Testing for voltage

[ptonsparky]

My problem is the level of voltage they indicate "would not be unheard of" when measured between the equipment and the ground rod that is attached to it. My math using .5 volt as a max allowed, measured from equipment to ground rod with 10' of #6 GEC indicates about 310 amps is flowing on the GEC. I have to agree that there is a problem, but so would a couple tenths of a volt.

Came in an email:

How to Use a Volt Meter for Electrical Safety when working around Electrically Powered Irrigation Equipment.
Caution!!!!!!!!! Before working around any electrical powered irrigation equipment, use a voltmeter to make one of the following two types of checks:
A voltage check measurement from a metal enclosure/metal component to the system?s electrical ground rod is used to determine if the metal enclosure/component is properly bonded and if a dangerous short or fault exists. This measurement should be very close to zero in most cases unless a problem of some type exists although depending on the system, lead length and connections, a couple of tenths of a volt would not be unheard of, but 0.5 volts was the level that became the go-no go reading.
1. Check for voltage between any metal component you will work around (CP sprinklers, drip irrigation filter stations, electric pumps, structural components, etc) and the systems electrical ground rod. Any voltage greater than 0.5 volt measured between a motor, control panel exterior, or structural component to the rod is not normal and could indicate a problem with the electric equipment or wiring. WALK AWAY AND tell the landowner/operator to CALL A QUALIFIED ELECTRICIAN.
A voltage check measurement from metal enclosures/components to ground (earth/soil), is technically a ?neutral to earth voltage? reading and could be different (higher) than the metal enclosure/component to ground rod measurement. This voltage check measurement can be used when the voltmeter leads are not long enough to measure from certain metal enclosures/components to the ground rod, this occurs frequently in the field in many instances. This voltage check measurement includes the contribution of the current flow on the supply utility neutral wire (off site stray voltage) and will fluctuate with current flow on both the utility and on-site neutral wire. In general, anything below 3-5 volts is considered normal. Once a reading goes over 3-5 volts, people can be shocked in wet, damp areas. Around 15 volts is considered a real concern area for humans because that?s where the current starts getting high enough through the resistance of your body to cause involuntary muscle contraction. From a practical safety standpoint, whenever that number is above 5 volts, it is an indication that something in the wiring system (either on or off farm) is not correct and needs attention.
2. If the voltmeters leads are not long enough to measure from the metal component to the ground rod, measure voltage from the metal components to earth (soil), this is commonly called stray voltage. Depending on conditions around the system, a measurement above 5 volts to the earth (soil) could be an indication of a potential problem with the electrical supply system, on-site electrical wiring or equipment. If the check voltage between a metal component and the earth (soil) is above 5 volts, WALK AWAY AND tell the landowner/operator to CALL A QUALIFIED ELECTRICIAN.
If you ever have any doubts about the safety of any electrical powered equipment, WALK AWAY AND ask the land owner or operator to CALL A QUALIFIED ELECTRICIAN.
ALWAYS PLAY IT SAFE!!!!

 

[K8MHZ]

Realize that the voltage difference is with the circuit open and (relatively) no current flowing when it is measured with a volt meter.

If you actually had 100's of amps flowing while connected, it would certainly cook the electrode conductor.

I also would like to know where you came up with 310 amps. The voltage difference between my home's panel electrode and a rod driven 6 feet away is between 1.2 and 2 volts on any given day, yet there is not enough current there to illuminate a 1 volt LED or light bulb, let alone shock someone.

 

[GoldDigger]

Realize that the voltage difference is with the circuit open and (relatively) no current flowing when it is measured with a volt meter.

If you actually had 100's of amps flowing while connected, it would certainly cook the electrode conductor.

I also would like to know where you came up with 310 amps. The voltage difference between my home's panel electrode and a rod driven 6 feet away is between 1.2 and 2 volts on any given day, yet there is not enough current there to illuminate a 1 volt LED or light bulb, let alone shock someone.

Sounds like your panel electrode is not bonded to the rod driven 6 feet away, and the indication is that a fairly high current is flowing through the earth itself, either through the plate electrode GEC or the earth itself from stray current.

The 310A is probably calculated from the voltage drop of .5 volts and the resistance of 10 feet of #6 copper. Not including any effect from the 3100 volts that would be measured across a good 10 Ohm electrode-to-earth connection at that same current.
That kind of current in a ground electrode would have to be coming from a bad POCO primary neutral!

Shows you how easy it is to go wrong when you treat the electrode-to-earth resistance as zero.

 

[K8MHZ]

Sounds like your panel electrode is not bonded to the rod driven 6 feet away, and the indication is that a fairly high current is flowing through the earth itself, either through the plate electrode GEC or the earth itself from stray current.

The 310A is probably calculated from the voltage drop of .5 volts and the resistance of 10 feet of #6 copper. Not including any effect from the 3100 volts that would be measured across a good 10 Ohm electrode-to-earth connection at that same current.
That kind of current in a ground electrode would have to be coming from a bad POCO primary neutral!

Shows you how easy it is to go wrong when you treat the electrode-to-earth resistance as zero.

Correct, the test rod is not bonded. It's function is to measure the voltage difference between the soil and my service's electrode, in this case lots of real metal water pipe.

If the described test was meant to be performed on a connected ground rod, there is no way 310 amps could flow on a #6 conductor and not fry it or at least heat it up a bunch. More likely, the voltage measured would be from a poor or open connection to ground rod being tested.

I used my personal test to illustrate that a high impedance test to an open electrode will show voltage that is not of concern.

As you mentioned, the OP's test also fails to address the resistance of the rod and as such, is not really a good test to check for touch potential from a metal panel to the earth.

 

[ptonsparky]

Correct, the test rod is not bonded. It's function is to measure the voltage difference between the soil and my service's electrode, in this case lots of real metal water pipe.

If the described test was meant to be performed on a connected ground rod, there is no way 310 amps could flow on a #6 conductor and not fry it or at least heat it up a bunch. More likely, the voltage measured would be from a poor or open connection to ground rod being tested.

I used my personal test to illustrate that a high impedance test to an open electrode will show voltage that is not of concern.

As you mentioned, the OP's test also fails to address the resistance of the rod and as such, is not really a good test to check for touch potential from a metal panel to the earth.

That is the point. According to their instructions we are to check for voltage between the equipment and the grounding electrode, not an isolated rod. Using the resistance for 10' of #6 and .5 volts difference I come up with about 310 amps. We both know that isn't going to happen.

 

[K8MHZ]

That is the point. According to their instructions we are to check for voltage between the equipment and the grounding electrode, not an isolated rod. Using the resistance for 10' of #6 and .5 volts difference I come up with about 310 amps. We both know that isn't going to happen.

Granted. Like I said, though, the voltage present may be due to a poor connection which also may pose a hazard if combined with a faulted piece of equipment. Do the math again with a few hundred ohms of resistance.

All checking the voltage at the connected electrode will do is tell you what your touch potential would be if you were standing on the rod, would it not?

 

[ptonsparky]

Granted. Like I said, though, the voltage present may be due to a poor connection which also may pose a hazard if combined with a faulted piece of equipment. Do the math again with a few hundred ohms of resistance. My problem may be I find that either the connection is good or non existant.
All checking the voltage at the connected electrode will do is tell you what your touch potential would be if you were standing on the rod, would it not?

I agree.