From: dh0496@win*&dstr$%em.net   
      
   bud-- wrote:   
   > On 1/26/2019 6:59 PM, Dean Hoffman wrote:   
   >> bud-- wrote:   
   >>> On 1/23/2019 7:17 PM, Dean Hoffman wrote:   
   >>>>      I work for a company that sells center pivot irrigation systems.   
   >>>> A common situation   
   >>>> is the pivot and electric well motor are powered from a commercial   
   >>>> utility.  The electrical service is three phase, 480 volts,  and there   
   >>>> will be 1300 feet of quad wire buried from the utility's meter to the   
   >>>> well and pivot.   
   >>>>     We sometimes see that one of the underground power wires has gone   
   >>>> bad.  Someone will simply substitute the equipment ground wire for the   
   >>>> bad power wire.   People think the ground rods and earth will keep   
   >>>> them safe.  I'd like to have a short illustration showing that it   
   >>>> won't.   
   >>>>     I found a chart in an article that shows earth resistance.  Farm   
   >>>> ground is 100 ohms/meter.  Thirteen hundred feet or 396.24 meters x   
   >>>> 100 ohms equals 39,624 ohms resistance in the dirt.  I added 50 ohms   
   >>>> resistance for the two ground rods that would be at the utility's   
   >>>> power pole and at the well.   
   >>>>    An online Ohm's law calculator put the current flow from the well   
   >>>> motor to the utility's supply at 0.012 amps if there was a short to   
   >>>> the well motor's frame.   That wouldn't blow even the smallest fuse in   
   >>>> the equipment. Am I at all on the right track with this?   
   >>>>                                 
   Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Thanks,   
   >>>> gentlemen   
   >>>   
   >>> With the service at the center pivot/well I suspect what is happening is   
   >>> a service phase conductor (meter to pivot service equipment) is failing   
   >>> and the service neutral is being substituted. The service neutral is   
   >>> originally earthed at the supply transformer and the pivot service.   
   >>   
   >>     And hiking a corn field with a wire locator in July or August isn't   
   >> any fun.   
   >   
   > Yeabut - how about a dowser???   
      
        I did know one or two guys who claimed they had the gift. One used   
   the old farmer CeeTee pliers if I remember correctly. Digging a new well   
   generally isn't done during growing season unless the outer casing   
   collapses. Guys just make do if the issue is the the well losing water.   
   The well pumps some air which escapes through the sprinklers nearest the   
   center point.  They might be able to have column added to lower the   
   bowls depending on the static water level.   
   >   
   >> The temptation to cheat "temporarily" gets the best of people.   
   >> A local mechanic claims there is nothing more permanent than temporary.   
   >   
   > I like that.   
   >   
   > Center pivot irrigation seems to be spreading like weeds.   
      
        I grew up on a farm in eastern Nebraska during the 60s and 70s.   
   Irrigation was mainly furrow irrigation. Land levelers were busy guys   
   making it possible for water to run from one end of the field to the   
   other.  I don't remember any pivots in my immediate area back then.   
   Pivots gradually moved into eastern and central Nebraska as farms got   
   bigger. Furrow irrigation requires a lot of labor and isn't very   
   efficient for water use. Guys can control and monitor their pivots from   
   their cell phones now. Furrow irrigation involved hiking the field to   
   see if the water had made it to the lower end of the field. No more   
   opening and closing gates on pipe to irrigate another section of the field.   
   >   
   >>>   
   >>> The 100 ohms/meter is the "bulk resistivity" of earth. You can't   
   >>> multiply by length to get a resistance. It is likely the resistance of a   
   >>> 1 ft cube of earth, measured between 1 ft square plates on opposite   
   >>> sides. If you are measuring between ground rods, the further you are   
   >>> from the rods the more earth is in the path.   
   >>   
   >>      That directly answers my question.   An ungrounded well 50 feet   
   >> from the electric supplier's meter isn't really any safer than an   
   >> ungrounded well 1300 hundred feet from the electric supplier's meter.   
   >>>   
   >>> You are using 25 ohms as the resistance-to-earth from a ground rod. It   
   >>> is the maximum allowed by the NEC. Unless the installer actually   
   >>> measured it, there is no reason to believe it is that low (or it could   
   >>> be less). Dry earth would be significantly higher resistance than wet   
   >>> earth. If 2 connected rods are installed there is no NEC requirement for   
   >>> resistance, so a common practice is to just install 2 rods. Ground rods   
   >>> are better than nothing, but not by much.   
   >>   
   >>      We started using ufer grounds a few years ago but there are bunches   
   >> of wells and pivots with just ground rods.   
   >   
   > Ufer is concrete encased electrode (20 ft) - in pivot base??   
      
      Yes.  I must've learned Mr. Ufer's name from a newsgroup or maybe   
   from Mike Holt's site. There are still bomb storage facilities east of   
   Hastings, NE.   
   >   
   >> The company has been furnishing 10 foot ground rods which might help a   
   >> bit.   
   >   
   > You can also get sectional ground rods. You drive a rod, attach a   
   > coupling, add a rod on top, and drive it. Can do multiple rods.   
   >   
   > Well casing might be good.   
      
       Casing in the old days was concrete.  Well drillers are using pvc now.   
   >   
   >>>   
   >>> Your example would be 277 V (phase-to-ground) divided by 50 ohms, which   
   >>> is 5.5 A, which is likely much less than any branch circuit protection.   
   >>> As gfretwell wrote, the NEC does not allow the earth to be used as the   
   >>> path to trip overcurrent protection. That is because an earth path will   
   >>> not reliably work.   
   >>>   
   >>> (Around here corner-grounded 480 V might be used, so disconnects are   
   >>> 2-pole.)   
   >>>   
   >>       Some of the surrounding utilities are using those. One is   
   >> apparently switching to a wye connection.  Variable speed drives on   
   >> a well motor can save $3/hour according to the University of Nebraska.   
   >> I'm hoping those catch on.  A vfd sees corner ground as single phase   
   >> power so it has to be over sized on those.  More dollars, of course.   
   >   
   > The argument I have heard for corner-grounded is cheaper 2-pole   
   > disconnects. Single phase VFD should negate that, and I wouldn't think a   
   > utility would like the unbalanced load.   
      
       Our electrical suppliers don't sell single phase disconnects for 480   
   volt. We have to put dummy fuses in the grounded leg. The power   
   suppliers want the grounded leg switched even though they put in single   
   phase meters.  The grounded leg and equipment ground are joined in the   
   power company's meter.   
   >   
   > -----------------------------------   
   > Interesting what backgrounds (like you) are around a.h.r   
      
       It beats talking to my coworkers. All of the different experiences   
   are a big help. I hope to maybe make something like a diagram on Mike   
   Holt's site.  It shows a power pole with a goofball kneeling next to it.   
      
   [continued in next message]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)