Ag service bonding

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Jameslopez

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I recently had to visit a dairy to consult about a possible PV installation. It surprised me when the dairy supervisor told me that he was worried about someone working in his Main because they might tie his ground and neutral together there and give him stray voltage problems. He informed me that his neutrals and grounds are separate in his main. I've worked in commercial electrical for most of my carrier but I never assume I know everything. I assumed that like most installations I've ever seen you have to bond your neutral to ground at your main and separate it thereafter. Is it allowable to keep the neutrals and grounds separate at the Main in a Agricultural installation? The panel was a old Zinsco 400A stand up switchboard and with a 3 phase 240v service which I'm also assuming has a stinger due to the fact that every third breaker space was empty
 
You might look at 547.9.
From what you say, it appears he has a site isolation disconnect and the install falls under 250.32.
Did you notice if he had a grounded and grounding conductor supplying his "main" ?
 
Thanks for responding. Now that I know a little bit more about what to look for I'll definitely check in the future but I didn't look at the number of conductors coming in overhead from the pole. I also was not looking for the Site Isolation Switch. After reading Article 547 and 250.32 it looks like if there is a Isolation switch on the pole then that is the new grounding point for the service and even though his 400A standup switch board is the point from which the rest of his site is fed, the neutral would not be bonded to ground there. Instead it would be bonded to a grounding electrode at the isolation switch. Am I correct on this? Also, the way I read 250.32 confuses me a little. It is written for Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s) and a Feeder is defined as "All circuit conductors between the service equipment...and the final over current protection device". Is the Isolation switch considered his service equipment and the 400A main the final branch circuit over current protection device? Why wouldn't 250.32 apply to all services? Maybe I'm just having problems understanding the definition of feeders and how it applies to 250.32.
 
A quick trip through definitions might help.
The service would stop at the service disconnect. The branch circuit is the wiring beyond the last overcurrent device. In between the two the wring would be a feeder, so you might have feeders feeding other feeders, etc.
If he has two or more buildings fed from one distribution point, then 547.9 will require a site-isolation device.
It appears from 547.9, the Code considers the conductors from the site isolation device to the buildings as feeders since it references Art 225.
 
A few code cycles back we were allowed to bring an EG from the Meter or what is now known as a site isolation device if used. That has changed back and forth several times over the years if I remember correctly.

Good luck doing an install for a Dairy. You may want to take a lot of time and document all or any potential problems now, because if any part of production goes South, you will be first on the list.
 
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