American Power Distribution Practice - Is a multi-grounded primary more desirable than a single-grounded primary?

I have not worked on distribution in a few years. I do not remember ANY medium voltage system ( less than 69kv) that has a neutral. 3 hots and a ground but not a neutral.

?? The system you see hanging on most three phase poles in the US had three phase conductors and a neutral (grounded conductor). Each piece of medium voltage equipment, along with many poles (often every third one) in areas without a lot of medium voltage equipment, will have a ground (grounding conductor).

As for Mike Holts support of a 5 wire system, he has gone overboard with his stray voltage tirades. He often equates stray voltage (very low voltages that are annoying but do not kill) with unintended energized metal objects (which can and do kill). One has NOTHING to do with the other. An unintended energized metal object is energized by a power source (such as a street light circuit) and has the capability of killing someone who comes in contact, such as the woman walking her dog in New York City recently. Stray voltage, on the other hand, is usually on the order of a few volts and can cause an unsettling shock, but not a damaging one.

I have a lot of respect for Mike's knowledge of the code and the reasons for things in the code, but on this issue he has missed the boat completely.

Charles Perry P.E.

The NEC does not govern medium voltage distribution construction of electric utilities. So any change would be irrelevent.

Charles Perry P.E.

Airport lighting uses medium voltage and a neutral - lots of single-phase lighting circuits. The neutral is supposed to be grounded, too - and trouble results when it suddenly becomes no longer grounded.

Bill

Well, there's more and more stuff in the NEC every code cycle - who knows what will be included by 20007?

Bill

Still wouldn't matter. The NEC could include any requirement they wanted and it wouldn't matter. Electric Utilities are held to the NESC, not the NEC. Two completely different codes.

Charles Perry P.E.

One of the reasons I posted these two articles is that, in my research, I found it fascinating that there could be so much strong emotions amongst professional engineers in the arguements for and against ungrounded vs. grounded distribution systems.

Apparently, in the US at least, the split also goes along with the two regulatory bodies NEC and NESC, that you describe and more roughly, splits the interests of utilities vs. the safety and well-being of homeowners and farmers.

My concern is with the reinforcement and codification of bad policy. For example, as one of the papers pointed out, NEC standards for RV vehicles were apparently just plain wrong in the 1950's when the neutral was allowed to served as ground and people were electrocuted just by touching the metal frame of their RV and standing on damp ground.

Apparently there were similar problems extended to marinas, which have their own unique problems with grounding issues. It took years of study before the regulations "made it right" with a safer insulation and a fully grounded system. In the meantime, how many unnecessary electrocution deaths occured to swimmers in all those early years when inferior and lax regulations were in place?

Beachcomber

The problem with your arguement is that the current system, as applied to medium voltage distribution lines in the US, is NOT killing people. Some stray voltage problems exist, but they are usually localized problems. These stray voltages are not killing people. They do scare people but they don't kill them.

In many of the stray voltage cases we have studied, the high levels of 3rd harmonic current in the neutral have been a major contributor. These currents are created by electronic loads and since they are triplens, they add, rather than cancel, in the neutral. I propose that the US adopt a European style regulation for all, and I mean ALL, electronic equipment requiring power factor corrected power supplies. Fewer harmonics, better power factor, everyone wins.

Charles Perry P.E.

Thanks for the reply. Just to clarify, I'm not advocating either point of view. I'm just interested in hearing from the experts in the industry, like yourself, as to what the current state of this debate is.

I will say this though, yes the stray voltages, where they exist, are not killing people, but apparently there are serious consequences to dairy farms, in particular, with the cows becoming sick, not producing milk, etc. I'm not sure that all of these problems are solved, but it does seem to be a good thing that utilities now pay attention to the problem when it comes up, instead of issuing a summary dismissal as a "customer wiring problem".

I don't think there is any best solution or design to an electrical system. I think the systems evolved with the culture and the unique characteristics of the region.

Here in the US, the formulative agency seemed to be the REA (Rural Electrification Administration) that decided during the depths of the depression that the new distribution system would be a grounded system, single-phase to most residential users, and a 240/120 split phase Edison 3-wire connection at the service entrance.

I'm sure they did this with the cost savings in mind (it does seem to be the most economical system in terms of using the least amount of copper wire). But that is logical as it was the depression, after all.

I like to think that the appointed reps who issue changes to the electrical codes should be open to new ideas and not just say "well that's the way we've been doing it for fifty years... why do we need to change it"?

Beachcomber

Sir I do not say this to be quarelsome

I do not say this to be quarrelsome but I am troubled by your assertion that the stray current problem is only a nuisance. Stray currents are causing losses in the dairy industry and many dairy farmers are unaware that their production is affected. My electrical business is overwhelmingly suburban residential but I have worked on two farms that saw an increase in production when the farm was isolated from the medium voltage Multi Grounded Neutral (MGN). In one case I simply opened the farms supply including the neutral that was common to the MGN and production shot up by more than a third. This was obviously not one of the extreme cases that has been widely publicized. The stock did not show any obvious sign that they were distressed. I was there to tie in an Engine Alternator Set (Generator) for outage protection and the dairy was running on it's PTO generator while I was working on the new transfer equipment. The farmer mentioned that the animals must like the tractors white noise because the milking was way up. I got suspicious and measured the neutral current with the new main breaker open but with the neutral reconnected. With the cows out to pasture and the PTO generator shut down the neutral current was over twenty five amps. The utility agreed to supply power at 240 volts from an ungrounded secondary. They just unstrapped their secondary at the pole pig. The neutral was disconnected at both ends of the drop and an outdoor dry transformer was installed to establish a local only neutral from a true separately derived system. We did have to clean up several ground faults caused by the older method of connecting separate structures. I did that even though the total current flowing on the existing Grounding Electrode Conductors was less than five amps on PTO alternator power. If that farmer had not decided to install a Generator during a long outage caused by a storm the stray current problem might never have been discovered.

I will agree that the stray voltage is more a problem for dairy farmers. We have done several projects for dairy farms, utilities in dairy areas, and even state commissions.

This is the telling part. I most, but not all, cases poor wiring is the culpret. The worst part is that your farm might be wired perfect but if your neighbor has bad wiring, you might experience stray voltage problems. These problems can be very hard, and expensive, to track down. I would say "you would be surprised at some of the wiring we see" but since you have seen it, you wouldn't be. Perhaps disgusted would be a more accurate term. I am always amazed at what passes as acceptable wiring at some facilities.

I can't see the entire electrical industry rebuilding all of the distribution lines for the dairy farmers. Those problems are best addressed individually.

Charles Perry P.E.

Charles How can you say that the local ground connections were the telling part when I had over twenty five amps flowing in the neutral with the main breaker open and the total current flowing on all four Grounding Electrode Conductors following isolation was less than five amperes. That farms wiring was in relatively good condition. The only thing I changed was the way the individual buildings were grounded and I did that after the neutral current had been reduced to no measurable current with the main breaker open. What is very clear to me is that the utilities are using customer premise grounding to carry a substantial portion of their neutral current especially on longer lines.