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A Shocking Question
By ghurd, Section Homebrewed Electricity Posted on Wed Oct 28, 2009 at 07:05:30 AM MST
Where are these volts coming from?

It is the busy time of the year for our RE system.
And the $3 HF meter connected as the battery SOC monitor started reading 0.4V too high. The batteries were run down quite flat and we really needed the power..
This system has been in place with no serious changes for a long time. The last change was the MPPT controller a few years ago.
The RE is completely ungrounded. 12V solar wired 12V into a SB2512iX, feeding 12V 660AH. There is no grid, no NG pipes. The well pipe is plastic, though there is no water near the electrical.
I break out "the back up charger". Meaning I pulled my truck up to the window, and ran jumper cables to the house batteries.
A while later, I go out to rev the motor a couple times. I grab the throttle cable and it feels like one of those little stray strands poked me. Ouch.
So I get a good solid grip on the do-hicky the cable goes to. YIKES!!!!
Something is strange. 12V doesn't shock me. About 18V gives me a good tingle. I get a meter.
Everything reads like it should... "12V".
Being a bit slow to catch on sometimes (you can see where this is going), I reach up and grab that do-hicky again.
Yup. YIKES!!!! again.
OK. I am barefoot on damp ground. But 12V doesn't bite me.
Meter from all the batteries negatives, to my finger is about 25V. I can almost believe that at the time because we have 2 12V systems somehow interacting.
But. Meter from all the batteries Positives to my finger is 40V DC.
And I can't imagine where that quantity would come from. Or how it would be linked to earth ground.
I considered the MPPT controller may be doing it, but it is not grounded. And it is connected to 660AH of batteries, then 12' of #10 cables to a G24 battery, then to where I am getting shocked. Seems like MPPT would be reading AC if there were any extra voltage not being absorbed by all that battery.
I considered something from one of the inverters, so all the load wires were disconnected. Still 40VDC.
Any ideas?
G-

A Shocking Question | 17 comments (17 topical)

Re: A Shocking Question (3.00 / 0) (#1)
by TomW on Wed Oct 28, 2009 at 08:02:18 AM MST

Shocking. Simply shocking.

Did you look for AC across the shock releasing points?

I would be wondering if it is wild AC from the alt bypassing its diodes?

Some older vehicles had a ground strap to keep passengers from getting static shocks when entering and exiting the velocipede. If yours has one it could be the ground path for current. Or maybe that baling wire you repaired the exhaust with is touching the ground?

Tom

Ignarus can exsisto rememdium. Sardus est forever

Re: A Shocking Question (3.00 / 0) (#2)
by Warrior on Wed Oct 28, 2009 at 10:46:47 AM MST

Hi there!!

Well, I would guess that 12 volts is the rms reading and what's shocking you are much higher voltage spikes!!..maybe even self inducing current in the stator getting past the diodes...

Even though 12 volts shouldn't shock you, I once touched both terminals on a disconnected car battery with one of my fingers wet, and although it wasn't a strong shock, I defenitely felt something...

I dare you to try it with shoes instead of barefoot...bet you won't feel a thing...

Take care!!
Warrior__ "Why can't Murphy's Law be used to my advantage??"
[ Parent ]

Re: A Shocking Question (3.00 / 0) (#17)
by ghurd on Fri Oct 30, 2009 at 10:09:49 PM MST

I totally forgot to check for AC.
Might be interesting given the switching regulator.

Toyota uses SS exhausts. THE Best thing Ever invented, if someone keeps a car for 250K miles.

Ground straps on cars? Like on antiques from before my time, like from Way back in the 80s?
LOL

TW- BTW, did get some soldering done today.
G-

[ Parent ]

Re: A Shocking Question (3.00 / 0) (#3)
by Ungrounded Lightning Rod on Wed Oct 28, 2009 at 01:38:08 PM MST

If the system is completely ungrounded it can float to anything it feels like. So some part of the system will couple to ground - for DC by leakage, for AC by leakage and/or capacitance - and the rest of the circuitry will be at some voltage with respect to that defined by the voltage difference between different parts of the circuitry.

Danger, danger, danger.

Even capacitive coupling can drive lethal currents through wet skin resistance. It only takes something like 20 ma up your left arm to do you in. If you can feel a shock when touching things the situation is dangerous.

You should drive a good ground rod then pick some part of the circuitry and ground it. (Probably the negative side of the battery. Also, if you are running an inverter and it's a model isolates the AC you'll need a ground on that side, too.)

An alternative to a ground rod is a foundation grounding system. If you bought the house or had it constructed to a purchased design (rather than building it yourself) there may already be one in place. Look for a wire emerging near a likely site for a utility feed if you don't have the plans and can't contact the builder.

ONE ground rod. ONE point gets grounded by ONE path. (Otherwise you're creating a greater lightning/surge current risk and potential for working current to travel along protective ground wires and start a fire.) If you have any metal plumbing it should be tied to the ground at ONE point (plus a bypass from cold to hot at the water heater unless copper hot and cold pipes are soldered together somewhere). Metal gas pipes should also be tied to either the electrical or water pipes in ONE place.

Once you have your grounding system set up, measure the voltage (both AC and DC) from the point you intend to ground to the grounding system. Then measure the current from ground to the grounding point - directly with a meter if the measured voltage was low, with a lamp in series with the meter if it was high.

If you get more than a few milliamps AC or DC (or light the lamp), hunt down and fix the leak before making a permanent connection.

Re: A Shocking Question (3.00 / 0) (#4)
by mixerman on Wed Oct 28, 2009 at 01:59:10 PM MST

can it shock you with pickup not running?

Re: A Shocking Question (3.00 / 0) (#5)
by DanG on Wed Oct 28, 2009 at 02:34:13 PM MST

Or can you hallucinate being shocked without the jumper cables connected? :)

[ Parent ]

Re: A Shocking Question (3.00 / 0) (#6)
by DanG on Wed Oct 28, 2009 at 02:44:45 PM MST

From your SOLAR BOOST<sup>TM</sup> 2512i(X) MPPT controller's op manual..

". ..transient voltage protection devices on the battery and PV terminals are designed to absorb static electric discharges, or other high voltage transients and can dissipate up to 1.5KW for 1ms. Steady state voltage in excess of 35V on either the battery or PV terminals will damage the unit."

Danger Will Robinson!

[ Parent ]

Re: A Shocking Question (3.00 / 0) (#15)
by ghurd on Fri Oct 30, 2009 at 09:46:10 PM MST

The battery bank and PVs are not grounded...
Except through the phone!
So everything was at 12V, except me.
G-

[ Parent ]

Re: A Shocking Question (3.00 / 0) (#16)
by ghurd on Fri Oct 30, 2009 at 09:49:33 PM MST

I guess Yes.
The jumper cables connected the truck to the house battery, to the phone, to the ground.

I must have the only mobile vehicle with a land-line phone connection?
G-

[ Parent ]

Re: A Shocking Question (3.00 / 0) (#7)
by cardamon on Wed Oct 28, 2009 at 05:34:38 PM MST

Neat. Your voltage readings make sense considering the battery voltage. If you take the positive as your zero point, you negative is at 15 and your finger is at 40, which would be 25 from negative to finger, of course thats not rocket science, what it is interesting is how you ended up at 40 volts. I would be curious what happens if the vehicle is off? This illustrates the importance of bonding in electrical systems. I dont see that earthing does much other than protect the premises from contact with higher voltage lines, IMO if it wasnt for the bare neutral in utility service drops, an ungrounded but bonded electrical system would be the safest. Anyway back on topic, yeah it can be tricky to keep even an ungrounded system from developing a potential with the earth. When i was digging my well, I was using an electric jackhammer run off a generator. I kept getting a tingle while jackhammering in the damp well, and I couldnt get rid of it no matter how many layers of plastic and foam I put under the genset. I takes so little current to feel a shock, even a very high impedance 'leak' will do it. Hey just look at dirt, its a horrible conductor but to send a few hundered microamps through it is easy. Say you have 50 ohms ground resistance, which is very good and rare to achieve with a just few grounds rods, with 120 volts thats 2.4 amps - not nearly enough to trip a OCPD but way way over the 4 milliamps that is potentially lethal.

Re: A Shocking Question (3.00 / 0) (#8)
by richhagen on Wed Oct 28, 2009 at 10:50:23 PM MST

Your 12V system is 12V from the positive to the negative of the batteries in the system. Since they are not grounded, they could be at any potential relative to the ground. Realistically it probably wouldn't differ by too great of a voltage because there is likely some highly resistive path to ground that will keep it from differing too much. Still, I would bet someone has electrocuted themselves that way in the past. If there is no other conflicting reference, I would agree that grounding the negative side of the battery terminal (if your system is fused on the positive side of the battery) is probably the best way to keep those voltages nearer to the ground that your damp feet are on, and keep you from getting shocked. Now as for the truck, I don't think you can cable it to a ground rod and still get much use out of it :-(. Of course if you are jumping your house battery from it, then it will end up referenced to your house batteries ground when connected anyway. Rich
'A Joule saved is a Joule made'

Re: A Shocking Question (3.00 / 0) (#9)
by Norm on Thu Oct 29, 2009 at 02:19:37 PM MST

You were barefoot summer before last....you still
haven't put your shoes on ? LOL !
( :>) Norm

Re: A Shocking Question (3.00 / 0) (#10)
by Norm on Thu Oct 29, 2009 at 02:26:19 PM MST

Have you tried switching vehicles to see if it
still does it?
( :>) Norm

Re: A Shocking Question (3.00 / 0) (#11)
by ghurd on Thu Oct 29, 2009 at 03:03:23 PM MST

I see absolutely NO reason to wear shoes.
Ever.
It kind of freaks out the UPS guy in January when I am shoveling snow.
I guess it kind of freaks out Norm too?

I "Think" I figured it out, remotely.
A standard telephone answering machine, hacked over to operate on 12VDC?
I DO recall crawling down over the battery trolley, and with my head twisted like Nadia Comaneci half way through a 10.5 scored routine, seeing the indicator LED being lighted. I figure the super low drain answering machine must have been connect to a more convenient terminal than 'power drawing' loads.

The battery bank, the loads, the charging, and about everything else is larger too.
Anyone who wants to try to stick their head down in behind there is welcome to try.
http://ghurd.info/images/Amish/batteries.jpg

I expected to get solid readings today or tomorrow.
"Events Occur" as they say.
G-

Re: A Shocking Question (3.00 / 0) (#12)
by Ungrounded Lightning Rod on Thu Oct 29, 2009 at 08:34:12 PM MST

At first I thought that was a BINGO! But then I re-read it. I don't think that's it.

His voltage is way positive. Tellco battery is minus 48. (When the wires are exposed to moisture the tellcos want the galvanic corrosion to occur on the big mounting bolts that hold things up, not the tiny copper wires that carry the paying signals.)

[ Parent ]

Re: A Shocking Question (Solved) (3.00 / 0) (#14)
by ghurd on Fri Oct 30, 2009 at 09:36:26 PM MST

Yes. It is the phone line.
Remember, the solar system is completely ungrounded.

Again ran out of time to get much playing done, but...

Tested the voltage from battery Pos to earth ground, through me. 38VDC today.

Disconnected the phone line, and got "0V". I think maybe he said an erratic 3V.

To confuse the issue a bit:

The answering machine came with a 9V AC wall-wart. It is connected to a Radio Shack car adapter, 12~24VDC to 9VDC,
and it is a Switching Regulator!

Looks like this, (except I paid like $2 on the clearance rack when they changed the packaging colors)
http://www.radioshack.com/product/index.jsp?productId=3812864

The factory answering machine's 9VAC wall-wart power went into a bridge rectifier inside the machine.

Also, the phone line wires could very well be reversed between the main phone system and the house where the answering machine is located. There are some 'complicated issues' about phones in the area.
I did a lot of 'work' to the main phone system, which is correctly wired.
The phone company later ran the wire (150'?) from there to the house for the answering machine.
I am not sure about the phone wires inside the house.

G-

[ Parent ]

Re: A Shocking Question (3.00 / 0) (#13)
by terry5732 on Thu Oct 29, 2009 at 09:45:13 PM MST

Is your phone line grounded? Most are

And they generally run about 48 volts

[ Parent ]

A Shocking Question | 17 comments (17 topical)

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