AC cord polarity?? Does it matter???????
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Nsgarch, Thank you for the informative response, and will look into it further. I did find some interesting articles after posting my last response, but still have inquiries pertaining to non-ferrous chassis having the same effect. Lots of veteran electronic engineers at my work, so I'll get to the bottom of this, yet. Thanks again... |
First, there is no such thing as "zero volts," we just agree to call something zero (the earth) and measure everything relative to it. It is completely arbitrary. Second, neutral and ground are bonded in the sevice panel and only there. Go to Home Depot and look at a panel. The neutral bar and the ground are bonded. Another term for neutral is grounded conductor. Third, in a house, circuits are taken off each "hot" conductor, and the currents in the grounded conductors (which are all tied together) cancel each other out. In other words, if, by chance, the currents on each hot leg were exactly equal, the grounded conductor could be disconected from the service neutral (and the ground) and every thing would work. All the current would flow through the hot legs. Fourth, the reason the grounded conductor measures hot relative to ground is because the conductor has resistance, and there is current flowing through it. Since there should be no current flowing in the ground, and the two are tied only at the panel, there will be a voltage difference at the outlet (but not at the panel). Fifth, Mr. Holt does not mention, although I am sure he knows, that all power is generated and transmitted as three phase, each 120 degrees apart in time. When each phase has equal current, they sum to zero and there is no net current to flow through the ground. Any current is due to the three phases being unequal and will be small relative to the total current draw. If any significant power is disipated in the Earth, the power company loses money. The Earth naturally has absolutely enormous currents flowing in it, making the Earth into a giant magnet. Polarity is a safety issue. |
>>"Fifth, Mr. Holt does not mention, although I am sure he knows, that all power is generated and transmitted as three phase, each 120 degrees apart in time. When each phase has equal current, they sum to zero and there is no net current to flow through the ground. Any current is due to the three phases being unequal and will be small relative to the total current draw. If any significant power is disipated in the Earth, the power company loses money."<< >>> >>"Mr. Holt does not mention"<< >>>>>>>>> The article was not written by Mr. Holt. ===== >>"When each phase has equal current, they sum to zero and there is no net current to flow through the ground."<< >>>> In my neighborhood there is only one 7200V HV line on top of the power pole, feeding many pole mounted transformers. Lets say for argument, (A) phase of the 3PH 4W 7200/13200V wye feeder leaving the substation. No balance here....Just one ungrounded 7200V conductor and the grounded conductor, the neutral (single phase).... No balance in the (B) phase neighborhood or even the (C) phase neighborhood. I will grant you at the point where the 3PH 4W HV feeder branched out going their separate ways, from that point where the neutrals were joined to the main feeder neutral, back to the subtation the HV neutral only carries back the unbalanced load plus*.... ======= * http://www.ibiblio.org/obp/electricCircuits/AC/AC_10.html#xtocid269101 |
Jea48, thanks for straightening me out. The author is Donald Zipse, and Mr. Holt reproduces the article. I should have been more careful. I agree with your point about each neighborhood being on a different phase, but my point is that the net return current of the entire system is very small compared to the load. While each neighborhood may be on its own phase, my third point still holds and the neutral current is relatively small. Mr. Zipse's proposal (comparing fig. 21 and 22) is to remove the neutral-ground bond from the panel and replace it with a dedicated ground wire back to the transformer neutral-ground connection. One problem I can see with this approach is that if this new 4th ground conductor was damaged or the transformer ground missing or damaged, as far as the customer could tell, the system would function normally. It much easier to visually verify the neutral-ground bond at the panel than to vouch for the integrity of a 4th conductor to an off site ground. I can appreciate the logical appeal of completely separating the neutral and the ground functions and wires. However the cost to implement would be huge, the benefit theoretical. The author himself states that his proposal was "soundly rejected" by both the NESC and the NFPA. |
