Foretravel Owners' Forum

I just thought I'd put this out here in case anyone else is in the same boat with an older coach.  I recently "got" to get the radiator re-cored, and did the R & R in my driveway. I noticed, as I was doing this oh so fun job, that every now and again I'd feel a pinprick sometimes, like there was something sharp on my creeper that was poking at me. I never did find anything though. Then, getting ready for our most recent trip, whilst checking the undercarriage and looking for any leaks and doing the general preflight inspection, once in a while well there it is again.. What the heck? Was that a bee sting, or a.... jolt? Naw. Couldn't be. Everything in the rig is fine, nothing wrong showing on the power meter, my little polarity/short checker tool that you plug into a socket said all clear no troubles here, boss. Well, I may be slow on the uptake, but I have a jaundiced and suspicious nature when it comes to electricity. So out comes the multimeter and I intermittently  measured about 40v AC between a wheel and the ground when plugged into shore power. During those fun occasions my little socket tester no longer said "all AOK"...My, how alarming. I have to wonder, given how galvanic corrosion can work, how much this ground short had to do with needing to re-core the radiator in the first place. Anyway, I figured I'd start by checking the shore cable front to back and quickly diagnosed a fault in the plug at the shore end. New plug at camping world, cut off old plug and install the new one, complete with a nice layer of solder on the joints just to be certain. I think, I hope, i dodged a bullet. This much power to ground could be a real horror show for things like wheel bearings, and, well, everything electrical. From here on out I'm going to use my outlet tester every time I plug into shore power.

Thanks for sharing.  Glad you found this without more that a minor jolt!

Wondering, is this condition one that would be detected by a Progressive EMS unit?

Jay, I could be wrong, but I believe that's what's known as "hot skin" in the RV world and is dangerous to more than just electronics.  If it *is* the same as "hot skin" it can kill, so very good that you found the problem and fixed it!

Michelle

If I ever had to change my cord and or plug I would go with the orange aftermarket brand,can research here on the forum plenty of info.

Chuck - the link came up as a "404 - not found" for me.

ETA - I looked at the "RV Safety" on that site and see both "Hot Skin", one of the oldest articles, and "Are Little Shocks Normal/OK" (or something like that) as one of the newest articles.

No~Shock~Zone » August 2013 (http://noshockzone.org/2013/08/)
Then click on 24 comments.
JohnH

Mark,

Yes.  If a "hot skin" condition exists,  Progressive Industries EMS devices will detect reverse polarity and prevent energizing the coach (downstream of the EMS) from a faulty (reverse polarity) source of power.  Once energized through the EMS, a "Hot Skin" coach fault downstream of the EMS will also cause it to trip.  "Hot Skin" is reverse polarity.  In other words, if either neutral or ground becomes "higher" than the other, detected by the neutral not being within some small electrical potential from the ground wiring - anything more than a few volts, the EMS will not connect to the power source or will trip it if connected.  Side note trivia:  Yamaha and Honda generators have "floating" Ground and Neutral wiring and because EMS is looking for near zero volts difference between G and N, a PI EMS (or TRC) will not connect to a Yamaha or Honda Generator.

By National Electrical Code (NEC), any system (eg - home or commercial building) is allowed to connect ground (a long copper rod driven into the ground) to neutral in only one place - ONLY within the power entrance panel to that home or business.  RVIA code says that the ground and the neutral must be separated from one another everywhere (floating) throughout the RV and its extension cording.  This is for the purpose of preserving the NEC and also for not allowing cross currents between the neutral and ground conductors, anywhere except back at the home or business incoming power source panel connections.  There will be wiring resistance between the breaker panel and the EMS and that resistance will develop some voltage difference between neutral and ground, but when it reaches a few volts (3 or so), the EMS will no longer connect or stay connected, because it believes there is a reverse polarity.

Now when a true reverse polarity condition occurs on a coach, through whatever wire or component fault, the ground or "skin" of the coach can become "elevated" to a dangerous voltage.  What is that dangerous voltage number, for an unsuspecting person?  Well it depends upon the fault and how much current that fault can supply, but in general, if it's 30 volts or more, it can, at worst, stop you heart in an instant.  Especially if the current runs, for example, from your left hand to your right foot (which happens to be on the ground) through your heart.  Strong heart and good luck - you live.  Poor luck and no help - a slight tingle can start atrial fibrillation that will be the death of you.

Two reasonably inexpensive  devices will protect you from these undetectable (invisible) and unanticipated conditions (that all too often occurs in the many, many different places that you happen to plug into).  Your coach, after all, is not like your house where you wire it once, test it once and it is good forever after.

• Progressive Industries EMS (there may be equal or better, but I haven't found it)

• Always use a shore power tester, that you test the power pedestal with, before you plug in.  I built mine for less than $50 or so.  I've offered plans before. There's a picture below.

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Some logical ways to protect your on board systems from the power surge insults and challenges, that will eventually damage them, is to treat every opportunity to be conservative with power source changes as an opportunity to preserve the longevity of your coach. 
For example:

• Always Open the shore power breaker (or pull the shore power plug) before starting your generator. 
• Always test unknown shore power before plugging into it, even if you have an automatic Electrical Monitoring System (EMS) or equivalent.
• If you don't have one, invest in a good EMS.  It has saved us numerous times in totally unpredictable, unanticipated ways.
• Wire your EMS to protect your own generated power as well as incoming shore power faults.  I've had generator output problems that would have caused very expensive damage long before I ever knew I had a problem. 
• Always Stop your generator before plugging into shore power.
• Always unload all of your large loads (AC's, microwave, hair dryer, toaster, resistive heaters, etc.) before you stop a running generator or open the shore power breaker.
• Always open the shore power breaker before plugging into the shore power receptacle or unplugging from it. 
• Always keep your shore power plug blades clean and lightly coated w/dielectric grease and don't drop the plug in the dirt or grass.
• Some people even turn off their inverter before starting/stopping their generator or before bringing shore power on/off the coach.  I don't because the inverter sensing circuitry is specifically designed to seamlessly handle these transitions and I'm lazy (as in I don't like reprogramming clocks that don't have memory backup), but it would be ultra conservative to follow their example.
• Etc., add your own.

HTH, let me know if you want plans for my Shore Power Tester!

Neal

Another scenario that the EMS may not pick up (I'm not sure on all the features), but a GFCI will prevent is this:  Broken ground wire between RV and power pole, and then a hot wire that is touching the frame or something metal in the RV. This could cause the "hot skin".

Reverse polarity is not necessarily dangerous if all the wiring is correct and up to snuff. But it CAN become dangerous under certain circumstances.

PS: I love my EMS hard wired 50amp with remote display. I'm installing one on my boat too!

In this case, wasn't the issue the OP's shore power cord plug end?  A hard-wired EMS would pick this up, but a portable plug-in at the power source would not, since the problem would be between the EMS and the coach, not coming into the EMS.

Remember the static discharge straps on trucks years ago? Would something like that protect us from a hot skin contrition?

Michelle beat me to it. After a "near melt-down" we not only got a EMS pedestal-mount, but replaced the cord ends, AND the connection in the wet bay!  ^.^d  A quick aside: after we bought this coach, I had always noticed a rubber tube scraping the ground, figured it was a "discharge strap". Finally tracked it down, it was the "slobber tube" that had come loose!

Michelle,
Good catch. 
The safe answer is yes (an HW EMS would catch it and a Portable would "maybe" not), because we don't know exactly what the OP found wrong with the cord and what else is "broken" on the coach that is causing one or both of the Lines (Line 1 or Line 2) to "leak" to ground (the coach frame)
If it was just an open ground or an intermittently open ground conductor in the shore cable, the Portable EMS would not recognize the condition, whereas the Hardwired EMS would see the condition instantly upon open (fault) occurrence.

The "maybe" (which I was thinking, but not getting wordy about when I posted) comes in where the OP's coach has a double fault (actually a more common occurrence) where some failed component on the coach is allowing the ground (frame) to become partially elevated in electrical potential and the intermittent open ground doesn't provide a continuous drain for that potential.  I was trying to read too much into the OP's intermittent condition, the 40 VAC measurement and some sort of plug end problem.  If there were a strong connection from Line to Ground, the coach frame would have been reading a full 110 VAC to ground whenever the ground conductor was "open".  If it was a failed component, like an air conditioner or refrigerator, that is not on a GFCI circuit, and the fault was a weak fault, then either a hardwired or portable EMS would see the intermittent open ground in the shore power cable when the ground and neutral conductors became mismatched by greater than 3 VAC or so.  In that condition, either one (HW or PT) would trip to shut off incoming shore power. 

Double faults are more common because the ground wiring masks weak faults to ground by continuously draining away the elevated potential.  Then it takes an interruption of the ground path (fault #2) to make fault conditions obvious.  We have a lot of non-GFCI protected loads on a coach, where weak faults to ground can occur.  Once the condition becomes obvious (elevated frame potential), I would be shutting off all AC breakers and watching the frame voltage as I reenergized the components, one by one, trying to find the component that has the "leak" to ground.  Then I would have both a faulty component AND the shore power cable to fix.  But that's just me.

John,
"Hot Skin" condition is just a surplus of electrons accumulated where they should not be (in the frame), so anything that will FULLY drain the electrons back to ground potential will remove the condition.  Ground straps on trucks were meant to discharge the electrons accumulated by the vehicle moving through air and over ground, picking up loose (weakly bonded) electrons and creating a static charge buildup on the truck. Touch a grounded gas nozzle to the metal of the fuel nozzle and fuel vapors from the open tank go into riot mode.  Gas pump design has considerably relieved that nowadays. 
A loose ground strap on a coach (from frame to earth) would be less certain to have good contact with the earth (ground) than a four wire shore power cord and an EMS that was monitoring the G and N for abnormal electrical potential between the Ground and Neutral conductors in that power cord.
HTH,
Neal

The other item to check on your power cord at the point where it plugs into the shore power socket, is how dirty are the contact pins.
I regularly sand with 600 grit to keep them a shiny brass colour.
I have seen plugs where the plastic housing has been melted because it got hot due to dirty contacts or weak contact pressure in the shore power receptacle.
I will also check the temperature of the plug after using it a couple hours, if you cannot hold on to it because it is hot, look for the problem.