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Old 03-22-2015, 09:26 PM   #101
Bus Nut
Join Date: Feb 2015
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Milliamps kill you but it takes volts to push them through a load (your body). I=E/R, and if E is lower, I is lower. It's not impossible to get electrocuted by low voltage systems, but it's a lot harder.

Interesting point about those fences. Around here all metal structures above a minimum size of surface area (varies residential/commercial/etc.) must be grounded. They just did work at my local train station to ground the fences there, and I was talking to the foreman about what they were up to. He said the same thing about playground fences. Concrete (which the posts are set in) is conductive but if it's dry can be 1Mohm or more - it won't hold a static charge but may not dissipate a live short fast enough to protect life/property so extra grounding is required.

As for AC/DC, electrocution is caused by fibrillation, and AC's rapid changes of direction greatly increases the effect of this. Again, either can kill you - but DC is a bit harder (and 120VDC and 120VAC are NOT the same "level"). Great article here, although a bit morbid:

AC and DC Electric Shock Effects Compared

I try not to be that guy saying "YOU should do XYZ", so I'll just share what I'm doing. I'm grounding my skin. We camp a lot in a variety of campgrounds, and this is just part of my "defensive driving" thing. I need to plan not just for my own skill but for the skill (or lack thereof) of others.

What keeps me up at night:
* Somebody's string of christmas lights get knocked over onto my bus by a stiff wind
* I accidentally back into an RV park site's power post
* I cross-wire something. Note: I custom-design my own surface-mount electronics as a hobby and own a copy of (and regularly consult) the NEC - I'm no stranger to electricity. But I'm human (sadly).
* My wife will toss an extension cord out a window and it'll be frayed somewhere.

I don't want my method of discovering one of these things has happened to be getting zapped grabbing the door handle while standing in the rain. I want sparks flying and breakers popping the moment any live voltage comes in contact with it - so I'm grounding it.

Note that I am not advocating grounding individual outlets TO the bus skin.
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Old 03-23-2015, 07:24 AM   #102
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mike, thanks for coming here and helping us. when it comes to AC voltage and wiring, im an idiot! but i wired my bus.......scary huh?

would you post a simple diagram showing the grounding you talk about? im confused by all this bonding, not bonding, etc. for instance, i have a solar charged battery bank and inverter which runs the bus now. all wires are run from inverter to breaker box and not bonded at the box. should i ground the inverter? where?

when i unplug the main lead from the inverter and plug it into the generator, should the generator be grounded? how, where? P.S. the generator has a tt30 plug that says it is not bonded.

when i unplug the main lead from inverter or generator and plug it into a campground power pedestal, should it be grounded/bonded? where? how?

i went to public school so pictures work better. lol
thanks again for your time.
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Old 03-23-2015, 08:08 AM   #103
Join Date: Aug 2012
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Originally Posted by jazty View Post
Ok, cool.. I'll rectify the situation. The outlets are all GFCI protected and haven't tripped yet, but I'll fix it anyhow.

PS: do isolated ground receptacles all come in orange? White would be nice...
Yes, they do make isolated receptacles in white.

Note that while NEC doesn't care if you double-bond receptacle grounds to building steel (they're only concerned with safety, not sound system hum) so it's not specifically a code violation. That's why I do a lot of sound system consultations where I need to get electricians to rewire all the receptacles to isolate their EGC grounds from the building grounds.

I'm pretty sure that under the right conditions (moisture and salt spray) creating a copper to steel to aluminum loop connected at both ends in your bus could result in corrosion at the connection points. It will act like a battery that degrades to metal. Note that this has nothing to do with AC voltage, it's the galvanic battery action of the dissimilar metals themselves. External sacrificial anodes won't really help, but galvanized bolts have been used for decades at this point of contact. The zinc itself becomes the sacrificial anode, and will eventually be consumed in the galvanic reaction.

More fun stuff to study.
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Old 03-23-2015, 09:28 AM   #104
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Ugh. I think a lot of us are trying to follow along here and not ask questions. That is another trait of public school upbringing. I looked out the window most of the time. Too many questions can sometimes muddy the water when texting.
I have not wired my bus yet. I plan to soon with a gen and the option for shore power. Keep up the great info! Thanks!
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Old 03-23-2015, 10:48 AM   #105
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Thanks for the clarification. Your information is always tops.

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Old 03-23-2015, 06:20 PM   #106
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Originally Posted by jmsokol View Post
The thing to be aware of is you only want a single point of bonding the EGC (Equipment Grounding Conductor) to the frame. That allows no AC current to flow through the chassis under normal circumstances. Of course DC current from the house battery and starting systems are a different story. There's no getting around the fact that virtually all vehicles systems use the chassis as the 12-volt return path. I think those DC currents are doing to be magnitudes larger than AC leakage currents that could likely occur.

FYI: Here's a pretty good tutorial of just how galvanic action can occurs between aluminum and steel. Aluminium Corrosion Resistance - Aluminium Design
Originally Posted by jazty View Post
Is this to say that metal device boxes should be grounded by the wiring run, but not touching any of the bus metal? I have my chassis grounded at the buses electrical panel, but I know that at least one of my device boxes is in contact with the bus structure as well. Would this be considered having two bonding points?
Originally Posted by jmsokol View Post
Yes, that's correct. In pro-sound systems double-bonding causes something called ground loop hum. And it can cause GFCI breakers to trip randomly. One way to correct that problem is to use isolated ground receptacles, and leave the metal boxes bonded to the metal of the bus. Now, I think that double-bonding ENG grounds would only add small amounts of circulating ground current under normal conditions that would require the addition of moisture to create corrosion. Still, it's one more thing to consider while troubleshooting grounding systems.

Big thanks Mike.

These three posts finished answering my questions regarding the downsides of bonding the grounds to the metal bus skin / chassis.

I will now be creating one point of bonding between the AC electrical and the bus metal.

I will still be isolating every single DC ground from the bus metal with dedicated ground wires.

Thanks again for your time.

"Don't argue with stupid people. They will just drag you down to their level, and beat you up with experience."

Patently waiting for the apocalypses to level the playing field in this physiological game of life commonly known as Civilization
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Old 04-26-2015, 01:42 PM   #107
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So I'm turning my short bus into an ice cream. The freezer only needs ~250W on startup. Could you recommend an inverter to use? Or what should I use for this? It's going to have to run directly off the battery?
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Old 04-26-2015, 06:55 PM   #108
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Originally Posted by nat_ster View Post
I will still be isolating every single DC ground from the bus metal with dedicated ground wires.
Not to be a PITA.. but.. it'll be really difficult to keep the DC system isolated from the chassis because of items like the starter and alternator that are case grounded. Or maybe you were referring only to the house DC system rather than the vehicle DC system. Isolating just the house DC system from the chassis is very doable. I can understand the motivation to run ground return wires instead of using the chassis because body grounds seem to fail so often, but I admit I don't see a reason to go to any great trouble to ensure DC ground isolation.

Thanks to taskswap, jazty, and jmsokol for the conversation about the interplay between equipment grounding conductor and conductive building structure. Not having done any commercial wiring experience (and single-family residential being approximately 100% NM cables in wood structures), I wasn't aware of the nuances there. I knew that metal building structure and conduit systems, like conductive systems in a non-conductive building, are connected to ground. But if I understand the discussion correctly, use of metallic conduit and device boxes can eliminate the need to run a separate grounding conductor. (there seems to be disagreement whether the separate grounding wire MAY be omitted, or actually SHOULD be omitted.)

If the conduit and boxes are to be used instead of a grounding conductor, then what detailing is required for the conduit system? I've really only played with EMT with set-screw type connectors and the occasional handy box. Does one simply set the screw snug, tighten up the lock nut on the connector, and use receptacles that have the little brass piece around the screw on one end of the yoke? I'm not sure I can count on the set screws and lock nuts to not work themselves loose in the mobile environment.
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Old 04-26-2015, 08:41 PM   #109
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You omit the ground wire because usually you're worried about "fill" and it's not necessary. Also, you never include a bare copper ground because of the possibility of galvanic corrosion between the galvanized steel EMT and the copper wire. You ground ONE end (the "source" end) of the conduit.

And yes, those connectors are all you use. But I personally have misgivings about this approach. EMT was designed for use in houses - the kind that don't move. Vehicles move, shake, and vibrate. There's a reason why wiring is almost always stranded, and you're not supposed to solder (crimp connectors are now standard almost everywhere). The same vibrations that can cause fatigue in the wires can work an EMT connector's set-screw loose very easily, and they're a relatively coarse thread soft steel (both the screw and the fitting). Even slightly loosening could render that connection very unreliable.

I believe Nat_ster was saying he'd be using plastic conduit, which basically makes it a chase. That's definitely what I plan to do. It's cheap, very easy to install, never rusts, doesn't become a conductor unexpectedly (or a non-conductor the same way), is easy to bend/cut, is lightweight, etc. It's, like, tailor-made for RVs. When used this way it's officially a "chase" rather than "conduit" - but all that means is you basically follow standard wiring practice as you would anywhere else. You can just pull regular wire through it, with ground, and you're done. If you install a pull rope with your wire bundle, it's easy to add more later if need be.
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Old 04-26-2015, 10:54 PM   #110
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For plastic conduit: the gray SCH40 PVC, or the corrugated blue "smurf" ENT tube, or is there another I don't know about? I like the idea of EMT because it's thinner but it does have all the down sides you described.

What about device boxes? I'm familiar with three types: the blue nail-on plastic designed for NM cable, the gray outdoor stuff intended for the PVC conduit, and a very limited selection of blue ENT boxes. I don't really want to use EMT.. but these plastic options all seem worse in their own ways.
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