US Electrician qualifications for a Brit

Y connected transformers are preferred on distribution systems. No problems with ferroresonance or backfeed.

Continue from where I left off...

Schuko is not bad, but it has one major disadvantage; it is unpolarised. This is not advisable with things like ES lamp sockets; though I think their days are numbered, I wouldn't be surprised to see them banned in Europe for new installations within a few years, for both safety and energy effienciency reasons. It's already a requirement that new buildings, or reburbished ones, have a certain proportion of lamp fittings that cannot take traditional incandescant lamps.

Plug design is one area where Europe got it badly wrong. In most of Europe the live and neutral pins are very similar in size and spacing. they are not identical, some are 4mm diameter, and some are 4.8. Some are at 19mm spacing, and some are something slightly different. To get around this problem the two-pin 'Europlug' was devised; it has two thin pins which are angled inwards slightly towards the tips. they are shrouded. The pins can bend slightly in the plastic moulding, the idea being that they can fit most European three-pin sockets. The problem is that it fits all of them badly, and is restricted to a low current, I think it's either 2.5 or 3A. I don't like this, I'd rather see standard sockets, and plugs that fit them properly. When you go to the

3-pin plugs they do dit properly, and can handle the full 16A load, but there are lots of different versions. There have been several attempts at standardisation, but agreement could never be reached. One suggestion was to abandon them all, and design something totally different. You'd better get that drawing of your design ready! Germany has the Schuko, with its top and bottom earth contact strips, France has something similar, but the earth contact is a pin in the socket which fits into a hole in the plug. Most Schuko plugs have such a hole, so they can be used with either type of socket. They are polarised when used in the French socket, but not in the German one. Italy uses something like a Europlug, but with an extra pin between the other two, and in line with them. This is also unpolarised. If you take this, but move the earth pin slighly upwards, to form a low, wide triangle, you get the Swiss one, which is polarised. Most of the other countries use one of thesetypes, except Demarrk, which has its own unique design, with strangely shaped pins. What a mess. I wouldn't recommend any new system to follow the European route.

that I'm not a great fan of the NEMA range. I think there are several problems with them:

There are too many of them, though this is not a problem with the connectors themselves, but simply reflects the number of different types of supply in America.

There's not really a logical pattern to them; you can't tell just by looking at an unfamilar connector what it is intended to be used for.

The vast majority, though not all, of the receptacles are unshuttered.

The blades are too thin; some of the poorer quality ones are quite soft, and bend easily. The metal is too thin to allow shrouded blades to be used. With a bit of effort I can just about touch the blades of a NEMA plug at the piint where they make contact; I doubt that I could do so accidently, but a small child certainly could.

Another problem, though again not the fault of the NEMA design, is that many of them are of very poor quality; there are good ones available, but there's also a lot of absolute junk. I find that if you speak to an American about this you generally get one of two responses; either

"The higher quality receptacles cost about a dollar more" This is for the better domestic grade, not the industrial grade ones. So what, that would add maybe $100 to the total cost of a house, hardly a major increase; isn't it worth it?

The other response is: "The cheap ones are good enough, they do the job" I'm sorry, but in my book a plug or socket which gets distinctly warm, if not downright hot, when carrying well under its rated current, or where the pins are obviously burned is not 'good enough'. I'm not a great health and safety fanatic, I think a lot of today's regulations go too far, but some of these things really are a fire hazard, and shouldn't be on sale. Fifty years ago we had a lot of equally bad stuff here, but we've got rid of it from the shops long ago, and you seldom see any still in use.

So, I'm really not totally happy with any of them. I'm not too worried about the shape of the pins; I'd like them to be nice and solid, with a large contact area, shuttered sockets, or at interlock so that the socket cannot be live when there is no plug inserted, shrouded pins, or other means to prevent partially-inserted pins from being touched. I think you could bring the pins closer than on post of the existing designs, to reduce the overall size.

| | Y connected transformers are preferred on distribution systems. No problems | with ferroresonance or backfeed.

I would definitely have three phase transformer secondaries configured as Y or star. The primary would be delta. The loads would be L-L or L-L-L.

On 11 Oct 2006 11:41:22 -0700 snipped-for-privacy@mail.croydon.ac.uk wrote: | Continue from where I left off... | | Schuko is not bad, but it has one major disadvantage; it is | unpolarised. This is not advisable with things like ES lamp sockets; | though I think their days are numbered, I wouldn't be surprised to see | them banned in Europe for new installations within a few years, for | both safety and energy effienciency reasons. It's already a | requirement that new buildings, or reburbished ones, have a certain | proportion of lamp fittings that cannot take traditional incandescant | lamps.

If by ES you mean Edison Screw ... I've already heard those are banned in some places like Germany, at least for pluggable lamps.

| Plug design is one area where Europe got it badly wrong. In most of | Europe the live and neutral pins are very similar in size and spacing. | they are not identical, some are 4mm diameter, and some are 4.8. Some

My understanding is that Europe was 220/127 before Edison, and the first plug design was based on that, and hence unpolarized because the power was L-L. When electric lights came along, rather than connect them to 127, they connected them to 220. Dangerous. Then my guess is they changed things over to 380/220 for a combination of reasons, like more efficient use of wire, and getting a grounded wire on the screw base shell.

| are at 19mm spacing, and some are something slightly different. To get | around this problem the two-pin 'Europlug' was devised; it has two thin | pins which are angled inwards slightly towards the tips. they are | shrouded. The pins can bend slightly in the plastic moulding, the idea | being that they can fit most European three-pin sockets. The problem | is that it fits all of them badly, and is restricted to a low current, | I think it's either 2.5 or 3A. I don't like this, I'd rather see | standard sockets, and plugs that fit them properly. When you go to the | 3-pin plugs they do dit properly, and can handle the full 16A load, but | there are lots of different versions. There have been several attempts | at standardisation, but agreement could never be reached. One | suggestion was to abandon them all, and design something totally | different. You'd better get that drawing of your design ready! | Germany has the Schuko, with its top and bottom earth contact strips, | France has something similar, but the earth contact is a pin in the | socket which fits into a hole in the plug. Most Schuko plugs have such | a hole, so they can be used with either type of socket. They are | polarised when used in the French socket, but not in the German one. | Italy uses something like a Europlug, but with an extra pin between the | other two, and in line with them. This is also unpolarised. If you | take this, but move the earth pin slighly upwards, to form a low, wide | triangle, you get the Swiss one, which is polarised. Most of the other | countries use one of thesetypes, except Demarrk, which has its own | unique design, with strangely shaped pins. What a mess. I wouldn't | recommend any new system to follow the European route.

There was no EC back then. Everyone was on their own. So development went in crazy ways. There were some historical plugs with crazy designs even in the USA. Fortunately, those disappeared.

|>From what I've said earlier, you probably won't be surprised to hear | that I'm not a great fan of the NEMA range. I think there are several | problems with them: | | There are too many of them, though this is not a problem with the | connectors themselves, but simply reflects the number of different | types of supply in America.

Exactly. I have the document that describes them all. Overwhelming. I'm trying to make web construct images of most of them. Some of that is already seen in some URLs I have shown.

| There's not really a logical pattern to them; you can't tell just by | looking at an unfamilar connector what it is intended to be used for.

There was a little bit of logic in a few. But as it expanded, logic got lost. That's probably because no one who had vision also had the power to make it happen.

| The vast majority, though not all, of the receptacles are unshuttered. | | The blades are too thin; some of the poorer quality ones are quite | soft, and bend easily. The metal is too thin to allow shrouded blades | to be used. With a bit of effort I can just about touch the blades of | a NEMA plug at the piint where they make contact; I doubt that I could | do so accidently, but a small child certainly could.

Yes, there are thin bendable blades around. But I also have many that are solid and sturdy. I would go a bit thicker, maybe as much as the blades in the BS1363.

| Another problem, though again not the fault of the NEMA design, is that | many of them are of very poor quality; there are good ones available, | but there's also a lot of absolute junk. I find that if you speak to | an American about this you generally get one of two responses; either | | "The higher quality receptacles cost about a dollar more" This is for | the better domestic grade, not the industrial grade ones. So what, | that would add maybe $100 to the total cost of a house, hardly a major | increase; isn't it worth it?

Contractors and speculators build homes under the principle of "cheapest everywhere". Maybe they save only $100 on receptacles and switches. But if you add that up for all the other things they go cheap on, it can add up to thousands of dollars of savings. If there is an argument why they should not go cheap, wouldn't it apply to the whole house?

| The other response is: "The cheap ones are good enough, they do the | job" I'm sorry, but in my book a plug or socket which gets distinctly | warm, if not downright hot, when carrying well under its rated current, | or where the pins are obviously burned is not 'good enough'. I'm not a | great health and safety fanatic, I think a lot of today's regulations | go too far, but some of these things really are a fire hazard, and | shouldn't be on sale. Fifty years ago we had a lot of equally bad | stuff here, but we've got rid of it from the shops long ago, and you | seldom see any still in use.

UL should be testing them. I don't know how that is working out.

| So, I'm really not totally happy with any of them. I'm not too worried | about the shape of the pins; I'd like them to be nice and solid, with a | large contact area, shuttered sockets, or at interlock so that the | socket cannot be live when there is no plug inserted, shrouded pins, or | other means to prevent partially-inserted pins from being touched. I | think you could bring the pins closer than on post of the existing | designs, to reduce the overall size.

My thought was 4mm to 6mm spacing for the lower voltage I proposed, and

10mm to 15mm for the higher voltage I proposed, at the 16 amp level.

I've also thought about the idea of using a coaxial pin construction where the end of the pin has the hot contact, and the sleeve has the grounding/earth contact. The opening would have the grounding wire contacts completely blocking the opening, effectively being the shutter. Both pins would be this way so there would be 2 connections of ground. The tip of the pin would have to break from the contact for power as it is removed, before it reaches the grounding shutters. Maybe this would be a good design for the lower voltage plug. It would still be possible to push the shutters open by sticking something in, but at least if the hot contact is made inside, it is likely that whatever does contact it is also grounded at the shutters that are squeezing on it.

Got news for you. The Valdez Marine Terminal at the end of the Trans Alaska Pipeline has about fourteen miles of cable tray installed in

1975-76 under the supervision of Fluor Daniels Engineering. The cables in the cable tray have no grounding conductors - that's right none!

Some of the replacement, low power consumption lamps, still come with screw bases.

In spite of its shortcomings, the ES was one of the great inventions of its time. Early lamp bulbs had to be "installed" by crews of boys on bicycles, who brought replacement bulbs to customers.

The screw vase was cheap, low tolerance, provided both electrical and mechanical interface, and could be used by anyone without tools. There was the story of a farm wife, whose house had just been wired, and was asked how she liked the new lights. She answered, "The light's fine but I burn my hands taking it from one room to the other."

(How many programmers does it take to change a lamp bulb?) ;-)

Lamps were an early example of the "Gillette plan", where you give away the product and make a good profit on the consumables. (Now the HP or Epson plan.)

My family lived in Saint Louis in the early thirties. The power company would regularly raise the voltage to burn out "old" lamps. They would give free replacements, which used more power than the old ones.

You can get printers, almost free, that way now! :-)

I guess there aren't any utility engineers reading this thread. I was holding off hoping one of them would comment.

L-L or delta transformers are _REALLY_ discouraged for new installations. Most new installations are Y-Y for three phase or L-N for single phase. L-L can cause all sorts of problems when a phase is lost. First off there is ferroresonance. That's when the magnetizing inductance of the transformer resonates with the line capacitance of the open phase. It's a real problem for underground service because of the higher capacitance of underground lines. It can still be a problem for open overhead, particularly at higher voltages, particularly 34.5kV. I have heard of lots of problems when utilities upgrade to 34.5kV primary and still have delta connected services they can't change, for example a customer with 240V center tapped delta. The other problem is with back feed to a faulted phase. Yes, motors can back feed and add to fault current. But usually the motors can't blow the line fuses on the transformer. But a delta connected bank will. It's not uncommon for every delta bank to blow one or two fuses when one of the primary phases has a hard fault (like crossed over conductors) and the source is single phase tripped. After the line is repaired and the feed restored, the line crew must then go around and refuse a bunch of transformer banks. Three phase tripping eliminates this, but many utilities like the single phase fault protection out on long lines.

|> | |> | Y connected transformers are preferred on distribution systems. No |> problems |> | with ferroresonance or backfeed. |>

|> I would definitely have three phase transformer secondaries configured |> as Y or star. The primary would be delta. The loads would be L-L or |> L-L-L. |>

| | I guess there aren't any utility engineers reading this thread. I was | holding off hoping one of them would comment. | | L-L or delta transformers are _REALLY_ discouraged for new installations. | Most new installations are Y-Y for three phase or L-N for single phase. L-L

I still see L-L, which I assume to be D-Y, all over the place. And in dry transformers, it seems to be the only way it is done.

| can cause all sorts of problems when a phase is lost. First off there is | ferroresonance. That's when the magnetizing inductance of the transformer | resonates with the line capacitance of the open phase. It's a real problem | for underground service because of the higher capacitance of underground | lines. It can still be a problem for open overhead, particularly at higher | voltages, particularly 34.5kV. I have heard of lots of problems when | utilities upgrade to 34.5kV primary and still have delta connected services | they can't change, for example a customer with 240V center tapped delta. | The other problem is with back feed to a faulted phase. Yes, motors can | back feed and add to fault current. But usually the motors can't blow the | line fuses on the transformer. But a delta connected bank will. It's not | uncommon for every delta bank to blow one or two fuses when one of the | primary phases has a hard fault (like crossed over conductors) and the | source is single phase tripped. After the line is repaired and the feed | restored, the line crew must then go around and refuse a bunch of | transformer banks. Three phase tripping eliminates this, but many utilities | like the single phase fault protection out on long lines.

How does a D-Y backfeed (not the motor backfeeds)?

Anyway, if you really really want all the harmonics issues you'll get with a Y-Y transformer configuration, then you can have your Y primary. The design of the L-L and L-L-L loads doesn't depend on the transformer primary that I am aware of.

What do you suggest as a means to prevent primary ground return through the secondary service drop ground?

--snipped a bunch--

It depends on where you are. Some utilities still have substations set up for delta connected primaries, hence they continue to add new services delta or L-L. But for a brand new substation, or for primary voltage upgrades, it's overwhelmingly set up for Y connected primaries.

Unless special ordered. But as you note, most dry types are are D-Y. Ferroresonance isn't a problem there since they are pretty much only used where the primary is feed from a three phase breaker.

--snipped more--

If a phase is opened on the primary, the opened phase wants to center between the other two phases. It can backfeed enough current to blow the primary fuses.

Zero sequence harmonics. The positive and negative phase sequence harmonics go through the D-Y connection also. But yep, more harmonics. The zero sequence harmonics go back to the substation, and usually there is a delta or zig-zag winding somewhere that will eat up them up.

Nothing is done. You trade that off with the voltage rise on a primary to secondary fault. In a delta system, there is no primary ground present, so it relies entirely on the secondary ground system. Since that often can be

10 + ohms, it's possible to get a LOT of voltage rise when that happens. One good thing is that the ground relays can be much more sensitive to trip quicker, but then there is a loss of selectivity.

|> How does a D-Y backfeed (not the motor backfeeds)? | | If a phase is opened on the primary, the opened phase wants to center | between the other two phases. It can backfeed enough current to blow the | primary fuses.

What are the phase angles of this current with respect to the remaining phases?

|> What do you suggest as a means to prevent primary ground return through |> the |> secondary service drop ground? |>

| | Nothing is done. You trade that off with the voltage rise on a primary to | secondary fault. In a delta system, there is no primary ground present, so | it relies entirely on the secondary ground system. Since that often can be | 10 + ohms, it's possible to get a LOT of voltage rise when that happens. | One good thing is that the ground relays can be much more sensitive to trip | quicker, but then there is a loss of selectivity.

But the delta system won't have a primary neutral to secondary neutral path because there is no primary neutral to have a path with. So all you have is the middle L-L voltage relative to ground (e.g. half of the L-L voltage) as capacitive coupling. The ground can eat that easily as the current is just small charging current. But when you have L-N, the primary N is connected to the secondary N since both are grounded there. Bad neutral connections don't result in a system failure, but rather, in ground currents that get picked up elsewhere at better connections. The end result is part of the primary return current is by way of the LV service drop neutral and the customer grounding electrodes.

Somewhere close to in phase, unless a motor is pulling it out.

It's a trade off of the all the time problematic neutral current with Y connected vs. the potential high voltage on the ground under failure with delta. The ground on a service may not be low enough in impedance to trip the primary protection, so it could just sit and cook.

Delta delta was used long ago- capacitance problems could mean that appreciable fault current could flow in the case of a phase to ground fault- without being detected. This can occur even where resonance or ferroresonance is not a factor. In addition, the problem with ungrounded overhead lines is that the line to ground potential is at the mercy of any overhead charged cloud- a cause of such phase to ground faults (this was discovered about 70 years agoso Delta at the HV level went out) . Hence the idea of a system which is Y grounded. Y-Y transformers have problems with third harmonics in the voltages while Y-Delta do not. Part of the problem can be eliminated by Y-Y delta for larger units such as autotransformers or grounding both Y s to the same point so that triplen harmonic currents flow through but voltages are normal. Where single phase supplies are considered there are advantages to a (grounded) Y secondary.

|> |> How does a D-Y backfeed (not the motor backfeeds)? |> | |> | If a phase is opened on the primary, the opened phase wants to center |> | between the other two phases. It can backfeed enough current to blow |> the |> | primary fuses. |>

|> What are the phase angles of this current with respect to the remaining |> phases? | | Somewhere close to in phase, unless a motor is pulling it out.

In phase to which of the 2 remaining phases?

If you have 3 lines running to a transformer primary that is a delta, where the secondary is wye, and one of those 3 lines loses power, what is the phase of the voltage being fed back into the dead line by that delta primary?

Suppose phase B goes out and phases A and C remain powered. The A-C winding in the delta would be powered at full voltage (minus any drops due to the extended loading effects). Windings A-B and B-C would be in series relative to phases A and C, resulting in the corresponding secondary windings having half voltage. Phase B coming it is tapped in the center of that A-B and B-C series. I would think that the voltage being applied to line B would be however far offset from the center point that the middle of A-C would be at, which would be 28.9% (1/(2*sqrt(3)) of the L-L voltage (half of the L-G voltage), and at a

90 degree angle from A-C.

Is this the backfeed you are referring to?

| It's a trade off of the all the time problematic neutral current with Y | connected vs. the potential high voltage on the ground under failure with | delta. The ground on a service may not be low enough in impedance to trip | the primary protection, so it could just sit and cook.

Are you referring to a fault directly to earth? I don't see how that would be any different between delta and wye connected loads where the source is a wye secondary and the ground is carried on the distribution but does not normally carry current. The choice here is how to load the circuit, not how to supply the circuit. The circuit is the same (source is a secondary in wye configuration). What differs is whether the load put on it to feed a LV customer is connected D-Y or Y-Y or for single phase, L-L or L-N.

| Delta delta was used long ago- capacitance problems could mean that | appreciable fault current could flow in the case of a phase to ground | fault- without being detected. This can occur even where resonance or | ferroresonance is not a factor. In addition, the problem with ungrounded | overhead lines is that the line to ground potential is at the mercy of any | overhead charged cloud- a cause of such phase to ground faults (this was | discovered about 70 years agoso Delta at the HV level went out) . | Hence the idea of a system which is Y grounded. Y-Y transformers have | problems with third harmonics in the voltages while Y-Delta do not. Part of | the problem can be eliminated by Y-Y delta for larger units such as | autotransformers or grounding both Y s to the same point so that triplen | harmonic currents flow through but voltages are normal. Where single phase | supplies are considered there are advantages to a (grounded) Y secondary.

So what about every step along the way being a D-Y transformer, with the star point grounded AND carried along (and earthed at various intervals), but all loads connect only L-L-L (e.g. a D-Y for each LV service) or L-L?

But, with the grounded neutral carried along, you also have the option of loads from line to ground(L-N). This is the usual practice. In many rural situations a line and the grounded neutral are carried down one road supplying single phase transformers. while on other roads the other phases and neutral are used in order to get a rough load balance. This is cheaper than running either 3 phase or 2 of the 3 phases. If you check your service in the back alley in a residential area in a town or city, the same thing is done. For example, in my block, one phase (7200V to ground) and neutral are brought across and run underground to distribution transformers to get

240/120V single phase. The neutral is common to both the HV and LV sides. Even if 3 phase is present on the local line in the alley (if overhead), the typical procedure is as above, using L-N rather than L-L to supply local distribution transformers. A transformer designed for line to grounded neutral operation is cheaper than one designed for line to line operation at the same voltage.

There may be areas where single phase loads are connected line to line but I haven't lived in any of those.

In the past many small town supplies were D-D with local single phase transformers connected line to line and when the lines were getting near capacity, the main substation transformer was either reconnected D-Y or more likely replaced while the single phase transformers were then connected line to neutral. This meant that a small town's could be changed with a minimal disturbance and cost as no new distribution transformers or larger wires were needed (insulation was more than adequate). --

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

| But, with the grounded neutral carried along, you also have the option of | loads from line to ground(L-N). This is the usual practice. In many rural | situations a line and the grounded neutral are carried down one road | supplying single phase transformers. while on other roads the other phases | and neutral are used in order to get a rough load balance. This is cheaper | than running either 3 phase or 2 of the 3 phases. If you check your service | in the back alley in a residential area in a town or city, the same thing is | done. For example, in my block, one phase (7200V to ground) and neutral are | brought across and run underground to distribution transformers to get | 240/120V single phase. The neutral is common to both the HV and LV sides. | Even if 3 phase is present on the local line in the alley (if overhead), the | typical procedure is as above, using L-N rather than L-L to supply local | distribution transformers. A transformer designed for line to grounded | neutral operation is cheaper than one designed for line to line operation at | the same voltage.

However, I still would not do it that way. If cost were the exclusive reason for all decisions, we could have a much lower cost electrical system than we have today. But it would also be much less safe.

The inverse of this is that we can have a safer system, but it will cost some more. And that is how I would design it: safer

Having the same metal wire used for a return of MV circuits _and_ being connected to the LV customer service drop and the premise electrodes and EGC does have dangers. Many of those can be avoided, and many technical problems I believe may be linked to this bad practice, could be avoided by having a separate grounding wire on the MV distribution that is not used for return current whatsoever, which can be used for grounding the secondary side of the MV->LV transformers (along with a grounding electrode).

| There may be areas where single phase loads are connected line to line but | I haven't lived in any of those. | | In the past many small town supplies were D-D with local single phase | transformers connected line to line and when the lines were getting near | capacity, the main substation transformer was either reconnected D-Y or more | likely replaced while the single phase transformers were then connected line | to neutral. This meant that a small town's could be changed with a minimal | disturbance and cost as no new distribution transformers or larger wires | were needed (insulation was more than adequate).

How were the MV->LV transformers grounded before ... and after? I bet they were grounded with a wire that was NOT carrying current before, and afterwards, they were grounded with a wire that was carrying current.

They had a safer ground wire before. They took a step backwards in safety for economic reasons.

So, tell me, how can I wire up an isolation at the end of the service drop which isolates my grounding wires from the distribution neutral?

I may not have interpreted your question properly but as I read it, here goes.

In the situation that I mentioned, a previous step down delta-delta transformer was changed to a delta-wye transformer. Then single phase loads referenced to ground could be take from each phase. Increased capacity. Certainly there will be neutral current in the case of unbalanced loads. Is this a major factor for safety? Not really, provided that the neutral is properly grounded and of adequate size. Note that the heaviest currents in the neutral (about 97-100%) were those due to the 120/240 Edison system customer loads -these would not change but account for 95-100% of the total neutral current. In fact, any primary neutral current will actually reduce the current in the neutrals (admittedly not by much). In your home you are dealing with 120V/240V loads which are rarely balanced so the neutral carries current- does this bother you? Yes, equipment is tied to a seperate ground for good reasons. Note that the current carrying neutrals of a MV LV or HV-MV system are also very well grounded and the fact that they may carry current in the case of unbalanced loads is recognised and accounted for.

Now consider the delta with a neutral tap on one side. Will this mean that the neutral is not carrying current- ideally so but ??? Suppose also that it was 12.5KV line to line. That means that 2 legs are at 6.25KV with respect to ground and the other is at 14KV with respect to ground. Is this better than having all 3 legs at 7.2KV to ground? Zig- zag grounding transformers were often used to get a neutral point which was equidistant electrically from all phases. The center tapped leg of a delta is a cheap, but poorer alternative to this.

Seeing that the user with a single phase supply sees no difference from the situation where the distribution transformer is connected l-l vs l-n on the primary- your last question is meaningless. Run a separate ground wire if you want. If you are looking at an industrial system taking 3 phase from a delta with a neutral on one side and single phase to neutral loads on the tapped side- where while neutral current can't flow, unbalanced voltages can result -then I would prefer a Grounded wye system. A separate safety ground wire to the frames of equipment is just as feasible there as with the household single phase system. Note also that ground fault protection is a hell of a lot easier with a Y.

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

----------------------------

snipped-for-privacy@ipal.net wrote:

You can wire up an isolation transformer but you will almost inevitably have to violate one or more code rules to achieve full isolation. I wired up a dry transformer on a dairy farm to separate the farms wiring from the Multi Grounded Neutral (MGN) of the medium voltage distribution system. The power company threatened to cut off service until the public service commission of the state government got involved and pulled out an old tariff for ungrounded delta service. The rule is that if the service is tariffed then the Utility must provide it. When they tried to have this "outdated" tariff rescinded the entire dairy coop system apposed it. So far that is the only NEC compliant way I have found to actually break the earthing pathway between the MGN and the customers premise. The US NEC requires that the grounded conductor of any wye connected transformer be brought to the service equipment enclosure (Customer Service Unit) and bonded to it. That conductor must also be grounded / earthed at the customers end. This means that it is inevitable that stray MGN currents will be flowing over the customer premise grounding electrode system. The reason that that practice continues is that the cost of enlarging the utility MGN or installing a separate insulated neutral in the medium voltage distribution system gives the utilities' management nightmares. With the increasing population density the inadequacy of the present neutral system will become more and more apparent as the MGN becomes more heavily loaded and stray currents increase.

There is a special type of utility transformer that is specifically designed to supply dairy farms that accomplishes the prevention of utility neutral current flow on the secondary grounded conductor without violating the National Electrical Safety Code that governs there work. I have no idea how it works.

Edison may turn out to have been right about the dangers of AC current after all. Edison abandoned ground return for electric current fairly early in the development of the Edison electric system.