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|01-13-2012, 12:03 PM||#31|
Join Date: Nov 2011
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Thanks for the very concise solution.
|01-13-2012, 12:48 PM||#32|
Join Date: Sep 2008
Posts: 1,497Rewards Points: 500
Anyway as to why that may sometimes shock you, then go away...
That nail is not a good electrical connection to the drain pipe. It could be something as simple as humidity or outside temperature.
With higher humidity, wood expands. And with temperature changes, a house can "shrink" or "grow". i.e. All the wood in the attic can move a little with humidity and temperature changes.
So the wood moves a little one way and that nail makes contact. Moves a little the other way and does not make contact.
|01-19-2012, 04:14 PM||#33|
Join Date: Jul 2008
Location: NW of D.C.
Posts: 5,990Rewards Points: 2,000
My two cents is below.
This is a good, challenging problem. If it doesn't come back in several days I guess it's fixed. I hope it doesn't come back.
>I clearly felt current when I touched the hot/cold taps to adjust the temperature.
Assuming you are an adult male you had at least 1 mA through your wet body at some unknown voltage less than or equal to 28-30 vac. More than 10 mA would have been painful.
This would put your skin/body resistance at most 28K to 2.8K as maximums. With wet skin the lower value is more likely.
A phantom voltage caused by ~200' of romex could give you 1 mA so if it's less than this distance back to the panel this cause is not too likely.
>He then took his voltmeter into the bathroom and measured 28-30V on the shower door (sliding glass door with brass frame sitting on the tub) but NOT on the taps where I had felt it previously.
Removing the screws that anchor the shower door into the wall changed nothing so the shower door is grounded with or without the screws.
If he used a lo-Z meter with input impedance ~ 3k then you would have felt up to 10 mA which would have been very painful so I think he used a conventional meter (input imp > 1 megohm).
>there was a direct correlation between the voltage reading and where the dimmer switch was set.
Yet the dimmer puts out 120 vac and you are seeing only 28-30v so there must be a voltage divider somewhere.
>turning on the tap water and switching the water diverter to the shower setting charged up the taps.
The water seemed to be providing a somewhat conductive path, perhaps over a very short distance.
>the problem disappeared entirely
It is/was intermittent.
>He plugged an extension cord into the GFI and ran it to the shower. Plugging the voltmeter into the hot lead in the cord and touching the other end to various areas of the tub shower showed a reading of 120V - I believes this indicates it is well grounded.
More so if he used a low impedance meter and better if he could have lit an incandescent bulb with this voltage. For ground testing the test current should be pretty high; a bad ground may read as low as 0.5 ohms on top of the wire resistance expected over the distance back to the panel.
>Plugging the voltmeter into the neutral lead of the extension cord and touching the other end to various areas of the tub shower yielded a reading of 28V, hence the diagnosis that it was an issue with the neutral. As the dimmer switch was adjusted, the reading correspondingly went up and down.
So 28 v is on the neutral lead. With all good connections it would take enormous current through a neutral to cause this much voltage so it's likely there is a bad neutral connection or (unlikely) this is a phantom voltage.
>The electrician found a rogue nail that had nicked the relevant switch leg. This nail was touching a metal band as well. The metal band was touching the cast iron waste line, which came into the shower tub on the same side of the room as the light switch (explaining why we couldn't reconcile the water pipes and wiring being on the same side of the bathroom). This lit up the nearby pipes and shower when the switch was activated.
The last corroded connection I measured came in at 160 megohms but yours could have been less.
Q: What single equivalent circuit would account for most or all of these symptoms?
I will be unsubscribing from this thread as soon as I post it so in the unlikely event of questions please PM me.
Last edited by Yoyizit; 01-19-2012 at 04:18 PM.
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