US Electrician qualifications for a Brit

No, I don't share your motivation and may we may share a lack of a solution.

I believe that the problem in the dairy industry was the so called stray currents. Considering that such currents, of a higher magnitude, can be produced from the LV system, additional, smaller currents from the MV system may not be the problem. I once read a book which dealt with this and this contained a detailed technical analysis which brought the concept of ground currents from MV systems into question. A greater problem was poorly maintained LV wiring. However technical opinions and legal opinions are two different things. If the MV system is grounded properly, there will be unbalance ground currents-true. A separately grounded LV system, including one using your isolation scheme will have much higher ground currents and, since the earth is a common conductor, there will still be coupling between MV and LV ground systems. Rather than isolate or try to do so, it might be better to spend money on ground rods- even a ground grid and tie everything to this grid. Note that in HV substations a grid is used in order to make the ground as nea an equipotential as possible and the fence is either outside the grid far enough to have a low step potential in case of a fault or inside by a distance such that anyone or anything outside and touching the fence is well within the grid. This is a bit extreme but the same idea of an equipotential grid to which all neutrals, enclosures, etc are connected may be a better alternative for locations considered critical. This will mean that the ground under a cow is at the same potential as the milking machine hardware attached to the udder. Your isolation scheme doesn't do this.

If you can get the utility to install a transformer without a grounded secondary then your home free. Such installations are usually unavailable. There is no particular danger in using such a supply to feed a dry transformer because that supply would be ground referenced at the next transformer or several spans away at the worst. It's just that if that kind of service is not identified as an available form of service in the tariffs of your states utility regulatory body the power utility will not provide it.

Don Kelly wrote:

Don I don't claim to have your theoretical expertise but I may have more practical experience in farm and ranch electrical problems. Here is one example of that experience. I had a customer who was having production problems in his dairy herd. I converted all of his feeders to four wire and megged his neutrals until I cleared all ground faults. There were only three. One was in the aging well pump, another in a water trough heater, and a third one in one of the homes light fixtures. Great, I thought, I should have this whipped. WRONG. As soon as the neutral connection was restored animal behavior resumed being erratic and production fell. During the repairs and updates the farm had been running on its power take off emergency back up generator to supply the dairy operation and a portable engine generator to power the refrigeration and like items by cord and plug in the home. While on this free standing power the production rose markedly when it should have fallen. The reason that we expected production to fall was the absence of ventilating fans in the dairy barns outside of the milking parlor area. Those of us that were capable of sharing our perceptions were definitely less comfortable in the barn during this period. To our surprise the production rose without public power and fell as soon as it's neutral connection was restored. Keep in mind that while I was chasing ground faults on the LV neutral a sub contractor was redoing the entire yard pole and building drop system. While the farms electrical system was disconnected from the Multi Grounded Neutral of the utility's medium voltage system production rose. When it was reconnected production fell. Now I was really confused. I went there to cure what I strongly suspected was a stray current problem. Having megged all neutrals clear of ground at 300 volts I couldn't see were the stray current was coming from but when the sub contractor approached to get his work order signed off I had a clue. With all stock out to pasture we shut down both generators and measured for current flow on every feeder conductor. The entire farm was deenergized at the yard pole but we had measurable current on all of the feeder Equipment Grounding Conductors. The readings on these EGCs were only slightly less than the readings obtained on the old feeder neutrals before starting the upgrade work. I finally came to the conclusion that the current was coming from outside the farm property via the utility's medium voltage MGN. When the utility MGM was disconnected from the premise wiring at the service equipment the stray current disappeared and production rose. I then purchased a used three pole switch and all milking and barn feeding was done with the power fully disconnected from the utility while negotiations with the utility began. Their first reaction was to attack my competence. I threatened to file suit for defamation and they agreed to send an engineer. After the engineer saw my readings using three different clamp on ammeters and a meter shunt he sent for the utility's measuring equipment. After measuring for three days they claimed the condition was normal. The rancher filed suit but more importantly he called his state senator who, as luck would have it, was serving on the committee that oversaw the states public service commission. When the Public Service Commission's engineer denounced the claim of normalcy the utility began to be civil. I then found out that the tariffs included single phase 480 as a service option to supply long runs of private overhead wire to feed customer owned transformers. The utility supplied that power from a different point on the property line. The only main bonding jumper on the entire premise became the one at the customer owned dry transformer that is located at the existing yard pole. There is now no measurable current flow on the main bonding jumper or any of the Grounding Electrode Conductors at any of the buildings. Production stabilized at 15% higher than its previous average. In a two hundred and fifty head herd that is huge.

I said all of that to say that isolation from the utility's MGN is sometimes a very good idea. All that went down over twenty years ago. It is true that modern practice calls for an equipotential plane in the floors of stock barns but it is also true that the stray current does not always originate in faults in the premise wiring.

--------------------- Thank you for the information. I did not intend to imply that problems are due to the customer's own system. There can be and are MV ground currents coupling into the customer's LV system as the earth is a common conductor. Your final solution is a damned good one. AC ground currents are fairly localised (and tend to follow the line) so having a more remote utility grounding point certainly helps- more so than the 4 wire setup which apparently didn't help. --

Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

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| No, I don't share your motivation and may we may share a lack of a solution. | | I believe that the problem in the dairy industry was the so called stray | currents. Considering that such currents, of a higher magnitude, can be | produced from the LV system, additional, smaller currents from the MV system | may not be the problem. I once read a book which dealt with this and this | contained a detailed technical analysis which brought the concept of ground | currents from MV systems into question. A greater problem was poorly | maintained LV wiring. However technical opinions and legal opinions are two | different things.

Given a constant resistance, MV is going to push current harder than LV. Certainly in many cases it can be an LV issue. When it is, it is easier to deal with and get fixed. But when it is an MV issue, then it is a lot harder to get fixed. Perhaps more initial incidents might be from an LV based cause. But those caused by MV persist longer because the utilities tend to not want to take the steps to fix it.

| If the MV system is grounded properly, there will be unbalance ground | currents-true. A separately grounded LV system, including one using your | isolation scheme will have much higher ground currents and, since the earth | is a common conductor, there will still be coupling between MV and LV ground | systems. Rather than isolate or try to do so, it might be better to spend | money on ground rods- even a ground grid and tie everything to this grid.

This depends on where the return point on the LV system is. If the system is derived at a pole transformer, which will be grounded, then there will be two return paths, the neutral of the service drop and the ground itself by means of neutral current flowing over the neutral to ground bond, to the entrance based grounding electrodes, and through ground to the pole grounding electrode, and up the pole grounding wire to the transformer.

The problem is the system has multiple points of grounding the neutral. The NEC doesn't allow this and for good reason. The NESC forces it on the service drop.

The fix is to derive an entirely new system with a transformer where the primary is connected to the two line wires at 240 volts for single phase or delta for at 208 or 480 volts for three phase. The neutral on this system will have exactly one bonding point (unlike the service drop system that has at least two). As long as there are no faults in that system, then any neutral currents can only flow back to the source by means of the neutral conductor. Even if the service drop neutral/ground wire is connected to the secondary bonding point (e.g. all grounds/neutrals are connected together), it introduces no additional paths on the secondary system because the secondary neutral has exactly one point for it to reach ground (even though after that point is then can branch out to get to ground by the electrodes at the building and through the service drop neutral to the pole grounding electrode).

However, this doesn't prevent any MV current from coming down the neutral and flowing through the building grounding electrodes, unless the primary is fully isolated (not even ground connected) from the secondary.

| Note that in HV substations a grid is used in order to make the ground as | nea an equipotential as possible and the fence is either outside the grid | far enough to have a low step potential in case of a fault or inside by a | distance such that anyone or anything outside and touching the fence is well | within the grid. This is a bit extreme but the same idea of an equipotential | grid to which all neutrals, enclosures, etc are connected may be a better | alternative for locations considered critical. This will mean that the | ground under a cow is at the same potential as the milking machine hardware | attached to the udder. Your isolation scheme doesn't do this.

I don't know if they do this, or not, but the milking machine could be made to work where there is absolutely no metal contact to milk where the milk flow is continuous back to the udder. This isn't hard to do, so I want to believe they have done this.

Still, this doesn't address the fact that these problems exist outside of the scope of milking activity. Apparently some step potential exists near any point of grounding electrode that has a metallic path back to an MV circuit. Apparently livestock can sense it, at least when standing or walking bare-hoof on at least wet ground. MV neutral currents flow out each grounding electrode and radiate outward from there.

If I were a dairy farmer, I would at least isolate the milking operation, which is where the livestock would be near grounding electrodes and when their sensitivity would have a significant impact. I don't know offhand what kind of power they need. But a plastic natural gas pipe, or a propane tank, connected to a generator, might do the job. The area might still need a ground ring to divert any exterior ground currents passing by. Or maybe I could "go green" and power everything from solar power sources. Or maybe from indigenous methane :-)

Adding a transformer at the end of a service drop I do think will improve things (because of the singular bonding). It's not all I want, but it is more than most have.

On Fri, 27 Oct 2006 04:06:36 GMT Member, Takoma Park Volunteer Fire Department wrote:

| I don't claim to have your theoretical expertise but I may have more | practical experience in farm and ranch electrical problems. Here is one | example of that experience. I had a customer who was having production | problems in his dairy herd. I converted all of his feeders to four wire | and megged his neutrals until I cleared all ground faults. There were | only three. One was in the aging well pump, another in a water trough | heater, and a third one in one of the homes light fixtures. Great, I | thought, I should have this whipped. WRONG. As soon as the neutral | connection was restored animal behavior resumed being erratic and | production fell. During the repairs and updates the farm had been | running on its power take off emergency back up generator to supply the | dairy operation and a portable engine generator to power the | refrigeration and like items by cord and plug in the home. While on | this free standing power the production rose markedly when it should | have fallen. The reason that we expected production to fall was the | absence of ventilating fans in the dairy barns outside of the milking | parlor area. Those of us that were capable of sharing our perceptions | were definitely less comfortable in the barn during this period. To our | surprise the production rose without public power and fell as soon as | it's neutral connection was restored. Keep in mind that while I was | chasing ground faults on the LV neutral a sub contractor was redoing the | entire yard pole and building drop system. While the farms electrical | system was disconnected from the Multi Grounded Neutral of the utility's | medium voltage system production rose. When it was reconnected | production fell. Now I was really confused. I went there to cure what | I strongly suspected was a stray current problem. Having megged all | neutrals clear of ground at 300 volts I couldn't see were the stray | current was coming from but when the sub contractor approached to get | his work order signed off I had a clue. With all stock out to pasture | we shut down both generators and measured for current flow on every | feeder conductor. The entire farm was deenergized at the yard pole but | we had measurable current on all of the feeder Equipment Grounding | Conductors. The readings on these EGCs were only slightly less than the | readings obtained on the old feeder neutrals before starting the upgrade | work. I finally came to the conclusion that the current was coming from | outside the farm property via the utility's medium voltage MGN. When | the utility MGM was disconnected from the premise wiring at the service | equipment the stray current disappeared and production rose. I then | purchased a used three pole switch and all milking and barn feeding was | done with the power fully disconnected from the utility while | negotiations with the utility began. Their first reaction was to attack | my competence. I threatened to file suit for defamation and they agreed | to send an engineer. After the engineer saw my readings using three | different clamp on ammeters and a meter shunt he sent for the utility's | measuring equipment. After measuring for three days they claimed the | condition was normal. The rancher filed suit but more importantly he | called his state senator who, as luck would have it, was serving on the | committee that oversaw the states public service commission. When the | Public Service Commission's engineer denounced the claim of normalcy the | utility began to be civil. I then found out that the tariffs included | single phase 480 as a service option to supply long runs of private | overhead wire to feed customer owned transformers. The utility supplied | that power from a different point on the property line. The only main | bonding jumper on the entire premise became the one at the customer | owned dry transformer that is located at the existing yard pole. There | is now no measurable current flow on the main bonding jumper or any of | the Grounding Electrode Conductors at any of the buildings. Production | stabilized at 15% higher than its previous average. In a two hundred | and fifty head herd that is huge.

How was that 480 volt service drop configured? Was it a 2-wire 480-0 single ended system, or an Edison-style 240-0-240 3-wire system at twice the voltage single phase is normally supplied at? Was the ground/neutral of the secondary side of the customer dry transformer connected to the one ground+neutral wire of the service drop? Did they do this with one pole transformer, or two?

If the grounded conductor of the 480 volt service drop was not connected to the derived 120/240 system, then I'd say the MV currents on the MGNs were the original problem. But if that connection is present, then I'd say that the original problem was that the 120/240 system itself was the problem through the fact that it had a MGN of its own (one at the transformer, and probably sharing all the MGNs of the MV circuit, too, and one at the service entrance. This can create a ground return on the LV side, even through the MV circuit itself.

It's sad that a utility has to go around making such claims. I'm sure it's typical corporate greed and hoping the complainer will just go away so they don't have to deal with it. But this is a well know problem that exists all over the country. I'm sure the utility knows about it, but hopes they can just blame the farmer for their own issues (which in fact at times really is the cause).

Do you remember if the MV circuit was single phase or three phase?

| I said all of that to say that isolation from the utility's MGN is | sometimes a very good idea. All that went down over twenty years ago. | It is true that modern practice calls for an equipotential plane in the | floors of stock barns but it is also true that the stray current does | not always originate in faults in the premise wiring.

There are so many places for electricity to go. It will find them _all_ given the chance.

And was that 480 volt tariff just for farms, or was it available in other places? If I'm going to put in a transformer, I think it would be a better idea to use 480, if I can get it.

| If you can get the utility to install a transformer without a grounded | secondary then your home free. Such installations are usually | unavailable. There is no particular danger in using such a supply to | feed a dry transformer because that supply would be ground referenced at | the next transformer or several spans away at the worst. It's just that | if that kind of service is not identified as an available form of | service in the tariffs of your states utility regulatory body the power | utility will not provide it.

I know they don't like to provide it because that means the transformer they are responsible for is more subject to damage due to ground voltage differential, such as from electrical storms (and not just from a strike of lightning, but also from the ground charge that follows storm cells).

But I do suspect a transformer designed for MV circuits will have better insulation levels between primary and secondary, and so could survive a case where a LV-to-LV dry transformer might not. OTOH, I absolutely do not want the utility to supply me power above 600 volts. If I were to take the approach of a full isolating transformer, using a pole pig to do it might be more reliable. But I'd have to take power from the utility at LV then step it back up to MV again on my own (e.g. the disocnnect would be at the LV service drop). Just imagine two yard poles with pole pigs on each and they are coss connected to each other. I saw a couple of nice

As Tom alluded to, buildings for livestock have to have equipotential floors - mesh or other conductors in the concrete floor connected to the building ground system. That should take care of the stray current problem in buildings. Stray currents could still be a problem outdoors (if the livestock can go there).

bud--

Don The four wire set up did eliminate three + amps of current but breaking the connection to the MV neutral eliminated over ten amps of stray current. Point is the motivation for having a truly derived electrical system is that it puts all of the systems characteristics under the owners control. The power utilities needs and the customers do not always coincide in the short run.

I no longer recall what the MV supply configuration was. The output side of their transformer had two fully insulated bushings. There was a case connection but only the MV MGN was attached to it. The tariff was for any customer who had a long run of private service conductors. This was the only tariffed service that did not include a grounded conductor. I don't know if the tariff was actually intended to supply ungrounded service but the fact that it didn't forbid it was good enough for me.

On Sat, 28 Oct 2006 03:12:23 -0500 Bud-- wrote: | snipped-for-privacy@ipal.net wrote: |> |> If I were a dairy farmer, I would at least isolate the milking operation, |> which is where the livestock would be near grounding electrodes and when |> their sensitivity would have a significant impact. I don't know offhand |> what kind of power they need. But a plastic natural gas pipe, or a propane |> tank, connected to a generator, might do the job. The area might still |> need a ground ring to divert any exterior ground currents passing by. Or |> maybe I could "go green" and power everything from solar power sources. |> Or maybe from indigenous methane :-) |> | As Tom alluded to, buildings for livestock have to have equipotential | floors - mesh or other conductors in the concrete floor connected to | the building ground system. That should take care of the stray current | problem in buildings. Stray currents could still be a problem outdoors | (if the livestock can go there).

The usually can go outdoors.

Which is more expensive, the equipotential floor (including cost of installation and disruption of operation) or the ungrounded service (and its cost of installation)?

What I want, though, is for there to be no MV system currents on my grounding electrodes whatsoever. I don't want the added corrosion. I don't want the added induced hum. I don't want the intermodulation with my RF fields. And I just don't want it for the principle of the thing.

May be rather limited in factory farms. If outdoors, maybe cattle orient up to minimize voltage between front and back of animal, and might be the equivalent of iron filings in a magnetic field. ;-)

In new buildings, which is what the NEC applies to, an equipotential floor shouldn't cost much.

bud--

---------- If the MV side of a transformer is connected line to ground and the LV load is perfectlybalanced, then there definitely will be a higher neutral current in the MV system. Consider a 7200/240/120 transformer. If the load is 2.4KVA at 240 V the secondary current is 10A and the secondary neutral current is

1. The primary current will be 0.33 A and the primary neutral current will be 0.33 A. This assumes negligable ground resistance but for a 100 ohm ground rod resistance the neutral to true ground voltage will be about 30V. Now consider that the secondary load is 120V 7.5A on one leg and 120V, 2.5A on the other leg. Secondary neutral current will be 2.5A while primary neutral current will be about 0.2A. Which dominates? Even at 1.3KVA on one leg and 1.1 KVA on the other, the secondary neutral current will be about 1.7A while the primary is still nearly 0.33A. If the load is essentially 240V or very well balanced between legs of the secondary, the secondary neutral current will exceed the primary current. However, a factor that exists is that there may be downline loads which produce MV neutral currents. This could be of concern.

-------

-------- Fair enough- I indicated as much.

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------

This implies an ungrounded primary except possibly back at the main substation neutral point. Either line to line or line to neutral (neutral grounded at onl one point-the main sub). If main sub doesn't have a ground point- then the line is floating. For distances measured in feet or even a few miles at MV , this may not be a problem except for protection purposes. Otherwise, we are back to the reasons why delta transmission is not used. Viable- yes- within limits- advantages- maybe, maybe not.

------- Any I have seen don't have such a contact. ----------

--------- Much of what is going to happen is dependent on a: location of the line with respect to the region of concern. While currents do radiate out from grounding electrodes( and current density and step voltage drops decrease rapidly), AC currents will tend to follow the line. This could mean that the problem is not due to local circumstances but due to what is going on downline. However, as Tom states, going to a 4 wire system did eliminate 3A , use of a higher voltage transformer at some distance from the farm, and stepping down eliminated much more "stray current". This may not always work as the source of the current may be elsewhere. Adding the transformer at the service drop may well be the answer- if the service drop is far enough away but other factors come in.

----------- Thanks for the information. I do realise that I am looking from a relatively academic point of view and I really do appreciate your experience, observations and your solution. There's more to this than what appears on the surface because there are many unknown factors.

Do you have any information on the downline loads and the proximity of the line itself to the dairy barn? This as well as the feed to the customer- 3 phase Y or single phase line to ground? If the latter, were all three phases on the line? Any other info would be of interest. There are factors that are not under the customer's control (i.e. bad grounds or unbalance downline) but can affect a customer.