Mobile Home

[don_resqcapt19]

Re: Mobile Home

Mike,

Any one of the engineers in this forum can attest to the fact that if the ground rod installed at the mobile home was close enough to the required ground rod at the service it could cause a parallel path for a fault current that could have enough resistance to impair the opening of the over current device.

How??? The fault current is still flowing through the OCPD and if high enough will cause the device to open the circuit. Additional current paths on the load side of the OCPD cannot reduce the current through the device.
Don

 

[jwelectric]

Re: Mobile Home

Surely we all know that a parallel circuit is a current divider, part of the current will take one path and part the other. Notice I did not say it would prevent a fault current from opening a overcurrent device, what I said is that it could impair the function.


The ideology that a second rod is required at a mobile home would bring forth questions of how far are these rods to be from each other, are they to be bonded together if under a certain distance.
Should two rods be allowed with-in 4 feet of each other? What would be the effect if the ground was wet? How much current could be carried through the wet soil? What hazard could this create?

The bottom line is that a ground rod is not required at the mobile home. There is a danger in installing one if the circumstances are right. 550.16 is very intense in what is mandated to be grounded and bonded with-in the mobile home and how it is to be connected to the service. Ground rods are not part of this.

 

[don_resqcapt19]

Re: Mobile Home

Mike,

Surely we all know that a parallel circuit is a current divider, part of the current will take one path and part the other. Notice I did not say it would prevent a fault current from opening a overcurrent device, what I said is that it could impair the function.

Again I ask how??? I see no way that additional paths on the load side of the OCPD could impair the function of the device. The multiple paths could very will increase the current flow and cause the device to clear the fault quicker.
Don

 

[OneWay]

Re: Mobile Home

The provisions of 550.33 will better explain what I meant by grounding the frame of the MH.Also, read 550.16(C)(1) for the method in which to apply the bonding conductor!$%@%#

[ August 18, 2005, 03:54 PM: Message edited by: OneWay ]

 

[OneWay]

Re: Mobile Home

Originally posted by don_resqcapt19:
Mike,

Surely we all know that a parallel circuit is a current divider, part of the current will take one path and part the other. Notice I did not say it would prevent a fault current from opening a overcurrent device, what I said is that it could impair the function.

Again I ask how??? I see no way that additional paths on the load side of the OCPD could impair the function of the device. The multiple paths could very will increase the current flow and cause the device to clear the fault quicker.
Don

Agreed! There is always the potential to alter the function of an overcurrent protection device when dealing with parallel circuits.

 

[jwelectric]

Re: Mobile Home

Okay my friend let me try to keep this simple.
The numbers that I am using is for demonstration only and are not intended to imply that the measurements are or could be accurate.

The grounding conductor has a resistance of 1 ohm. The two ground rods have a resistance between them of 10 ohms. A short to ground of 120 volts takes place causing a current flow of 132.0132 amps. I used the product over sum method to find the total resistance of the parallel path.

120 amps of this current would flow over the grounding conductor while the remaining 12.0132 amps would flow between the two rods.
You are the electrician that is standing out at the outside panel trying to figure out this problem. The breaker is reset and you are in contact with the metal panel while standing on the wet ground with wet shoes and the fault occurs again.

What do you think? Is this breaker going to give it up quick enough to stop you from getting hurt?
How could we prevent this? Would bonding the two rods together work? How far would we have to go before this would no longer be required?
This is the clincher, what if the grounding path was lost completely? Now whoever was working on this panel while the ground and their feet is wet will be between this 12 amps that will never clear itself.

Yes I can envision something like this happening, can you? An old mobile home that has a 50 amp cord is tied down with straps that go over the top of the trailer. It is raining and cold when I get the call. Nothing in this mobile home is working and it is a single mom with children. They need heat so I respond to the call only to get set on my rear. The dogs going in and out from under the trailer had pulled the cord loose in the panel and it was shorting to the panel. From the panel to the frame on to the metal siding and into the metal tie down straps through the earth (about 4 feet) to the panel that I was going to raise the cover of.

Two rods at a mobile home could very well be a death trap for someone.

I have said many times ?common sense is the best tool any body in the electrical trade could have.?

 

[OneWay]

Re: Mobile Home

JW, good example yet what I am referring to is Provision 550.33(C)(1)"A bonding conductor SHALL BE connected between the distribution panel and accessible terminal on the chassis.
Provision 550.33(C)(2)"The bonding conductor SHALL BE solid or stranded, insulated or bare, and SHALL BE 8 AWG copper minimum, or equivalent. The bonding conductor SHALL BE routed so as not to be exposed to physical damage.
This method if applied correctly may prevent your scenario froom occurring.

 

[derf48]

Re: Mobile Home

After going through the last 7 pages of this thread I would like to add my two cents. First of all, are you all politicians? nobody likes to answer a question, they simply write what they want.

I have gone back and forth on this issue as both sides have good arguements. Then I read 550 and tried to remember it says what it says, not what I want it to say. The real question here is Do we ground the MH at its location. Ground, by definition, means to connect to the earth (I'm paraphrasing here). 550.16 requires the MH to be grounded through the green colored insulation conductor...to the service ground in the service equipment. This mandatory requirement, IMHO, supercedes the requirement of 250.32 for a GEC and a GE. The definitiions of GEC and GE, found in article 100, tell us their function is to establish a connection to the earth. 550.16 ammended that!!!

I always required a ground rod at a MH. After re-reading the code, I believe I was wrong.

Fred

 

[jwelectric]

Re: Mobile Home

That is 550.16 (C) Mr. Oneway

It was this #8 that fed the chassis that allowed the metal tie downs to be energized. It had been raining for a couple of days and the ground at the panel was almost liquid. I had on a pair of tennis shoes that were wet through.

Had these tie downs been isolated from the metal of the trailer then I wouldn?t have been hit at the panel. The fact that nothing worked I was thinking that the fuses had blown in the outside panel so there is where I went to first.

The grounding, neutral and one phase had pulled free. The phase was touching the side of the panel and the fuse had blown for the phase that was still connected. There wasn?t enough current being pulled to blow the other 60 amp fuse.

 

[augie47]

Re: Mobile Home

j.w, oneway
most thought provoking

not to make light of a serious subject, j.w., but I've been there and I believe a flaw in your scenerio is that you would be unable to get to that panel due to the 56 chevy on blocks in front of it. :roll:

sorry, devil made me!

 

[don_resqcapt19]

Re: Mobile Home

Mike,

You are the electrician that is standing out at the outside panel trying to figure out this problem. The breaker is reset and you are in contact with the metal panel while standing on the wet ground with wet shoes and the fault occurs again.

What do you think? Is this breaker going to give it up quick enough to stop you from getting hurt?
How could we prevent this? Would bonding the two rods together work?

That has nothing to do with the function of the OCPD. Your previous posts said that the parallel ground paths could impair the function of the OCPD. I still see no way for this to happen.
As to the shock issue, in the case of a mobile home, the power supply is required to include an EGC, so the rods would be bonded together.

But that aside, how is it any different from any service with an outside main? There is a rod at the service and one at the utility transformer.
Don

 

[jwelectric]

Re: Mobile Home

So Don what you are saying is that it is you professional opinion that a parallel path would have no affect on the operation of an over current device such as a fuse or a breaker.

Is the above statement true?

 

[petersonra]

Re: Mobile Home

it is absolutely true that the OCPD does not care a whit how many paths the current flowing through it takes.

it trips when it sees an excess current flow, either instantaneously (as in a short circuit) or over a period of time (an overload).

its strictly a matter of how much current flows through the OCPD.

[ August 18, 2005, 05:10 PM: Message edited by: petersonra ]

 

[jimwalker]

Re: Mobile Home

Taking one more stab at this.
1. what exactly electrically is differant between a mobile home and a garage with a sub panel ?
2. if lightning hits this mobile that is 30 feet away will this grounded conductor send it to ground as good as a rod 1 foot from it ? Also note that while this disconnect is within 30 feet of the mobile ,the wire might be running to the far end of a double wide,so distance may very well be walking on 100 feet between mobile panel and didconnect.
3. the tie down anchors attach to the frame and the frame is attached by #8 to the panel in mobile.But they do not meet nec as far as being a rod or rods.
4. yes nec say the frame and metal parts are to be connected to the ground bar at the service panel at the pole.I do not see it as saying we are not required to have a rod at the mobile alsnly that there shall be a connection between them.
5. as to this idea of an extra rod at mobile being a hazard then lets think about them anchors in the dirt,while not legal rods try telling the anchors not to conduct electric.And if this is a hazard here why is it not a hazard at a garage ?
6. maybe its time nec rethinks this situation.
7. lets tell lightning to follow the wire to the pole.

ok now pick this reasoning apart

one more thing,this mobile with 60 amp fuses.Just how old is it and what part of KY is it in ?

[ August 18, 2005, 05:36 PM: Message edited by: jimwalker ]

 

[don_resqcapt19]

Re: Mobile Home

Mike,

So Don what you are saying is that it is you professional opinion that a parallel path would have no affect on the operation of an over current device such as a fuse or a breaker.

Is the above statement true?

Actually I said it won't impair the function of the OCPD. Yes, it will have an effect, because the parallel fault clearing path will slightly increase the fault current causing the OCPD to clear the fault quicker. That is an improvement, not an impairment.
Don

 

[don_resqcapt19]

Re: Mobile Home

Mike,

The grounding conductor has a resistance of 1 ohm. The two ground rods have a resistance between them of 10 ohms. A short to ground of 120 volts takes place causing a current flow of 132.0132 amps. I used the product over sum method to find the total resistance of the parallel path.
120 amps of this current would flow over the grounding conductor while the remaining 12.0132 amps would flow between the two rods.
You are the electrician that is standing out at the outside panel trying to figure out this problem. The breaker is reset and you are in contact with the metal panel while standing on the wet ground with wet shoes and the fault occurs again.

If this is the panel at the mobile home, and if the grounding electrode is close to the panel, and if you are standing close to the grounding electrode, you will receive less of a shock than if there was no grounding electrode at that location. First the shock voltage will be limited to the voltage drop in the fault return path. This voltage will be about 40 volts if the EGC is the correct size. The addition of the earth as part of the fault path will reduce this drop very slightly. If you are touching the panel or any bonded conductive part and earth, you will be subjected to this voltage. If however, you are standing very close to the grounding electrode, you will receive a smaller shock. The voltage around the rod will drop off over a 2-3' area. In this area the voltage of the ground, will be somewhat elevated over that of "true earth" (ground remote from the grounding electrode). The reduction in the shock voltage will be greater the closer you are to the rod.

If you are at the remote panel the same thing will happen, but the only voltage drop available here, is that of the grounded conductor back to the service.

 

[ryan_618]

Re: Mobile Home

Originally posted by jwelectric:
So Don what you are saying is that it is you professional opinion that a parallel path would have no affect on the operation of an over current device such as a fuse or a breaker.

Is the above statement true?

I think that is Kirchoff's opinion too, isn't it?

[ August 18, 2005, 06:23 PM: Message edited by: ryan_618 ]

 

[OneWay]

Re: Mobile Home

JW, Sorry to here about such a close call as that, I"M curious as to wether this service is protected by fuses or a main brkr.