John, I think you've got it; should work as you describe. As you imply, it
is much better that there be a ground connection, but the GFCI will, if
working correctly, trip when the kid goes for the bike. Which brings another
point--I have seen defective GFCI's that would no longer trip. That's why
they have a test button on them. It puts a high resistance to ground and
actually tests the operation of the GFCI. We should make a habit of testing
them often.
The only thing I see wrong with your description is the apparent impression
that the resistance of the ground fault is particularly relevant. Please
keep in mind that for the relativey high voltages we are talking about (120)
that it is going to make practically no difference whatever to the person
being shocked whether the ground fault is 2000 ohms or 20 ohms. Or 20000
ohms for that matter. If you apply Ohm's law to it, you'll see that a
dangerous level of current can flow through the human completing the
circuit at relatively high ground fault resistance. The resistance that the
human is supplying to the circuit varies greatly depending on a number of
conditions. In the example you supplied, of a kid standing in a puddle of
water grabbing the metal bicycle frame, let us say with both hands, the
circuit resistance through the kid is fairly low, so he is going to get
quite a shock even if the fault resistance is relatively high. Conversely,
on a dry day with rubber soled sneakers on, he wouldn't even feel a tingle
if the fault resistance were at or near zero. A ground fault of much higher
resistance than the 2000 ohms that Edgar measured could, under some
circumstances, be lethal. And under a LOT of circumstances it would be very
uncomfortable.
One thing that still haunts me about this thread is the reliablity of the
campground-supplied ground wire. In your hypothetical example, we have one
that is completely broken. A handy dandy checker would catch that so
therefore it is good practice to always check the outlet when hooking up.
But what if instead of being completely open, it is instead a corroded
connection of, say, 5000 ohms? The neon light in the handy dandy tester will
see 5000 ohms as a ground and will happily light, indicating a ground. Then
if your water heater element has a 2000 ohm fault as Edgar's did, you've got
a voltage divider putting the coach frame at a potential of five sevenths of
the line voltage above ground. This is close to a hundred volts, still
potentially lethal and definitely dangerous. But at least a GFCI should tip
immediately when you hook up. I still have the nagging feeling that it would
be prudent to try to provide a dedicated ground wire as part of the hook-up
procedure, even knowing that it is less than perfect. If it just provided
enough of a ground to cause your GFCI to trip, it was definitely worth
connecting.
I'm amazed at how many first-person anecdotes are coming in relating to
being shocked in the motorhome. I think we've hit on something here, and am
very glad it got brought up and discussed.
Electricity can be your best friend or your worst enemy.
Very best regards,
Travis
- -----Original Message-----
From: John Dolan
To: gmcmotorhome
Date: Friday, July 09, 1999 1:04 AM
Subject: Re: GMC: Don't get electrocuted! (was:Temporary Shore Power)
>Open for comments from all... just trying to clear up a myth that the
>label writers may have created while trying to help GMCers protect
>themselves at the same time as they limit their liability when they
>installed a CFI in the GMC motorhome.
>
>>> Providing a ground for a GMC doesn't sound very
>>> appealing to me. Henry
>
>>Could you elaborate just a bit more on your ground comment? Do you mean
>>electrically (technically) speaking? Physically (as in having to drive
>>in a ground rod in a client's parking lot)? This is not a set-up
>>question and I'm sure there are pros and cons on each side and I don't
>>have a clue . John
>
>>>Hmmm ... I was thinking of becoming the ground path when one wasn't
>>>present. Not a pleasant thought. Henry
>
>
>Ok... Now I follow you. Then it fits in with what I wanted to get on
>the table for discussion. Lets say the most dangerous kind of a short to
>ground occurs in someones GMC, that being a hot wire (120 vac) to chassis
>ground. And lets assume that the short is a lower resistance short than
>Edgar's (2000 ohm) and even lower than Gary's (500 ohms). But it is high
>enough so that the circuit breaker doesn't trip. Let say something like
>20 ohms is the resistance of the short to ground. So the short only
>draws something like 6 amps and the circuit breaker does not trip.
>
>Now lets assume there is an inline GFI in the circuit (either one that we
>built, or one that we bought, or one that came with our GMC). And let's
>assume we are at a campsite where there is a standard 20 amp oulet but
>the ground is defective and we don't have a plug tester or don't use the
>one we have.
>
>So we plug in and the happy GMCers start turning on everything in the
>motor home including the unit with the short to ground. Now since the
>ground is defective (lets say it was just broken this morning when the
>previous camper drove off while he was still hooked up to his shore power
>line) He stopped quickly as someone yelled, and he just got out, coiled
>his cord and left.
>
>So we are enjoying the early evening, as some clouds blow in and it
>begins to rain. The CFI doesn't trip because there is no ground
>connection at the socket that was yanked earlier this morning, but not
>obviously damaged otherwise. But there is a fair amount of voltage
>present on the frame of the GMC, but since there is no path to ground
>nothing happens.
>
>So our situation is outside the wording of the label on some GMCs that
>have OEM CFIs or we hear from a friend, that they were to be used on a
>grounded power distribution system, but we were not quite sure what the
>label meant (would it not work or what) Well we have already seen one
>thing different from Mark Grady's daughter's experience the CFI did not
>trip in our case when the unit with the short was turned on.
>
>It rains all night, but the morning dawns bright and sunny but still cool
>enough for a bike ride after breakfast. The kids can hardly wait to get
>out and get the best, newest mountain bike and they all fly out the door
>landing many feet from the door.
>
>Now comes the situation where the first kid starts to take the best bike
>down off the bike rack, while standing in a puddle of rainwater a couple
>of inches deep. I feel the CFI will trip instantly and before much
>current runs through the kid... but it is not 100% safe. In fact current
>has to run through the kid for the CFI to trip and I think that is what
>the label writers were trying to avoid. The only thing worse that I can
>think of is, the same senario but without a CFI.
>
>Regards,
>John
>
>--
>"I do whatever my Rice Krispies tell me to..."
>John said, from inside a 1974 Glacier on the
>Potomac, just north of the White House.
>
