From: dh0496@win*&dstr$%em.net   
      
   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?   
   >>                                                             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.  The temptation to cheat "temporarily" gets the best of people.   
   A local mechanic claims there is nothing more permanent than temporary.   
   >   
   > 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.  The company has been   
   furnishing 10 foot ground rods which might help a bit.   
   >   
   > 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.   
      
   > So imagine one phase of a 60 HP motor gets grounded. The fault current   
   > of 5.5 A may eventually trip the motor overload protection (running   
   > current at 60 HP is about 75 A). Or if the motor is not loaded to 60 HP   
   > - not. Or if the ground is before the motor starter (and overload   
   > protection) - not.  Divide the 277 V phase voltage between the 2 rods -   
   > 138V at each rod. In general 70% of the voltage drop away from the rod   
   > is in the first 3 ft from the rod. From your "grounded" equipment 3ft   
   > away from the rod there will be at 96 volts from equipment "ground" to   
   > earth. You don't have to be far from the rod to have more like 138 V.   
   > That assumes that the utility earthing is the same as at the service,   
   > which very well may not be true.   
   >   
   > If the service is at the pivot/well and, in effect, earthed through the   
   > well, 277V may appear near the meter can and there could be hazardous   
   > voltages on the equipment away from the well.   
      
       I hadn't thought about that. A shorted motor on the pivot could   
   put dangerous voltage on the well motor or panel.   
   >   
   > If the resistance-to-earth is lower there is a higher probability of   
   > tripping overload protection (but not likely the branch circuit   
   > protection). But the contact-voltage hazard does not change.   
   >   
   > IMHO there is a significant shock or electrocution hazard. In addition   
   > to lawsuits from victims/survivors, OSHA could be very unhappy, which   
   > means companies/individuals become very unhappy. It is, in any case, a   
   > NEC violation.   
   >   
   > ------------------------------------   
   > For the 70% drop away from a rod:   
   > Imagine concentric 1" thick cylinders of earth, the length of a rod,   
   > away from the rod. They are in series, and the resistance-to-earth will   
   > be the sum of the resistance of each cylinder. Compared to the cylinder   
   > at 1 ft, the cylinder at 2 ft has 2 x as much earth, thus 1/2 the   
   > resistance. The cylinder at 4 ft has 4 x the earth, thus 1/4 the   
   > resistance as the cylinder at 1 ft.  The resistance rapidly decreases   
   > away from the rod, hence most of the voltage drop occurs near the rod.   
   >   
   > For your amusement, approximate 3-ph currents @480 V:   
   > 60 HP      77 A   
   > 100 HP    124 A   
   > 1/2 HP    1.1 A   
   > 2  HP      3.4 A   
      
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