| Author | Message | ||
| Steve (Steve)
Rating: N/A |
In Bus Conversions Magazine this month there is an article on bus ac generator and shore power and inverter wiring. My question is if the neutral and the safety grounding wire are connected at the source to wit shore power, generator, inverter, why must the neutral and safety NEVER be connected togeather at the control panel??? And what protects my bus A/C D/C electrical systems and the generator from high voltage surges while pluged into shore power if all the sources neutral and ground safety wires are connected togeather anyway ??? | ||
| David Hartley (Drdave)
Rating: N/A |
A transfer switch is supposed to have the ability to disconnect the neutral along with the hot leg. That ground leg is supposed to stay at chassis ground of the rv. Most all transfer switchs have 2 double pole relays. One handles the hot leg(s) and the other switches the neutral legs of the generator and shore line. Codes say that a Ground is a physical ground isolated from any power source. (ie: hot or neutral) But there is part of the problem. Manufacturers have a habit of tying the Neutral and Physical ground together. Even the power company does this! There is supposed to be a grounding rod at the main disconnect panel. Oh about 20 feet of 1/2" copper rod. Some electrical panels are designed so that the ground and neutrals can be separated with a grounding kit. They generally do not come with anything usable. What keeps from burning up your generator is; a separate manual plug..Or a very good isolating type transfer switch ( kind that moves neutral ). Your generator may have the neutral and chassis ground connected together. That's usually OK because it's only using the chassis as the return path. Your shore cord however should have separate Neutral and Ground leads. The pole that you are plugged into may have combined N & G and your onboard panel may have them combined (by default). But let the transfer switch do the selecting and switching the neutral to shore or generator. Some people use the "Three Plug Rule*" Shore cord ends in 50 amp plug. Generator line ends in 50 amp plug. *Inverter Feed ends in 50 amp plug.* *note: Inverter may be style with internal automatic switching. So you woul only have the first two plugs. A short panel feed line with a 50 amp plug on the end is physically plugged into the source that you want to use. Or, Just buy a really good automatic switch and a surge arrester sized accordingly ( 50 amp version ). OK, Guys chime in here, I almost explained it.. | ||
| Sean Welsh (Sean)
Rating: N/A |
Steve, The reason you shouldn't have neutral and ground tied together anywhere inside your coach when you are connected to shore power is straightforward: Appliances in your coach draw current from the "hot" leg of the power, and "return" that current on the "neutral". Somewhere back in the shore system (usually at their main panel, or at the transformer) the neutral and ground are bonded together. If there is also a bond from the neutral to the ground in your coach, then some of the return current will flow through the neutral, and some of it will return through the ground system. How much current flows through each is a matter of their relative resistance -- usually, the ground system is lower resistance and therfore will carry more current. Since your ground system is bonded to your coach body and frame, you will now have 120vac current flowing through your coach body. Depending on how far away the campground's neutral/ground bond is, a person touching your coach may provide a better path to ground, and will certainly provide at least something of an alternative path. In other words, having the ground and neutral bonded inside the coach is a shock hazard. Also, if the campground has a GFI, it will trip the instant you plug in. On the other hand, if there is no connection to shore power, but you still have AC power available on-board (from, say, an inverter) then you must have a ground-to-neutral bond somewhere, preferably as close to the power source as possible. Otherwise, faults to "ground" in your system will not have a way to return to the source neutral and trip the breaker. This is the reason there is so much discussion of ground-to-neutral bonding "relays" -- you need a way to have the bond when you are not on shore power, but break the bond when shore power is connected. Regarding protection from surges, that is not the function of a ground system. The grounding system exists to divert fault current from personnel. If you are concerned about surges and over-voltage on the shore system, you might consider purchasing an aftermarket surge suppressor. -Sean | ||
| Jon W. Rating: N/A |
We manufacture the electric boxes RV's plug into under our own name and we private label them for other companies. www.jamestownadvanced.com The products are tested and certified to UL standards for temporary power outlets. Because local codes vary we are required to keep the ground isolated from neutral lugs, but because some local electrical codes have a "bonded" neutral we must supply a kit with each power outlet for the installer to join the neutral and ground. | ||
| Brian Brown (Fishbowlbrian)
Rating: N/A |
Thanks for the straightforward explaination, Sean. It's very helpful to use "electrically-challenged" folks. I can wire stuff to code, by sometime fail to grasp the concepts of those moving electrons. Sean, or anyone else: My coach (not wired by me) always trips GFCI breakers, so it has some issues with this. Could one check for offending neutral-ground bonds in an un-energized system with a continuity tester by turning on each breaker? It could also be a bonded sub-panel, but it's not readily obvious, if so. Thanks, BB | ||
| Sean Welsh (Sean)
Rating: N/A |
Brian, Tripping GFCI's may or may not be an indication of a problem. I can tell you straight away that there are several inverters that will always trip GFIs, such as the Trace SW series. I have this problem myself. To see if your inverter is the culprit, disconnect it at its input -- make sure to disconnect both the hot and the neutral. Also, if you have a relay to lift the ground/neutral bond when shore power is connected, the relay will almost certainly be slower than the GFI. So the instant you plug in, some of the return current will flow through the ground momentarily (before the relay can lift), tripping the GFI. If you have an open-frame type of relay, you can sometimes defeat this nuisance trip by holding the relay open with the eraser end of a wooden pencil, or with a tongue depressor or similar dielectric item, while you engage the shore power (it might take two people to effect this solution). If neither of these situations applies to you, then turn off all your breakers and unplug all your appliances, then plug in your coach to see if it trips. If it does not, turn each breaker on in sequence, then plug each appliance back in, until the GFI trips. The last thing you turned on or plugged in will be the culprit that has a ground fault. Bear in mind that certain 240-volt appliances, and cooktops are the chief offenders here, have the ground and neutral wired together internally. That will automatically cause a trip, and, moreover, it creates the same hazard that I mentioned earlier. If you have one of these latter items, you are in a pickle. It may be possible to open it up and isolate the neutral return of any 120-volt circuits inside the unit from the frame, in which case you can run a white neutral wire to this isolated point, and run a green safety ground to the frame. Lastly, if unplugging all your appliances and disconnecting your inverter does not solve it (and you don't have the bonding relay issue cited above), then you may have a wiring error in the coach, possibly including a subpanel that has a single buss bar for both ground and neutral, or a panel where the neutral buss was not properly insulated from the frame. A continuity check using an ohmeter between ground and neutral will confirm this, but it won't help you find it. You'll need to trace the wires and disconnect subfeeders until you find the culprit. HTH, -Sean | ||
| Brian Brown (Fishbowlbrian)
Rating: N/A |
Thanks for the tips, Sean. No inverter yet, nor 220 anything. I'm the guy that bought the finished conversion with the goofed-up shore cord/ transfer switch ( http://www.brownland.org/bus/elec/xfer.jpg )you walked me through a few months ago. I'm sure you'll appreciate the main disconnect I discovered today when I was out at the bus: http://www.brownland.org/bus/elec/maindisconnect.jpg The converter's lack of wiring knowlege obviously worked its way into the coach, as well. At least the panels are all close together so I can re-wire them. I should just bite the bullet and upgrade to true 50A, 4-wire, 120v service while I'm at it. Thanks, Brian | ||
| FAST FRED Rating: N/A |
For good info on surges and many RV topics take a look at http://www.phrannie.org/phredex.html Lots of information to get up to speed with. FAST FRED | ||
| ChuckMC9 (Chucks)
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Hey BB - I thought you were saving time/money by getting an alreaady 'done' conversion!? Hee hee. Sorry.  | ||
| Brian Brown (Fishbowlbrian)
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Good point, Chuck. I still feel like I am saving some time off a full-conversion, even fixing a few items done wrong, seeing how we were camping in her a few weeks after I got her. Now, whether we immolate ourselves via somebody’s shade-tree conversion practices remains to be seen... As far as $, it sure seems like any type of bussin' just opens up a giant sucking-sound 'o green stuff. Call it bu$$in! BB | ||
| R.J.(Bob) Evans (Bobofthenorth)
Rating: N/A |
Bu$$in - ain't that the truth. We bought a "completed" conversion too. I keep telling my bride that once I get the bus up to my standard of maintenance the flow of $$ pouring into it will at least lessen. I'm not sure she believes me. I'm not sure I believe me either. | ||
| R.C.Bishop
Rating: N/A |
Personal opinion only, not pickin' on anyone or saying there is no chance of doing it absolutely the perfect way the first time........ ....my experience in life, and construction in particular (I have 50 years plus in the latter) is that it is NEVER just the way it COULD be done to have the best possible outcome. That is the reason in the construction industry there are plans called "As Built" that superscede any Architectural or Structural design plans. AND, biggest thing...that doesn't mean it will please everybody, for what ever reason, including being to exact code....to being practically the greatest.... the safest and just plain difference of likes and dislikes. I've always felt that if one doesn't learn something new today, they are not making much progress. SO...in our conversion, we "live with it" a while, and if we find it is not what it should be, we change it, and if it is, we "lock it in". As FF says.....do it your way. But be safe!! FWIW | ||
| ChuckMC9 (Chucks)
Rating: N/A |
RCB, you said it better than I. I'm in the process of a multi-year prototype. Maybe in 2010 will it be ready to lock it in! | ||
| Chuck Newman (Chuck_newman)
Rating: N/A |
Sean, Does Brian have a whole house GFCB or is he talking about a single branch circuit GFR/GFCB? On a related issue, I would like to nail down this GFI / inverter tripping issue before I invest in a inverter. This subject came up several months ago. One case turned out to be a defective extension cord. As I recall the other never was resolved. Specifically in your case, due you have nuisance GFI trips on some or all of your GFI's. Are they wired as ground fault circuit breakers or ground fault receptacles? Does your 4024 run in full time mode for AC loads, or do you switch bypass during shore/generator periods? If you bypass the inverter with shore/generator, do you still have the tripping problem? Is the problem predictable and repeatable that we can do some tests with instrumentation at Busn USA? Thanks, Chuck Newman Oroville, CA PS: Did you get past your engine problems? | ||
| Sean Welsh (Sean)
Rating: N/A |
Chuck, I'll let Brian speak for himself, since I don't really know his layout other than the handful of photos he's posted (showing some really questionable wiring practices on the part of whoever did his coach before he bought it). However, I assumed his question related to his coach tripping the shore GFI (if any) when he plugged in his shore cord, which is why I answered as I did. Also, let me say here that I thought your post on GFI's and AFI's on the other thread was an excellent and clear explanation, and ought to clear up the confusion some people still have on the subject. One of the best descriptions of those devices that I have ever read. With regard to tripping of GFI's and the Trace SW4024, let me clarify the remarks I have made about this here and elsewhere: I do not, on my SW4024, experience any nuisance tripping whatsoever of GFI receptacles that I have installed downstream of the inverter. Bear in mind that Xantrex specifically lists some GFI brands/models in the owners manual as having been tested with the unit, and I did stick to the tested models. The problem I have experienced is upstream of the inverter. Specifically, if I try to plug my coach into a GFI-protected shore receptacle, it will trip 100% of the time. And I have tested extensively to be sure that the SW4024 itself is the culprit -- turning off and disconnecting all downstream loads, ensuring that the SW4024 is the only load on the shore line, and eliminating the neutral/ground bonding relay from the system (for purposes of the test only). It does not matter what type of GFI is on the shore receptacle -- it trips all brands of Ground Fault Receptacles and all brands of GFI circuit breakers that I have encountered. Repeated calls to Xantrex got me nowhere. They always agree that it shouldn't do that, but they never agree the unit has anything wrong or to take it back for repair. Every owner of a Trace SW model I have ever spoken with has described the exact same problem. And, while I have never disassembled my unit to instrument it and prove this assertion, my belief is that there is enough capacitance in the unit that it is flatly impossible to have balanced hot/neutral current within the trip delay timing of any GFI out there (i.e. the caps take longer than that to fully charge). The only way I know to defeat this is to eliminate the GFI at the shore receptacle (I've often been able to find an unprotected outlet nearby), or to use a 1:1 isolation transformer on the shore line (only a theory -- I have not tested this premise). Consequently, I have been keeping my eyes open for an inexpensive 1:1 transformer in the 15-30amp range, so far with no sucess. I was not aware of this particular problem when I designed my power system. If I had been, I might very well have elected to add a bypass switch for the inverter. I would then at least be able to bypass the inverter and run those loads directly from shore power. As it stands now, if the only shore power available is GFI-protected, I can't make any use of it at all. The new code requires GFI's on campground outlets, so this problem will only get worse for me (though we seldom stay at campgrounds with hookups anyway). I do carry a "cheater" non-GFI receptacle with me, so that if I get stuck in such a campground for a few days (and full-hookup campgrounds generally forbid or strongly discourage generator use), I can temporarily replace the GFI for the length of our stay. No help, of course, if the GFI is in the breaker -- I may start carrying extra breakers with me as well. -Sean | ||
| Chuck Newman (Chuck_newman)
Rating: N/A |
Sean, Excellent description and certainly a very interesting problem. Several issues come to mind. In my experience with several models and capacity of industrial inverters / chargers, most of the capacitance is in the DC side. I have several units that will arc when connecting the batteries due to the inrush of the capacitors. That being said, it is totally plausible the AC input is excessively capacitive. That is the method used on GFI's to trip the unit with the local test button -- it adds capacitance to the incoming AC and thereby momentarily unbalances the transverse current long enough for the differential amplifier to sense the condition. If all the 4024's experience this aberration, it is probably a constant inherent in the design of the AC section. That would account for the lack of tech support. The simply don't have a fix for it. This does not help solve the problem, but in reading 4024 manuals (at least two versions exist), and utility grid documentation concerning this unit, I get the distinct impression it was initally designed for the off grid and co-generation environments -- both of which use the 4024 AC input continuously connected or not, depending on system design. In contrast, the older Heart and other units were apparently designed for mobile/recreational use -- connect to AC, disconnect from AC. A colleague in the Power Generating industry has used a DR3624 for about 3 years. I queried him about general reliability and specifically high current inductive loads (air conditioners), and he has had no problems with the unit. I was more interested in any maladies with the modified sine output, but will check with him regarding the GFI issue in the inverter input. Apparently Xantrex has not sold a lot of the DR units to the RV crowd. But now I'm curious about Xantrex inputs in general in relation to a feeding GFI on the camp ground post. I don't have a copy of the Code in front of me, but as I recall the GFR requirement only applies to the 15/20 amp duplex receptacle on the post, not the 30 amp and 50 amp receptacles. If that has not changed, I presume you generally only use the 15/20 amp outlet or frequent locations that only offer same. Anyway, now I fully understand your situation as it occurs. I did find a buried short piece in one of the 4024 manuals concerning switching sources on AC input 1 in less than 100 ms. Xantrex states the unit cannot rephase in that duration and the 4024 will go into an overload condition. I have not seen this condition in any other UPS inverter design, both cheap and expensive. Perhaps the input capitance scenario is another design bug that is not an issue in fixed (grid) installations. This problem has gotten my attention. I don't have a 4024 to test on. Let us know if you find a silver bullet. Thank you for the in-depth explanation on the problem. Chuck Newman Oroville, CA | ||
| Sean Welsh (Sean)
Rating: N/A |
Chuck, We can certainly play with this in Rickreall. Ironically, the Rickreall rally last year is where I first discovered this problem. All the outlets there are 15-amp GFR's, and that was our first outing with the coach beyond the Infinity shop (the conversion was not yet complete). I have since recommended to Infinity that they install a GFI-protected shore outlet in order to test future projects. I no longer have a copy of the code, so I don't know what the '05 edition has to say about GFI's on campground outlets over 20amps. Suffice it to say that we are encountering more and more 30-amp receptacles that are fed by GFI breakers. (As yet, I have not encountered a 50-amp outlet so protected, but it can be done. Few places we stay ever have 50-amp service, though.) Also, my "main" shore cord (the one I use most often) is a 12-gauge, 50', 15-amp deal. Our electric system is so efficient that 15 amps is usually all we need, unless we need to run more than one roof air. I only drag out the 6-gauge 4-wire job when I absolutley have to -- manhandling it is a PITA, and faking it for stowage is an even bigger PITA. To top things off, we often park the "wrong way," meaning we head in to many spaces rather than back in, just to have a better view. This puts the pedestal on the wrong side, so the 50' cable comes in handy. As far as AC-side capacitance goes, remember that the 4024 is a grid-tie unit, designed for back-selling of power to the utility. This may be one reason for the additional line-side filters. The caps are probably part of the 100-ms transfer delay requirement as well (though we tie shore and genny to two different inputs, so the delay requirement does not apply). Interestingly, the /MC2 model, which is targeted specifically to the RV/Marine market, is physically identical and thus also has this problem, even though the grid-tie software has been removed. -Sean |