| Author | Message | ||
| Iver (Mciv)
Registered Member Username: Mciv Post Number: 6 Registered: 11-2006 Posted From: 70.69.188.254 Rating: N/A |
Ok, I plan to have a plug from my main panel which will plug into either a shore power or generator receptacle. Fairly simple. However, when I am plugged into the generator receptacle, and the generator is NOT running, but the inverter IS running, how do I deal with a potentially "double bond" situation. The generator bonds the neutral and ground. The inverter automatically bonds the neutral and ground when it is operating and unbonds when ac power is applied. The generator supplies power to the main panel. The inverter receives power from the main panel and supplies power to a separate inverter panel. Has anyone else thought about this? I guess I could simply not leave the main panel plug in the generator receptacle? Thanks, Iver. | ||
| Tom Caffrey (Pvcces)
Registered Member Username: Pvcces Post Number: 1129 Registered: 5-2001 Posted From: 65.74.67.130 Rating: N/A |
Iver, the inverter unbonds when the generator is running and it bonds when the generator is shut off. This means that you only have one bond, as you should. Not to worry. Tom Caffrey PD4106-2576 Suncatcher Ketchikan, Alaska | ||
| Sean Welsh (Sean)
Registered Member Username: Sean Post Number: 667 Registered: 1-2003 Posted From: 67.142.130.44 Rating: |
Tom, The generator is bonded whether it's running or not -- therein lies the problem. When the inverter, seeing no input power, makes its bond, there will actually be two bonds -- one in the inverter, and one in the genny. Depending on how he wires the rest of the system, that can cause half of his neutral return to run through the ground system, including the bus chassis. The simple solution to this is a two-pole (or three-pole, depending on whether his coach is 120-only or 120/240) contactor between the generator and inverter, energized by the generator itself. Thus the genny neutral will not pass through to the inverter unless the generator is running. Iver -- let me know if you need more detail on this. -Sean http://OurOdyssey.BlogSpot.com | ||
| Tom Caffrey (Pvcces)
Registered Member Username: Pvcces Post Number: 1130 Registered: 5-2001 Posted From: 65.74.67.130 Rating: N/A |
Hi, Sean. Thanks for the correction. I had not thought it through, I guess. It makes me think that I had better check our own generator circuit. I haven't had much trouble, but without a GFCI on the inverter input, I don't suppose that I would notice, unless I was looking for it. I'm not sure, but in our case, I think that the inverter might break both the hot and neutral from the generator when inverting and then bond the output. Also, in our case, all of our generator circuits run through the inverter, so there is no chance of a double bond. Always learning. Tom Caffrey PD4106-2576 Suncatcher Ketchikan, Alaska (Message edited by pvcces on September 17, 2007) | ||
| John MC9 (John_mc9)
Registered Member Username: John_mc9 Post Number: 504 Registered: 7-2006 Posted From: 66.217.106.35 Rating: N/A |
Ok... This is starting to make my head swim...... Re: "The generator is bonded whether it's running or not -- therein lies the problem. When the inverter, seeing no input power, makes its bond, there will actually be two bonds -- one in the inverter, and one in the genny. Depending on how he wires the rest of the system, that can cause half of his neutral return to run through the ground system, including the bus chassis." The way I've always understood all this, is the ground/neutral bond -must be- at the main source of power. The "source" of that power will either be on board the vehicle (boat, RV, etc), or at shore (power-pole). And..... that it matters little what methods are used to generate that power that brings it to the point of connection (source), the only importance is that the bond is at the source (connection point). Isn't the theory behind that code, meant to insure that a "distant bond" (second bond) doesn't prevent fuses/breakers from tripping? What difference would it make if one, the other, or both generator and inverter are supplying power, when each are supplying the same breaker panel (source/ point of connection)? The ground bonding for each are still at the source.... To the best of my understanding, the important point is to insure that the ground bond at the vehicle is removed, if and when the supply is no longer vehicle based, and is from an external source..... Please correct me here? (I'm not too old to be re-educated).... | ||
| Iver (Mciv)
Registered Member Username: Mciv Post Number: 7 Registered: 11-2006 Posted From: 70.69.189.107 Rating: N/A |
Ok, now I'm worried again.... "Depending on how he wires the rest of the system, that can cause half of his neutral return to run through the ground system, including the bus chassis." I have a dedicated breaker in my main panel supplying power to the inverter. The inverter supplies power to a separate panel to circuits which are not connected to the main panel. (My inverter won't accept any backfeeding.) The generator has a 50amp receptacle and I plug the main power into it. If that is a potential problem, I could leave the plug out of the genset receptacle until I planned to run it. That would be nuisance to say the least. Sean, what would you do??? Thanks, Iver. | ||
| Jack Conrad (Jackconrad)
Registered Member Username: Jackconrad Post Number: 639 Registered: 12-2000 Posted From: 76.1.180.218 Rating: N/A |
Here is a thought, do away with the receptacles. Install an Iota 50 amp/240 volt transfer switch (available on Ebay for less than $100). This switch has the shoreline as the default. If you are not plugged in to shoreline, even though this is the default, there is no neutral/ground bond because the cord is not plugged in. Since the generator is not the default, the generator neutral/ground bond is only in the system when the generator is running. The inverter is only providing the neutral/ground bond when there is no power from shoreline or generator. The only problem that arises is if you plug your shoreline into a GFI protected outlet. The inverter neutral/ground bond will not open fast enough to prevent tripping the GFI. This can be eliminated by adding a SPST switch in the neutral/ground bond circuit of the inverter. This has worked for us, YMMV. Jack | ||
| Iver (Mciv)
Registered Member Username: Mciv Post Number: 8 Registered: 11-2006 Posted From: 70.69.189.107 Rating: N/A |
Ok, assuming that I could have a problem with my wiring, how do I test to find the problem? Other than standing in a pail of water and grabbing the door handle??? Again, all the stick and staple units seem to use this simple system, (no transfer switches, no contactors and no switches). Are they simply ignoring the potential problem?? I always appreciate any and all advice, Thanks, Iver. | ||
| Jack Conrad (Jackconrad)
Registered Member Username: Jackconrad Post Number: 643 Registered: 12-2000 Posted From: 76.1.180.218 Rating: N/A |
To test for leakage, I pour some water on the ground near the bus, then set my digital meter to AC. Stick one meter probe in the wet ground and place the other meter probe on the bus body (mkae sure you have a good probe to bare metal contact). This test only checks for leakage. Jack | ||
| Tom Caffrey (Pvcces)
Registered Member Username: Pvcces Post Number: 1132 Registered: 5-2001 Posted From: 65.74.67.130 Rating: N/A |
Iver, the main reason that you could have a problem with two bonds is if the generator output supplies the inverter AND some other circuits. All of the grounds are always connected together. If the generator ground is connected to some of the neutrals while the inverter ground is connected to other neutrals, that will effectively tie all of them together, even when the generator is not running. Any GFCI connected upstream of these bonds will see different currents in the hot and neutrals, because part of the neutral current will travel on the grounds. This should cause the GFCI to trip, unless the difference is reduced to a few milliamps of current. If ALL of the output from the generator feeds through the inverter transfer switch AND the transfer switch disconnects both the hot and neutral when it starts providing power, you should be able to avoid this problem. Since we use only one plug and two sockets to switch between shore and generator power, that pretty well lets us avoid the problem. I hope this helps. Tom Caffrey PD4106-2576 Suncatcher Ketchikan, Alaska | ||
| Sean Welsh (Sean)
Registered Member Username: Sean Post Number: 668 Registered: 1-2003 Posted From: 72.171.0.140 Rating: N/A |
Sorry for the delay -- I've been a bit preoccupied. Isn't the theory behind that code, meant to insure that a "distant bond" (second bond) doesn't prevent fuses/breakers from tripping? Not at all. It is to prevent neutral current from flowing over the ground system. What difference would it make if one, the other, or both generator and inverter are supplying power, when each are supplying the same breaker panel (source/ point of connection)? The ground bonding for each are still at the source.... It's number of pathways. Specifically, there needs to be one and only one pathway for the return current. I'll leave aside the issue of having both the generator and inverter energized at the same time, which is a whole different can of worms (unless the inverter is the synchronizing type built for this purpose). When neutral and ground are bonded in two locations, no matter how "close" those locations are, some of the current returning on the neutral will diverge onto the ground path. Since the ground in a coach is bonded to the frame, this means that the frame will become energized. Current takes as many paths as are available to it to avoid resistance (and not only the "path of least resistance," as is sometimes mistakenly written). Now, if both neutral/ground bonding points are inside the coach, and the coach is not connected to a shore-side ground, this is unlikely to be deadly, as the amount of current leaking into the frame will be fairly low -- and here is where distance between bond points comes into play. But it will never be zero, and that's what the code, and safe practice, requires. Electric fields, such as the one induced in the chassis by a double-bond situation, are insidious things. They can easily become deadly under the right circumstances. I operate an emergency communications truck for the Red Cross with a 50' telescoping mast on it. In mast safety training, it's drilled into you that if the mast contacts a power line, even if you jump cleanly from the truck onto the ground, merely separating your feet from one another by a matter of inches can cause deadly amounts of current to flow through your body from the slight potential difference at different points in the field. (The escape technique is to hop, kangaroo-style, with both feet together until well away from the truck.) In a double-bonded coach with 50 amps flowing on the neutral, you could get a nasty dose of current just by grabbing the wrong two pieces of the frame at the same time. What's worse is that this is entirely out of the protective realm of the circuit breakers -- the entire 50 amps could be flowing directly through you and the breakers wouldn't even see it -- they're in the hot side of the circuit, which is unaffected. A GFCI would protect you in this situation, but few of us have installed a whole-coach 50-amp GFI. (Ironically, even having such a GFI would force you to do the bonding correctly -- a double bond would trip the device instantly.) Iver -- to answer your question: No, just leaving the cord unplugged is not sufficient. It would avoid the double-bond situation, but only as long as you remember to do it. The code, and safe practice, dictates that such a major safety issue not be left open to the possibility of operator error -- it's all too easy to forget to disconnect the plug before using the inverter. The simple solution, as I wrote above, is to insert a contactor in between the genny and the inverter (unless the inverter already includes this feature internally). Energize the contactor from the genny itself, and run the neutral through it along with the hots. This way, the ground-neutral bond in the generator will be isolated from the system unless the generator is actually running and on-line. HTH. -Sean | ||
| Sean Welsh (Sean)
Registered Member Username: Sean Post Number: 669 Registered: 1-2003 Posted From: 72.171.0.140 Rating: N/A |
Sorry, I missed this one: Ok, assuming that I could have a problem with my wiring, how do I test to find the problem? Other than standing in a pail of water and grabbing the door handle??? This one's easy. With your inverter "off" and both shore and generator physically disconnected (if you're using the plug-and-receptacle method, this is straightforward), take your ohmmeter and measure between the ground and neutral bus bars. The reading should be infinity. If you get any other reading at all, you have a ground-to-neutral connection someplace. Sometimes, this is in a miswired appliance or fixture, so unplug or disconnect things one at a time until the reading goes to infinity to find the problem. Now measure between ground and neutral at the generator receptacle. You should read close to zero, meaning there is a proper bond in the genny enclosure. Now turn your inverter on, and read between the ground and neutral bus bars again, being careful not to contact any hot parts inside the panel. Now you should read close to zero, meaning the bonding relay in the inverter has closed properly. With the inverter still on, go to the plug end of your cord, and read between ground and neutral on the cord. If you read infinity, you're good to go -- the inverter has already done the work for you of lifting the neutral connection when it went on-line. If, on the other hand, you read something close to zero, you will have a double-bond problem if you connect the plug to the (non-running) generator receptacle. Again, all the stick and staple units seem to use this simple system, (no transfer switches, no contactors and no switches). Are they simply ignoring the potential problem?? Umm, most stick-and-staple rigs don't come with inverters, so it's a non-issue. This whole ground/neutral bonding discussion (that we seem to repeat on these boards endlessly over the years, even though all there is to say has been said many times) is completely moot if there is no alternative source of AC power on board, i.e. an inverter. The few rigs that do have an inverter factory-installed have, indeed, addressed this problem. Usually, a factory inverter setup comes with a transfer switch (either internal or external to the inverter), which is wired in such a way as to preclude the possibility of double-bonding. I can't think of a single coach that comes from the factory equipped with an inverter that also uses a plug-and-receptacle connection for the generator. Hope this clears things up a bit. -Sean | ||
| John MC9 (John_mc9)
Registered Member Username: John_mc9 Post Number: 512 Registered: 7-2006 Posted From: 66.217.102.143 Rating: N/A |
Sean - "most stick-and-staple rigs don't come with inverters, so it's a non-issue. " Ah-hah! And that's why the issue has never been so defined and explained to us average RV'rs, as well as you have here! You mentioned (one post up): "if both neutral/ground bonding points are inside the coach, and the coach is not connected to a shore-side ground, this is unlikely to be deadly" This was the general explanation I had heard: The bonds are fine inside, as long as the source(s) is inside. If the source is outside, the bonds cannot be inside, and must be outside. No mention of different power supplies inside the vehicle, or the need to separate those neutral/ground bonds, depending on what on-board power supply is in use....... Interesting! | ||
| Iver (Mciv)
Registered Member Username: Mciv Post Number: 9 Registered: 11-2006 Posted From: 70.69.189.15 Rating: N/A |
Sean, Thank You. You have explained it so well that even I can understand it. I will take my multitester out tomorrow and follow your instructions. At least I will know if I have a problem or not and should be able to fix it if necessary. Thanks again, Iver. |
