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#210521 - 07/07/13 12:41 AM mobile home bonding Q  
Lostazhell  Offline
Member
Joined: Feb 2004
Posts: 1,428
Bakersfield, CA (Originally Or...
I took a call yesterday that took me to a mobile home DEEP into the mountains. Customer states that when he tries to reset his kitchen circuits the breakers will trip.

I found that the 12/3 homerun for the kitchen counter circuits had likely been attacked by varmint someplace. Both phases were shorted together and to top it off, were electrifying the metal on the outside of the mobile home, including the panel! shocked

I ran EMT from the panel and poked into the back of the first outlet on the circuit from the outside. Cut off the faulted wiring on both ends.

There was no bond between the neutral and the ground bar in the mobile home distribution panel. There is a ground rod and water bond to the ground bar, which was in decent shape. The meter and main breaker are about 250'+ away and feeds to the mobile home as 3 wires only in PVC 40. The neutral is bonded to the can and a ground rod at this location.

I installed a piece of #6 THHN between the ground bar and the neutral bar in the mobile home distribution panel to prevent the panel can from being energized should a fault occur again.

I left the bond at the main alone, considering that no ground wire is coming from there to the mobile home.

Did I do the right thing?

( I've been dealing with oilfield for the last 8 years and I'm helping our service dept out for a bit as they're incredibly short handed and overwhelmed.)


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#210527 - 07/07/13 02:12 PM Re: mobile home bonding Q [Re: Lostazhell]  
renosteinke  Offline
Cat Servant
Member
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
Since trailers are typically fed by a cable, perhaps shielded by PVC pipe, I don't think you created any hazard - but I do believe you violated code.

As I see it, your first overload device is at the pedestal serving the trailer - so that's where you bond the neutral to the ground. Likewise, the pedestal is where the ground rod ought to be. From there, four wires should go to the trailer.

250-ft is a mighty long run, and I doubt it agrees with PoCo specs. The pedestal is typically set right next to the trailer. With the distance you describe, the trailer certainly qualifies as a 'separate structure,' requiring a ground rod at the trailer as well.

As you're no doubt aware after this service call, trailers are not built is any manner that facilitates your adding a ground rod or bonding the panel to the frame. As trailers these days are typically supplied with a cable exiting the panel, as well as a UL sticker, I can only assume that the frame and sheathing are somehow bonded during the construction of the trailer.



#210530 - 07/07/13 02:56 PM Re: mobile home bonding Q [Re: Lostazhell]  
Lostazhell  Offline
Member
Joined: Feb 2004
Posts: 1,428
Bakersfield, CA (Originally Or...
John.. This wasn't in a trailer park, this was up in the Sequoia Nat'l Park.:P There was no pedestal, the meter and disconnect are on the utility pole along the dirt road that feeds a small tract of trailers/mobile homes. From the disconnect to the trailer are 3 - #1 THWN. No ground wire is in the conduit. It's a long uphill run to the trailer that I paced off being between 250-260'

I know the frame of the trailer is bonded to the panel as I pulled the skirting off in the panel location. I found a ground rod and the water bond (most of the water piping is PVC, but a steel section was bonded). My problem is that there was a direct phase to ground short and no OCPD opened up. The tin on the mobile home became energized, the metal panel can/ground bar and everything else lit up a hot stick. I measured 117V between the ground bar and the isolated neutral bar. I measured 115V between the metal of the home and the isolated neutral bar.

This tells me that there is not a sufficient path for current back to the transformer from the metal structure of this home in order to operate an OCPD during a fault.

I know that the neutral is only supposed to be bonded in one location, but how else do I keep these people from getting lit up if something else gets chewed on?? GFCI ALL the breakers? (The panel is twined out, so this would be quite a feat.)


#210531 - 07/07/13 04:14 PM Re: mobile home bonding Q [Re: Lostazhell]  
HotLine1  Offline


Member
Joined: Apr 2002
Posts: 6,799
Brick, NJ USA
Lost:
Read thru this from '11 NEC and see if your project qualifies:

(B) Grounded Systems.
(1) Supplied by a Feeder or Branch Circuit. An equipment
grounding conductor, as described in 250.118, shall
be run with the supply conductors and be connected to the
building or structure disconnecting means and to the grounding electrode(s). The equipment grounding conductor
shall be used for grounding or bonding of equipment,
structures, or frames required to be grounded or bonded.
The equipment grounding conductor shall be sized in accordance with 250.122. Any installed grounded conductor
shall not be connected to the equipment grounding conductor
or to the grounding electrode(s).
Exception: For installations made in compliance with previous editions of this Code that permitted such connection, the grounded conductor run with the supply to the building or structure shall be permitted to serve as the ground-fault return path if all of the following requirements continue to be met:
(1) An equipment grounding conductor is not run with the
supply to the building or structure.
(2) There are no continuous metallic paths bonded to the
grounding system in each building or structure involved.
(3) Ground-fault protection of equipment has not been installed on the supply side of the feeder(s).
If the grounded conductor is used for grounding in accordance with the provision of this exception, the size of the grounded conductor shall not be smaller than the larger of either of the following:
(1) That required by 220.61
(2) That required by 250.122
(2) Supplied by Separately Derived System.

FWIW, I would have no issue with what you described. As long as there is no parallel grounding path.


John

#210535 - 07/07/13 06:56 PM Re: mobile home bonding Q [Re: Lostazhell]  
renosteinke  Offline
Cat Servant
Member
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
Randy, I dealt with a similar fault once, but there was no problem with that breaker tripping! You've put a whole new twist on things.

First step, I guess, is to try to improve the bonding. Tedious, but I'm thinking of a series of small jumpers from the siding to the frame.

I'm guessing the next step is to look at that feeder closer. Perhaps there is a damaged section, or something else acting to limit current flow.

While you were measuring 117-V on the siding, I'd be curious how many amps were flowing. That high of a voltage suggests a very low amp draw.

I have encountered voltage surge issues twice that were found to be caused by by cracked lugs (or bussbars). Perhaps there is such a problem in your panel that is restricting current flow.



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