Open Ground Circuit On Inverter Power
Dear RV Doctor:
We enjoyed your seminars at the FMCA convention in Redmond, Oregon. I have a Magnum inverter in a 2007 Monaco motorhome. On shore power and generator power, all the receptacles check “good” using a three-light plug-in tester. However, when the electrical system is on inverter power, the receptacles show an open ground on both circuits. I have checked connections in the inverter block and it is wired correctly, plus all connections are tight. The same is true for the main breaker panel. I have owned this motorhome since new and have used the inverter only twice for short periods of time, so it may have come from the factory with this problem. Since the electric worked fine on shore power and generator power, I never gave it a thought to test the circuits. The only reason I did was because I was interested in finding out which circuits were powered by the inverter, with the idea of installing a residential refrigerator at some point.
Bill Willard, F196819, Gilbert, Arizona
Thanks for attending the seminars in Redmond; I hope to see you at one of the Family Reunions in 2011 as well! I’m not a huge fan of those plug-in testers. I, too, used one for many years until one of my electrical engineering buddies found some inconsistencies while comparing and using them. I always rely on my digital volt-ohm multimeter (VOM) for all electrical measurements, just to be safe. But since you state your circuits work fine and show the correct polarity when using the generator or the shoreline, chances are your plug-in analyzer is working. But to be sure you indeed have an open ground situation on the inverter output, it’s better to use a VOM while troubleshooting.
There should be an integral ground connection from the main panelboard distribution box through the inverter and ultimately to chassis ground. Here’s a paragraph from the Magnum manual for your inverter:
Wiring The Inverter AC Ground
Combine the ground (green) wire from the main panel’s ground buss bar and the ground (green) wire from the sub-panel’s ground buss bar (or the ground connection on the outlets, if using the output breaker versions). After these grounds are combined, use field wiring leads to connect them to the inverter’s AC GROUND (green) wire.
Be sure you can read continuity between the negative buss bar in the main panel and the negative terminal on the Magnum inverter. The ground path should remain constant throughout the entire AC circuitry. The hot and neutral conductors will be switched through the automatic transfer switch, but the ground conductor should not be switched.
Your problem could be the result of a ground wire coming loose somewhere in the system between the inverter and the breaker box(es). But measuring with a VOM should determine whether there is an open circuit and where it exists. Since an open ground circuit is potentially dangerous, it may behoove you to contact a certified RV technician to perform further testing and measuring, just to be safe.
Time For Tires
Dear RV Doctor:
It has come time to buy new tires for our 2004 Allegro Bus. Is replacing them with the same Michelin 275/80 R22.5 XZA2 tires really necessary? Or are there other tires and brands that you would recommend?
George Burns, F348579, Saugus, California
Because your question is of such a subjective nature, I decided to get an opinion from one of the top experts in motorhome tire information and safety, Walter Cannon, executive director of the RV Safety & Education Foundation (RVSEF). Here’s what he had to say:
“It is important to stay with the same size and load range (load range is not given in this question). We always recommend staying with a brand you are familiar with, but there is no reason one cannot change brands, just not the size. If the performance of the Michelin has been satisfactory, then I would see no reason to change.”
I fully concur with Walter. All things being equal (size, load range, weight rating, wheel spacing, etc.), it is permissible to change brands. Many RVers do so to save money on tires that may be “on sale.” But be sure to check the age code on the sidewall prior to the purchase. Molded into the sidewall of every tire is a four-number code that indicates when the tire was manufactured. The first two numbers indicate the week the tires was manufactured, and the last two numbers indicate the year it was made.
It is important that new tires be truly “new.” It has been reported that many tires available at the retail level, though never used, may have been warehoused or stored for a lengthy period of time prior to being sold. Remember, it’s the actual date of manufacture that is important to you, even though the warranty typically begins at the time of installation.
Blowout Plug Redux
Dear RV Doctor:
In reference to your mention of using a blowout plug to winterize the motorhome’s water system that appeared in the November 2010 “House Calls” column (page 18), I used compressed air for winterizing in the past as well, but I was told that this method doesn’t get the water out of the check valves in the water pump and could subject the pump to rupture. I would be most interested in your comments on this.
Bob Kendall, F388311, Murray, Utah
Keep in mind, the entire fresh water plumbing system is factory-tested by the motorhome manufacturer and tested again by the dealer at pressures nearing 80 psi. So, anything less than that will not create a problem within the system. In fact, in earlier years the test standard was 100 psi. The backflow preventer located at or near the water pump outlet becomes nothing more than a closed valve when air pressure is introduced through the city water inlet, so there’s no danger to the pump from that side. Now, it is important that the water pump indeed has a check valve located at or near the pump outlet. SHURflo makes one line of pumps that does not come equipped with an internal check valve. In those cases, the manufacturer will install a separate check valve fitting at the pump outlet. I’ve never seen a water pump damaged by injecting air into the system through the city water entry.
But you make a good point about the remaining water inside the pump on the other side of the check valve. One remedy, after emptying the fresh water tank, is to remove the outlet tubing on the pump and briefly run the pump to eject as much moisture as possible. Running dry for those few seconds will not damage the modern RV water pump. What little moisture that may remain inside the pump head will not likely cause any damage should temperatures dip below freezing.
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