Important - check this every time you set up

Originally posted by Old Dingo&Mrs.Dingo I've been following this thread with great interest, and I will be the first to admit, I know very little about electricity, except the more I learn about it, the less I seem to know.   If we keep the safety chains and tongue jack on the ground, without an insulating wheel or plate, how can the "hot skin" be a danger?  (assuming the chains and jack are resting directly on the earth or grass, and not on a concrete pad)   The human body isn't as good a conductor as the steel jack or the safety chains.  Electricity chooses the most direct (easiest) path to ground, and we are using the same "ground" in both cases.  What about a external ground strap from the chassis to a conductive "pin" in the ground, installed before the unit is connected to the "power supply"?   Maybe a plastic mat in front of the step?  (I use one anyway to limit the dirt tracked into the 'pod)  Shouldn't we be advocating these steps, as well as testing, as part of a safe "power post connection procedure", knowing that everything can change the minute someone backs into a power post??

All not true. Jacks and chains on the ground do not "ground" your RV. In fact, the earth itself is a pretty poor "ground". That is, even a ground rod driven 8 ft into the soil can have a resistance of up to 100 ohms and still be code compliant. The actual "ground" wire in your extension cord is technically called an EGC (Electrical Grounding Conductor" which must be connected by a low-impedance path (less than 1 ohm) back to the service panel's Ground-Neutral-Earth bonding point. This creates a local "ground plane" which is what gets rid of any leakage currents in your RV's electrical system which is what causes hot-skin voltage.

The idea that electricity only follows the path of least resistance is an old wives tale. Actually, it follows ALL paths at the same time. Remember that the human body with wet hands is around 1,000 ohms resistance. The jacks sitting on dry dirt might only be 1,000 to 5, 000 ohms resistance, maybe higher. If you have an open ground and any amount of current leakage (everything leaks) your RV can easily have 40 to 120-volts on its chassis and skin. If that's the case, then touching the door handle or steps while standing on the ground will allow current to flow through your body. How much current is dependent on both the voltage of the RV's chassis and skin plus the available current from the leakage. It could be a high-impedance (low current) leakage, in which case it will be self limiting to maybe 10 mA or so. That will give you a shock but not be lethal. However, if the current ability increases, then your body will draw as much current as your impedance to ground will allow. Since voltage divided by resistance equals current (E/R=I) even 40 volts across your 1,000 ohms of wet-skin body resistance could allow 40 mA (milliamperes) of current to flow through you. And if you've got 120-volts on your RV's chassis/skin from a hard short, then you can receive 100 mA of current through your body. Only 20 mA is needed to cause your hands to grab on an not let go of an energized conductor. And 30 mA for a few seconds is almost guaranteed to cause your heart to go into fibrillation which causes death in minutes without immediate CPR.

So the only thing protecting you from this is your EGC (Electrical Grounding Conductor) ground pin on your extension cord, which must be connected back to your service panel's Ground-Bonding point. Anything else won't work and offers a false sense of security. Don't think that putting a rubber mat on the steps or wrapping the door handle with electrical tape will protect you. Remember that anything attached to a hot-skin RV is also electrified, including your tow vehicle.

See why I want to do more clinics on RV grounding? This is stuff that even RV Technicians fail to understand. And certainly, most consumers who own RVs are not aware of shock dangers. If you ever feel the slightest tingle while touching your steps or door handle, then turn off the pedestal breakers and unplug immediately. There's something really wrong which could kill the next person who touches your RV. Never let an electrician or technician convince you that a "small shock" is OK because it's not. See http://www.noshockzone.org/are-little-shocks-ok/ for my article on the subject.

This is normal leakage currents that occur in ALL appliances and gear plugged into 120-volt AC. Here's what's happening. For a piece of electrical gear to be sold in the US it has to pass UL Ratings. UL allows up to 0.75 mA of hot-to-chassis fault current leakage from an "ungrounded" appliance like your iPhone charger, and up to 3.5 mA of hot-to-chassis fault current from a "grounded" appliance like a microwave oven or charger/inverter. Generally there's as much leakage from hot-to-chassis as there is neutral-to-chassis, so you end up with around 60 volts on ANY chassis that's not tied to the ENG (ground wire).

These normal fault currents are additive, so the more circuit breakers you turn on, the more leakage currents will show up on your RV chassis (and skin). So a single appliance (such as a microwave) could be leaking 2 mA to its chassis, but turn on 10 of them and you'll get 20 mA of total leakage current. This is what I call a high-impedance current source since it has the potential of supplying on a small amount of fault current to your body. The reason why the measured voltage is low with only one breaker on and gets higher the more breakers you turn on is that your meter and grounding nail is sinking some of the current to "earth" which forms a voltage divider. So that shows that you have no significant leakage current sources in your RV. If you did, the measured voltage to ground would be 120-volts when you hit the breaker connected to the wire that's shorted to the frame, etc....

So here's the thing. There's a high likelihood that even if you had a disconnected EGC (ground) on this particular RV right now, that there would be insufficient fault current available to cause a lethal shock. But that's just at this very moment. At any time one of your appliances could develop just a little more leakage current to its chassis which ends up on your RV chassis and could kill you or a loved one the next time you touch the ground and RV at the same time. The job of the power cord's safety ground (EGC) is to drain these potentially dangerous currents to the G-N-E bonding point in the service panel and prevent ANY voltage from occurring on the chassis (and skin) of your RV.       

Believe it or not, the job of the ground rod (GEC or Grounding Earth Conductor) is not to trip your circuit breaker in the event of a short circuit. The ground rod is primarily there to absorb the energy of near lightning strikes. That's why just poking a ground rod in the earth doesn't actually "ground" your RV. It needs the low impedance EGC ground wire path back to the panel to create a true safety ground.

Fred, what you describe is what I found with my pod last year.  Mine started off with about 60 volts with all breakers on, and that voltage decreased as each breaker turned off.  At the time it drove me nuts trying to figure out what my "fault" was, I expected the voltage to go away when I turned off breaker X, therefore the appliance(s) on breaker X were the problem.  Having it go down with each breaker was very confusing.

Let's have a big THANK-YOU to techntrek for bringing this problem to the attention of the forum.

Now all of you need to spread this important safety info to any other RVing groups you belong to.

Love dem crabs, hon!