Resi Pole Mtd XFMR stray current?

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ron

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High_voltage_illistration.htm

http://home.comcast.net/~ronaldrc/wsb/High_voltage_illistration.htm
Considering the picture from Ronald Coleman's website above, and that many residential pole mounted transformers seem to only have one conductor from the primary side that goes from pole to pole, isn't the earth return used for many residential distribution systems?
Is there commonly current returning on earth, essentially being considered stray?
 
Bob,
I trust that you are correct. I have never been up at a pole mtd xfmr to see accurately, but the one outside my home only appears to have a single conductor running on the top of the pole ... between poles. Of course there is a driven rod at the bottom of the pole.
 
ron said:
I have never been up at a pole mtd xfmr to see accurately, but the one outside my home only appears to have a single conductor running on the top of the pole.

I am sure one of the utility guys will have an answer.

I was under the impression the NESC prohibits using the earth as a only return path.

Seems like touching the down wire to the electrode while standing on damp ground would be a bad time.
 
ron said:
. . . and that many residential pole mounted transformers seem to only have one conductor from the primary side that goes from pole to pole, isn't the earth return used for many residential distribution systems? Is there commonly current returning on earth, essentially being considered stray?
The diagram he has is something that could happen in the worst of circumstances but is very unlikely to happen. All of the electric utilities that I am familiar with use single (primary) bushing transformers to serve residential customers. The transformer is fed from a single phase (normally 15 kV class, not 5 kV as he shows) as shown in his diagram and the primary coil is normally connected to the inside of the tank where the external ground is attached. The tank is then attached to the pole ground (normally a #6 Cu. or equivalent) that goes to the ground rod. The secondary neutral has a grounding strap that goes from the center bushing to the tank to establish a grounding point for the neutral. The transformer legs and neutral are then connected to the secondary that feeds all of the customers that the transformer serves. The transformer legs are normally #2/0 Cu. or larger and the primary neutral is normally also the secondary neutral.

In order for Ronald Coleman's scenario to work, the grounding jumper must be removed and the grounding strap must be removed as well as the transformer secondary neutral connection to the primary neutral. I am not saying it can't happen, I am saying it is not likely to happen.

To address the question of stray current (the proper term by the way, not stray voltage). Consider that all (most) of the electric utility's MGN (multiple grounded neutral) are grounded at least four times per mile and at every equipment pole (transformers, capacitor banks, lightning arresters, etc.) Also, most electric utilities will put lightning arresters at switch locations and dead ends. Each of these locations will then be connecting the earth in parallel with the earth back to the transformer that feeds the circuit. That transformer is in the substation and there will be some current return through the earth. That is why the equipotential plane is so important around swimming pools and agriculture milking parlors. :smile:

iwire said:
I was under the impression the NESC prohibits using the earth as a only return path.

Seems like touching the down wire to the electrode while standing on damp ground would be a bad time.

The NESC does permit the SWER but we do not use it and I know of no investor owned electric utility that does use it. It permits a single wire to be used for the primary and use the earth for the current return.

Touching the down ground is no different than touching the kitchen faucet. :smile:
 
Earth return is not used. Look for the system neutral wire which is usually below the primary conductor. Since the neutral is grounded to earth, a very small portion of the primary current will flow through the earth back to the source ie the substation transformer. Current will take all available path back to the source
 
charlie said:
.

Touching the down ground is no different than touching the kitchen faucet. :smile:

Would there not likely be a large step potential hazard if I touched this current carrying conductor running down the pole?

My kitchen floor is fairly insulating. :smile:
 
iwire said:
Would there not likely be a large step potential hazard if I touched this current carrying conductor running down the pole?
No, the current carrying conductor is the MGN, not the earth. :smile:
 
Charlie,
All of the electric utilities that I am familiar with use single (primary) bushing transformers to serve residential customers. The transformer is fed from a single phase (normally 15 kV class, not 5 kV as he shows) as shown in his diagram and the primary coil is normally connected to the inside of the tank where the external ground is attached. The tank is then attached to the pole ground (normally a #6 Cu. or equivalent) that goes to the ground rod. The secondary neutral has a grounding strap that goes from the center bushing to the tank to establish a grounding point for the neutral. The transformer legs and neutral are then connected to the secondary that feeds all of the customers that the transformer serves. The transformer legs are normally #2/0 Cu. or larger and the primary neutral is normally also the secondary neutral.
I'm trying to picture your description and comparing it to the pole outside many homes. There is a single MV line connected to a single primary bushing on the pole mounted xfmr. Visably, I cannot locate a neutral (grounded conductor) that travels with that single MV ungrounded conductor from pole to pole. In your description I didn't pick up on a description of the neutral that goes between poles. If the system is grounded at least 4 times per mile via a rod, then is it possible that return current is brought back through that primary path in the earth, like a SWER?

Consider that all (most) of the electric utility's MGN (multiple grounded neutral) are grounded at least four times per mile and at every equipment pole (transformers, capacitor banks, lightning arresters, etc.) Also, most electric utilities will put lightning arresters at switch locations and dead ends. Each of these locations will then be connecting the earth in parallel with the earth back to the transformer that feeds the circuit. That transformer is in the substation and there will be some current return through the earth.
From the description above, it sounds like the earth is the primary path of return current ... in part like a SWER.

The picture below is not a local pole near me, but an example I found on the internet. Where is the primary neutral? Is it the rod that you can see going to earth?
http://www.flickr.com/photos/connormolloy/1626974578/
 
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ron said:
. . . I cannot locate a neutral (grounded conductor) that travels with that single MV ungrounded conductor from pole to pole.
1. Where is the primary neutral?
2. Is it the rod that you can see going to earth?
1. The primary neutral is usually the top conductor of the secondary on a rack or the bare conductor of a cabled secondary (Marc is on it.)
2. No, the rod is only for grounding the equipment and the MGN. :)
 
In the picture posted by Mark, the top wire of the group of the 3 wires is usually the system neutral.
 
charlie said:
No, the current carrying conductor is the MGN, not the earth. :smile:

:)

No, I don't mean for that system, I mean if pole outside Ron's home did not have an MGN and did use just the earth for the 'return'.
 
I think there is some confusion about the touch voltage on the "down conductor". I think the questions were based on a SWER system and Charlie's answers were based on a MGN system. There is a step and touch hazard with the down conductor of a SWER system. It is my understanding that where such systems are used, the area around the grounding electrode is fenced off to keep people out of the step potential hazard area.
 
080517-1306 EST

What makes this discussion confusing is the introduction of ground rods.

First, assume no ground rods, meaning no connection to earth anywhere for the moment.

If we start with the knowledge the pole transformer primary must be supplied by two conductors, one for each side of the primary, that come from a substation transformer somewhere, and that the customer has a center tapped secondary, then this constitutes 5 wires, but only 4 exist. So one primary and one secondary must be common.

Thus, the non-hot side of the primary is made common to the secondary neutral.

If we look at the transmission line from the substation there will be two wires linking a multitude of customer pole transformers. Each pole transformer will service one or more customers. Between these customers there will be three secondary wires going to all customers serviced by a particular pole transformer, but between this group of customers and another group there will be only the primary neutral and hot wires ( my assumption).

This system would supply power to the customers, but it is potentially very unsafe because nothing is earthed.

Now introduce earthing points to neutral at each customer main panel, separately at each pole transformer, and at the substation transformer. This provides multiple parallel paths thru the earth with the primary neutral and secondary neutral, but at a much higher resistance than neutral wire.

Under normal circumstances some small current will flow thru the earth.

Consider the last customer group on this primary distribution system. Between this group and the one closer to the substation the primary neutral is broken. This creates a high resistance neutral thru the earth from this group of customers to the next group and a high voltage source. Thus a large voltage gradient in the earth between the two groups. Maybe over a 100 ft distance in a close packed area there is up to 15,000/100 or 150 V per foot of linear ground distance. With a normal secondary load there will be very low voltage in the isolated group.

.
 
gar said:
080517-1306 EST
Consider the last customer group on this primary distribution system. Between this group and the one closer to the substation the primary neutral is broken. This creates a high resistance neutral thru the earth from this group of customers to the next group and a high voltage source. Thus a large voltage gradient in the earth between the two groups. Maybe over a 100 ft distance in a close packed area there is up to 15,000/100 or 150 V per foot of linear ground distance. With a normal secondary load there will be very low voltage in the isolated group.
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The voltage gradient will not be 15,000 volts ? 100 ft. Practically all of the 15,000 volts to ground of the primary will be in the transformer primary winding. The voltage drop through the earth will be the load current times the ground resistance. If the load is 15 kVA, the current will be one amp. If the ground resistance is 50 ohms to the next customer where the neutral is grounded, the voltage gradient will be 50 volts ? 100 ft or 1/2 volt per foot.
 
080518-0720 EST

jghrist:

True plus magnitizing current.

But now assume an internal short from hot to neutral on the primary side that caused the neutral break, not likely, but not impossible. Then one has to assume a low source impedance with respect to the assumed ground resistance to determine the actual voltage available across the ground path.

My point is that it is not impossible to get some large ground gradients. Although not likely, and maybe not as large as I illustrated but still large. A fallen hot line may be a greater likelyhood of producing large ground gradients.

.
 
gar said:
080518-0720 EST
But now assume an internal short from hot to neutral on the primary side that caused the neutral break, not likely, but not impossible. Then one has to assume a low source impedance with respect to the assumed ground resistance to determine the actual voltage available across the ground path.

My point is that it is not impossible to get some large ground gradients. Although not likely, and maybe not as large as I illustrated but still large. A fallen hot line may be a greater likelyhood of producing large ground gradients.
Yes, with the neutral broken and a phase-to-ground fault, high voltage gradients are likely. Not only for a transformer fault, but for any primary fault to ground. The voltage gradients cannot be determined from only the ground resistance because the current flows through the earth in a manner determined by the soil structure and the geometric configuration of the grounding electrodes (ground rod and any other buried metal connected to the ground rod).
 
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