie, How is it not bonded to the service GES, if it is connected to the neutral bus of the service? Aren't they bonded together at the servce somewhere? If not shouldn't they be?

 

[bennie]

Re: Pool Bonding

Hello pgr. This topic points out the difference in bonding and grounding. The neutral bus is a supply point not a bonding (shorting) point.

Feature two ground electrodes 100 feet apart. One is the water line, the other is a driven rod.
The water line will have low impedance compared to the rod. More current will flow in the conductor to the water line.

This difference in current, will produce a different potential from each one of the electrode conductors, to the earth. This difference is a common mode voltage producing a common mode current ground loop.

Now feature one run of #4, from the neutral bus, to the water line, then on to the rod. This is bonding. No common mode voltage, except for micro-volts between electrodes out side of building.

 

[Ed MacLaren]

Re: Pool Bonding

Feature two ground electrodes 100 feet apart. One is the water line, the other is a driven rod.
The water line will have low impedance compared to the rod. More current will flow in the conductor to the water line.

This difference in current, will produce a different potential from each one of the electrode conductors, to the earth. This difference is a common mode voltage producing a common mode current ground loop.

I don't see what problem is caused by this? Could you explain it?

In this sketch, both conductors going to the grounding electrode system are connected at the same point on the grounded (neutral) conductor.

Ed

 

[bennie]

Re: Pool Bonding

The neutral/ground bus is the connection point for the equipment ground wires, the neutral/ground conductor, and the ground electrode conductor.

The neutral bus is not a common bus for the other ground electrodes required to be bonded together to become one electrode.

The first 5 feet of water pipe is the bonding bus. This prevents common mode current flow within the premises.

 

[bennie]

Re: Pool Bonding

For substantiating my statements refer to...250.24 (C), Grounding Electrode Conductor.

"A grounding electrode conductor" This is a singular statement, not plural, or more than one.

 

[don_resqcapt19]

Re: Pool Bonding

Bennie,
What about 250.64(F)?

(F) To Electrode(s). A grounding electrode conductor shall be permitted to be run to any convenient grounding electrode available in the grounding electrode system or to one or more grounding electrode(s) individually. The grounding electrode conductor shall be sized for the largest grounding electrode conductor required among all the electrodes connected to it.

 

[bennie]

Re: Pool Bonding

Don: That section only validates my statements further. Carefully read each word.

 

[don_resqcapt19]

Re: Pool Bonding

Bennie,
250.64(F) makes it very clear that I am permited to run individual grounding electrode conductors from the main bonding jumper to each and every one of my grounding electrodes.
Don