Open neutral?

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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110505-1512 EDT

You need quantitative data of the present conditions that can be consistently measured. That is slab to water voltage.

Then you could experiment in a small area with maybe two rods 4 feet apart driven under the slab. These would be connected to the EGC and see what effect this has on your previous test measurement in this localized area. For experimentation ground rod material would be convenient.

What is the ultimate solution? I do not know. Your groove, cable, and back fill might work, but the NEC, etc. experts need to advise on what must be done. If you can get the cement to water to be near zero volts, then what is the remaining potential gradient from the slab to the earth as you walk from the slab to the earth? What is near zero volts needs to be defined. My guess is under 500 millivolts.

Also you might need a filter that selects only 60 Hz at the meter input to minimize measurement error from higher frequency sources, such as close high power broadcast stations.

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I was informed that during the construction of the pool ground rods were driven around the pool and bonded to pool steel frame. The #8 copper bonding wire is connected to the ground rods (around pool), light, ladder, steel frame and also tied to the grounding at the sub panel which also has a ground rod below the panel. I may try disconnecting the connection at the sub panel and see if the voltage difference changes from the cement to pool water. I'm going use a Fluke 289 meter next time to get more accurate voltage readings. If disconnecting the equipontential wire from the 100 amp service doesn't solve the problem, my next set will be adding the #8 bare around the pool and bonding to four points of the metal frame. It is my understanding after speaking with an inspector, ground rods should not be installed near the pool and tied to the equipontential grounding plane since this increases ground current on the pool. Any ideas?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110506-1112 EDT

In all of your posts you have not provided us with any voltage measurements from the water to anything else. What values have you read? In particular at this time between the water, the cement, the earth immediately adjacent to the cement, and any other points you may have measured.

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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110506-1134 EDT

An additional point. If you have a 10 A load on a typical circuit there will be a measurable difference in voltage between the neutral and the EGC. For example: I can estimate the length of a #12 wire within a foot or two in a length of a few feet with 10 A thru that wire.

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don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
While there is no need for ground rods, the pool bonding system has to be connected to the electrical grounding system at some point. This will be a connection to the EGC for any and all electrically operated pool equipment.
 
While there is no need for ground rods, the pool bonding system has to be connected to the electrical grounding system at some point. This will be a connection to the EGC for any and all electrically operated pool equipment.

Ok, I agree with your statement. This makes sense, but why would an inspector advise not connect pool bonding to the sub panel EGC? Although like you stated, the EGC going to the pool equipment would connect the equipotential grounding plane. I'm a little confused on the issue. Regardless, I don't see how connecting the pool bonding to the sub panel EGC would cause problems. It's all connected via each pool equipment EGC. Can someone explain?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110506-1503 EDT

Anything relative to the pool and the immediate proximity relative to the EGC for the pool should be bonded with heavy gage wire to the pool, not back to some outside point. Depending upon various impedances I might guess at heavy meaning 10 times the cross sectional area of the largest hot wire coming to the pool area.

Under short circuit conditions of a hot wire to any part of the pool area there should be the least possible voltage drop between different elements connected to the pool, and step distance away.

50 ft of #12 wire has a resistance of 0.075 ohms. 5 ft of #2 has 1/100 this resistance. Consider these two resistances in series with 120 V applied. The voltage drop across the 5 ft of #2 is about 1.2 V.

If you have two separate EGC paths back to a common point and the EGC wire size equals the hot wire size, then the voltage between the two different EGC paths with a short from hot to the end of one of the EGC paths may result in a voltage of about 60 V between the EGCs from a 120 source.

If I understand from what you have described about what your inspector wants, then I think he is correct.

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don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Ok, I agree with your statement. This makes sense, but why would an inspector advise not connect pool bonding to the sub panel EGC? Although like you stated, the EGC going to the pool equipment would connect the equipotential grounding plane. I'm a little confused on the issue. Regardless, I don't see how connecting the pool bonding to the sub panel EGC would cause problems. It's all connected via each pool equipment EGC. Can someone explain?
They look at 680.26(B) and see:
...An 8 AWG or larger solid copper bonding conductor provided to reduce voltage gradients in the pool area shall not be required to be extended or attached to remote panelboards, service equipment, or electrodes.

And then say you can't connect the bonding grid to the electrical grounding system.
 

readydave8

re member
Location
Clarkesville, Georgia
Occupation
electrician
110506-1134 EDT

An additional point. If you have a 10 A load on a typical circuit there will be a measurable difference in voltage between the neutral and the EGC. For example: I can estimate the length of a #12 wire within a foot or two in a length of a few feet with 10 A thru that wire.

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Can I use this to measure scrap/leftover NM of various sizes? Have you got a formula or chart (I'm not good at math but suppose I can figure it out from vd formula).
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110508-1255 EDT

readydave8:

Yes.

At 20 C (68F) (room temperature) some resistance values per 1000 ft for copper wire are:
#14 .... 2.525 ohms/1000 ft or 0.002525 ohms per ft
Thus, the voltage drop across a 1 foot length of #14 at 10 A is 25 millivolts.

#12 .... 1.588 ohms/1000'
#10 .... 0.9989 "
#08 .... 0.6282 "
#06 .... 0.3951 "

How to do the measurement is important. You need to do a 4 terminal measurement. What is that? See photos P19 thru P21 at my website
http://beta-a2.com/EE-photos.html

From my post numbered #15 at http://forums.mikeholt.com/showthre...ighlight=four+terminal+resistance+measurement
A 4 terminal resistance measurement is nothing more than a resistive element in which the test current is injected into the resistance thru terminals separate from the voltage measuring terminals. The voltage measuring terminals are located inboard of the current terminals. This arrangement eliminates any error from the voltage drop thru the termination of the current to the resistance. Since the voltmeter draws very little current there is little error from its connection to the resistance.

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