neutral ground bonding - using coach to power the house

[Tim Strommen]

...The safety message is:
A person could become a better path for that power to follow, a shortcut of sorts, an easier path for the power than staying in the wires and metal and going back via the "proper" or expected, route, if that person was to be touching the wrong combination of things...

It's not actually that simple - yes, the path of least resistance is where most of the power will go, but power will split its power even to a poor conductor (so long as it's good enough).

When you have parallel paths for power, the resistances are fractionally additive,

R_total = 1 / ( (1/R_{first-conductor}) + (1/R_{second-conductor}) + .... + (1/R_{nth-conductor}) )

but the voltage across all of the resistances are EQUAL (this means if you hook yourself up as a parallel conductor where the potential between both connection points is 120VAC, you will get 120VAC at your fingers).  How much current you carry is determined by how much resistance you present to a circuit.  If a ground wire and neutral wire are 10mili-Ohms each end to end, and you are 1MegOhms - the neutral and ground for 120V will conduct up to roughly 12,000-Amps - but you can conduct up to 0.1mili-Amps.  More likely you have a lower resistance like 3,000 Ohms to 4,000 Ohms (source Wikipedia "electrocution" Article) - in which case you can conduct up to 30mili-Amps, or three times as much current as is needed to kill a person.  Obviously, the conditions will vary infinitely/daily but this should illustrate the importance of ensuring that the ground only carries current to trip fault devices - it should not become a parallel circuit load conductor.

...How about some plausible examples of how we might get electrocuted with more than one neutral/ground connection?...

...typically, unless it is connected to shore power, the bus itself is not grounded, as it is sitting on rubber tires.

The bus my not be grounded to earth - but the CHASSIS is bonded to ground!!  The rubber tire thing only keeps you safe so long as you or someone you care about doesn't hold onto a metal handrail or latch handle at the door, and step into or out of your bus at 0-dark-thirty with slippers onto moist or snow covered ground... then if you are double bonded, the person gives the current yet another path and ZAP.

-Tim

[niles500]

To make it more simple  each and EVERY neutral/ground bond should have only one bond at the source - any configuration which violates this rule downstream of the source invites disaster - FWIW

[artvonne]

 The rubber tire thing only keeps you safe so long as you or someone you care about doesn't hold onto a metal handrail or latch handle at the door, and step into or out of your bus at 0-dark-thirty with slippers onto moist or snow covered ground... then if you are double bonded, the person gives the current yet another path and ZAP.

-Tim

  So im still a bit confused here. The RV generator is bonded at the Gen to take the grounds back to the source so you wont get shocked stepping onto the ground. When you plug into shore power, your grounds are tied to the main panel in the house/campground, so you wont get shocked stepping onto the ground. I get all that.

  When your using the gen to power you house, do you want the gen bonded, or un bonded?

  From reading this, it would appear the answer is yes or no, that we need to "check" the grounds? It would also appear that many of us have put much greater reliance on campgrounds to assure we are properly grounded than is warranted.

[Melbo]

The ground is supposed to TRIP the breaker when there is a fault not provide safety

HTH

Melbo

[Sean]

...
  When your using the gen to power you house, do you want the gen bonded, or un bonded?
...

Ideally, you want the ground and neutral bonded in the generator, which is the source in this case, and the neutrals isolated from the ground in the panel being powered inside the house.

The ground is supposed to TRIP the breaker when there is a fault not provide safety

This is only partially correct -- ONE of the purposes of a safety ground is to conduct fault current back to the neutral, closing the circuit and thus tripping the breaker.  Note that this only works if the ground and neutral are, indeed, bonded someplace.

However the safety ground system has several additional purposes.  For one it ensures that any accessible conductive components of an appliance are at the same potential as the earth, so that an appliance user can not experience a shock.  This is one reason a driven ground, connected to the earth, is required.  If the only purpose of a safety ground was to conduct fault current back to the neutral to trip the breaker, no such driven ground would be needed.

The safety ground system also conducts dangerous voltage originating from elsewhere safely to earth, minimizing the potential for equipment damage or persons being shocked.  For example, a lightning strike, or an arc or direct contact from a high-voltage source such as a power line.  Even an electrostatic buildup will be safely drained to earth by a proper safety ground system.

-Sean
http://OurOdyssey.BlogSpot.com

[artvonne]

Ideally, you want the ground and neutral bonded in the generator, which is the source in this case, and the neutrals isolated from the ground in the panel being powered inside the house.

  Are you saying to "remove" the bonding screw inside the house main panel, (unbonding the neutral from ground) when using an RV to generate power to your house?

  I have a 200a transfer switch that only switches the hots. If I used the RV to power the transfer switch, and the generator in the RV is bonded, how would you suggest grounding/bonding the transfer switch?

  I was reading something written back in the early days of electricity and some of the controversy about added grounds. Someone said something about bringing the ground up to to all the circuits and appliances puts half the power that can kill you in your hands.   
 
 

[Seayfam]

Artvonne
Go out to your meterloop and see if you have a main breaker in a unlocked panel next to the meter. Or look for a main disconnect at the service entrance. If you have a main out there, look inside the panel and see if there is a bond from neutral to a ground rod.  "If a breaker is out side NEC says bond there"

If there is a bond there, then you can isolate your neutrals in your distribution panel in the house. Also make sure the distribution panel is grounded.  "This method has been code for 15+ years here"

Now if you are set up this way, you can install your 200A 3wire transfer switch in line before your breaker panel. Also you can leave the generator bonded. "Using your RV"

Now if you don't have a main disconnect outside, or in it's own panel, and the only main is with all your breakers. "Don't isolate them" And if you ask me, I'll tell you to run extension cords from your bus. Or a separate panel isolated from the power company as RVsaftyman is wanting to do.

I hope this has cleared it all up for you!!

Gary

[Iver]

Interesting reading and fairly easy to understand.....

http://members.rennlist.org/warren/generator.html

Iver.
   

[bobofthenorth]

I don't have a dog in this fight - no intent to stay in a house where a winter storm might shut me down ever again - but it seems to me that there is a simple solution that would be code compliant for those of you that still do have reason to power your house from your bus.  If you have a simple contractor's panel setup on a piece of plywood with enough four conductor cable appropriately sized to feed that sub-panel directly from your bus generator I believe that would meet the requirements of the code.  When the storm shuts you down string out the cable, fire up the gennie and plug extension cords from that sub panel to whatever you need to keep running in the house.  Somebody else can/will comment on the legality but it seems to me it would be legal and safe.  More importantly it would likely spend 99.99% of it's life with the cable wrapped around the plywood, stored in the back corner of the garage so it wouldn't ever be a code issue anyway. 

If it was me I'd just move into the bus until the storm blew over.  And then I'd head south until the snow melted off the roof.

[Sean]

  Are you saying to "remove" the bonding screw inside the house main panel, (unbonding the neutral from ground) when using an RV to generate power to your house?

That depends on how you are connecting the generator.

If the generator is to be connected using some type of permanently installed inlet with a transfer arrangement, then, no, you should not remove any bonding jumpers.  Instead, the loads must be fed from a sub-panel with isolated neutrals downstream of the transfer gear, and the transfer gear must switch the neutral.

If you are connecting the generator to the panel with a temporary pig-tail wired directly in to the panel (no permanent provisions allowed with this arrangement), then you need to remove the incoming hots as well as the neutral and tape them off.  The temporary pigtail connects to the same lugs that the permanent wires, now removed and taped, were using.  In this sort of arrangement, if the "incoming neutral" actually originates as a jumper inside this panel, then, yes, you would remove and tape the jumper as well.

  I have a 200a transfer switch that only switches the hots. If I used the RV to power the transfer switch, and the generator in the RV is bonded, how would you suggest grounding/bonding the transfer switch?

Sorry, but you simply can not use a transfer switch like this with an RV generator, ever.  Not in your RV, and not to connect your RV to the house.  Any transfer gear connecting to a bonded generator MUST SWITCH THE NEUTRAL.  Period, no exceptions.  Anything else is dangerous.

As I wrote earlier, generators made explicitly for fixed backup use are not bonded; they have floating neutrals.  This type of generator is connected with a transfer switch that does not switch the neutral but rather passes it straight through.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

[Melbo]

Bob

That was my suggestion at the beginning of the thread --- simple straight forward and yes code compliant as a TEMPORARY use panel.

Melbo

[artvonne]

   If I appear to be asking dumb questions, its only because I got electrocuted from a light switch once and have a very healthy respect for it. Too many people have the mistaken idea that 115 volt house current wont kill them. And they feel that way because without any load on the circuit you can let it pass through you and only feel a small shock. But if you become part of a circuit that energizes something, say a 60 watt light bulb as I did, you could truly get killed. In my case, I was instantly thrown across the hall and stunned. My fingers were burned and my arm hurt really bad and ached for over a year. I have no doubt that had there been more resistance on that circuit i probably wouldnt be writing this right now.

  Having the RV power my house would be wonderful. Having someone walk up and grab the camper door and pass the entire load the house is drawing through thier body at 230 volts would likely be the last move they ever made. This stuff isnt a joke. And the fact so many professionals have differing opinions just makes it all the more confusing. When I rewired my last property, I ran power to out buildings using sub panels after bringing power in after the meter to the main panel. I understand all the grounds and bonding with that setup. But when it came to asking about the transfer switch even the electrical inspector got excited. He wouldnt tell me how to do it, said to talk to the generator company and have them do it. He went on to say people shouldnt have generators hooked up to their house, they were dangerous. When I asked the generator company, instead of simple answers, they said they couldnt offer any information without sending out a technician. Understandable, they want to make money.... No, I think they dont want to be sued, and there is quite enough risk here.

  At any rate, this thread has opened my eyes even further to the dangers, and to the knowledge I need to study more. One thing that bothers me about running extension cords to circuits inside the house, backfeeding them. is that most are grounded back to earth. That could put live current onto every circuit that has the ground tied (bonded) to the neutral, theoretically bypassing every breaker in the panel. Its really something to think about.
 

[Seayfam]

Sorry, but you simply can not use a transfer switch like this with an RV generator, ever.  Not in your RV, and not to connect your RV to the house.  Any transfer gear connecting to a bonded generator MUST SWITCH THE NEUTRAL.  Period, no exceptions.  Anything else is dangerous.

Sean, I changed my last post, the part with the transfer switch to a 3 pole switch. "I don't want to give anybody the wrong idea"
Now I have a question for you. If you are wired like I explained, then your breaker panel in your house is considered a sub panel correct? And in that sub panel all your grounds and neutrals are isolated. This means nothing in the house is bonded. If I connect a standard 2wire transfer switch before this panel, and my generator is still bonded.
How am I getting a double bond?

Gary

[artvonne]

  Im trying to go back to the beginning of this thread and re-read it. If I am understanding the problem correctly, the best setup would give the Bus its own transfer switch that carries all generated power away from the bus, and which isolates the generator from the Bus entirely?? I assume this would be the type of TS that also switches the neutral? That you could then carry three wire (two hots and neutral) to your home transfer switch and have your ground bonded there?

[Tim Strommen]

  Im trying to go back to the beginning of this thread and re-read it. If I am understanding the problem correctly, the best setup would give the Bus its own transfer switch that carries all generated power away from the bus, and which isolates the generator from the Bus entirely?? I assume this would be the type of TS that also switches the neutral? That you could then carry three wire (two hots and neutral) to your home transfer switch and have your ground bonded there?

I think this is what we're getting at (all contained in the structure to be powered by the bus - not on the bus):



-T