If you read my last post, we began a discussion about grounding and bonding of portable generators. To begin, we talked about a standard residential electrical service and specifically discussed the ground wire system, the neutral wire system, and the single bonding point between the two systems. Lastly and most importantly, we also described what we referred to as a sub-panel and how important this is to us as RVers.
So, here we go with the next point: the breaker panel in your RV is a SUB-PANEL. When you pull into the campground and plug in your shore power, you are becoming a sub-panel to the main panel which is permanently installed at the campground. Therefore, the neutral wiring system and the ground wiring system in your motorhome are not bonded together. All the ground wires terminate in a ground bar within the breaker panel, and all the neutral wires terminate at a seperate bar within the breaker panel. The neutral and ground systems are kept isolated from each other. The bonding or connecting together of the two wiring systems is taken care of in the campground wiring system not in the RV.
Keeping in mind that the above discussion dealt only with the power supplied by the shore power cord, the next step is to look at what happens if your RV has an on board generator set, particularly with an automatic transfer switch. With an on-board generator we are faced with a problem because the breaker panel in the RV has an isolated neutral bar and ground bar, yet proper wiring calls for a single bonding point SOMEWHERE! Thus, the neutral and ground wiring systems coming from the on-board generator is bonded at the generator. This point is important for safety, but it is also important when you consider a portable generator powering an RV.
When you look at most portable generators, you will find that the generator has what is known as a “floating neutral”. What this means is that the neutral from the generator, and the ground from the generator are NOT bonded together at the generator! Why is this? Consider that many portable generators are sold to home owners who are going to use the generator as an emergency power source for their home. When you realize that the generator will probably be connected to the home’s main breaker panel, and remembering that panel is already bonded by code, it would be a code violation for the generator to also be bonded. So, generator manufacturers do not bond the neutral and ground systems. However, when one of these generators is connected to an RV, it is connected to a breaker panel that is NOT bonded. Since the generator and the breaker panel is not bonded, we have a code violation and a safety issue.
There are several ways to address this problem, depending on the generator and we will talk about these methods next post. Until then,
Happy RVing
Larry
Drew
Larry,
You could also find a way to bond the neutral busses together between the gen. and the rv, but I really don’t think anyone’s going there. I’ll just keep using my Progressive unit and doing preventative maintainence.
Larry Cad
Dave, I have been thinking about your comment, trying to come up with a response. Think of this, for many years, electrical equipment didn’t have a third ground wire. The ground is a relitively recent electrical innovation. I remember when I was young, if you had a radio on the kitchen counter and, while touching it, you received a shock, you simply unplugged the radio and switched the plug around in the two prong outlet. My point is that the ground is not a “necessary” component in electrical systems. It is there for personnel safety. Without it equipment still functions properly, albiet not necessarily safely. What I am trying to say is that an open ground wire on your RV will not cause the disaster you described. I can see such a situation occuring, under some circumstances, IF it were an open neutral, but not an open ground. One possible scenario would be if your ground wire were open, and the neutral in the power pole were open. Perhaps your ground had been open for some time, and you just happened to encounter a campground with an open neutral. I can see this combination causing problems. This kind of situation points up the usefulness of one of the aftermarket power monitor systems such as the ones sold by Progressive Dynamics. The good devices monitor voltage, and look for open neutrals and grounds.
Dave Beach
Keep going Larry: I’m looking forward to the next piece.
As a side issue, I had a failure of the ground wire right in the “onboard side” (inside the wall) of my main incoming service cord. That prompted the two 110 leads to basically bridge to 220V and it proceeded to cook my microwave and (new) flat screen TV. I ended up getting a new microwave after trying to find any fried parts. Fortunately, the TV was salvageable by the repairman whgo showed me a voltage sensitive “safety fuse” wired into the circuit board..he soldered in a new safety fuse, and I was like new again!
Larry Cad
Joe, inverters are a whole different subject. I’ll see about doing a series on inverters sometime in the future.
Larry
Larry Cad
Drew, what you are suggesting is technically ok, except it totally ignores the point of my posts which is that there is supposed to be a bond between ground and neutral. You are proposing connecting the generator case (ground) to the frame of the RV (also ground). Where is the connection between ground and neutral as required by code?
Larry
Drew
Larry,
You could bond the generator frame to the frame of the rv and be fine (although a pain in the neck due to the bolting of hardware needed to really accomplish this).- That’s in the scenario that you use a gen. that’s peripheral and not on board. If you use an on-board unit- the grounds from the rv and gen. are already bonded. You can verify by using a volt-ohm meter and probing between the ground prong of the shore power cord to the female at the j-box.
Drew
Steve
Larry,
Thanks for the insight. Many years ago I was in charge of facility maintenance. Because of this position I decided to learn the National Electric Code. It was amazing of the things I didn’t know or understand. The point you bring up is hard to explain to people that are not fully knowledgeable about the electric power supply of utilities and how to properly distribute the power. In the building I work in we buy 13,200, step it down to 480 and 208. Having a proper distribution, and SAFETY is a difficult job. I don’t have an RV yet, but will think about this point you made every time I plug int.
Thank you for the information
Steve
Joe Loughry
when you are done would relate this to an inverter – i have noticed some weird hapings that seem too relate to this ground/neutral bonding/