Replies

1. 
   johnnyd | Oct 03, 2003 10:00pm | #1

   What do you mean by "floating neutral"?  Is the entrance panel grounded to a rod?

   If the entrance panel is grounded to a rod, I think you've done it correctly.  The mobile home panel is then a subpanel to the entrance panel, and the ground and neutral in the mobile home panel should be separated.

   
   

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   Edited 10/3/2003 3:05:57 PM ET by johnnyd

2. 
   akv | Oct 07, 2003 07:50pm | #2

   The neutral is separate from the ground in the mobile panel. The service entrance panel is mounted on a metal tube frame embedded in the ground, but I can't find any other ground rods that the entrance panel is connected to. Is this deemed adequate for grounding, or should I install additional ground rods on the entrance panel?

   Thanks for the help.

   Alan

1. 
   johnnyd | Oct 09, 2003 05:59pm | #3

   Really think you should get some expert advice on this.  Your local inspector may be willing to provide advice, or pay an electrician for some design work.

   Worries me that there is no ground rod.  You need to provide an additional path (in addition to the neutral) to carry current in case you short out to a device chassis, and to make sure any over-current will properly trip the breaker.

   Try to get 4LORN1 in on this.

2. 
   4Lorn2 | Oct 11, 2003 07:48am | #4

   I have seen at least one case where the inspector considered a deeply imbedded metal frame the panel was mounted to to be an adequate ground. This particular frame was made up of two 2' galvanized steel pipes, could have been plumbing or rigid conduit, imbedded about 6' each with two pieces of galvanized Unistrut bolted between them. The top 3' of each post was set in concrete so I guess  it would act as a sort of Uffer ground.

   The ground conditions were good as the assembly was set in an old cypress bottom with very high water table and little sand or clay. Even so I, if it had been my job, would have installed a, around here a single rod is considered enough, ground rod just to make sure and cover any liability.

   A lot of this depends on the conditions in your area and the judgement of the AHJ, Authority Having Jurisdiction, the inspector/s. Most inspectors will answer a phone call and give you a good answer. If your in an area with little or no inspections ask any reputable electrical contractor familiar with the area.

   Worse outcome isn't all that bad. You might need to install two ground rods and about 15' of GEC, Grounding Electrode Conductor. Typically a #6 or #4 bare solid copper conductor, a set of copper plated rods, two clamps and maybe a lug. Should do it for around $30. Adding more ground rods won't do any harm but it might not do much good either. I wouldn't shirk the requirement for the exterior panel but neither would I go crazy.

   For fault currents the neutral, from the first point of disconnect to the transformer, is usually a far better path than any earth ground so the earth ground only acts as a redundant path. Most of what I read says that the ground rods in many locations, mostly due to the water table and soil conditions, offer only minimal drain for fault currents  from the first point of disconnect and the transformer.

   Lightning protection is also fairly poor in many systems. A lightning bolt that travels across five miles on open air pretty much goes where it wants. Even if it travels some distance along the ground system many conductors are too small and often too sharply bent to guarantee the current stays with any one conductor. The same is not, for the most part, true of a dedicated and properly designed lightning protection system. Of course these dedicated systems are far and above what the code requires.

   The mobile home frame, when installed with metal straps and steel earth augers every few feet and assuming reasonable soil conditions, is usually well electrically coupled to the earth. There is a rather large contact area. The only thing I could add is to make sure the interior panel has a solid connection to the frame. When in doubt I add a piece of solid #4 or 6 down from the ground bar, there should be a separate and electrically isolated ground and neutral bars, in the panel to a solid copper lug I bolt onto the frame. A 5/16" hole drilled in the I-beam web. Grind or scrape a space larger than the flat of the lug through the paint around the hole. I use a 1/4-20 nut and bolt and a spot of anti-oxidant grease to hold it and slow corrosion.

   Some electricians use a self-drilling screws for grounding but I have little confidence in the wider threads. This is still better than some connections I have seen come from the factory so it pays, IMHO, to check.