Can I get an reasonalbly accurate measurement this way?

Discussion in 'General Electronics Chat' started by Codesmith, Jul 19, 2006.

  1. Codesmith

    Thread Starter New Member

    Jul 19, 2006
    5
    0
    I recently discovered that
    1) None of my outlets in my appartment are properly grounded, except for two in the kitchen which have to be grounded and GFCI to meet building codes.

    2) My computer's powersupply is leaking current to the bare metal of my case on very humid days. Enough so that I get a mild but constant shock, but not enough to be painfull.

    I wanted to know how much current and amperage I was exposing myself to so instead of touching the bare metal of my case I touched it with one prong of my multimeter while holding the other end.

    I got readings of 60 V AC and .3mA AC. The results were consistent from reading to reading as long as my bare feet were touching the rug in the same way. Lifting one foot cause me to get a different, lower result which was consistent across repeated measurements.

    My question is this. Assuming that my mutimeter is accurate in this amp range, are the results I am getting a reasonably accurate representation of the actual voltage and current that is entering my body when I touch the case.

    If not then can someone explain to me exactly why the current or amperage I am actually exposed to would be different that what I am getting from my multimeter.

    PS Please don't give an opinion with explaining it in a semi-rational manner.

    I tried to ask about my PSU at a PC hardware forum and the discussion about whether the PSU was defective got derailed by people arguing over this measurement business.

    I already had people tell me that

    1) No current at all could be passing through my body when I felt a shock because of the large amount of matterial it have to travell through beneith my floor. (Hint more paths for the current to travell = less resistance)
    2) You can only measure voltage between two points only if you already know the potential of at least one of the points.
    3) You can only measure amperage in a circuit, and a human body can't form a real circuit.

    I really can't take much more of that. I am hoping anyone on this forum will either no the basics or if they don't will at least become less verbally abusive when someone politely disagrees with them or ask them to clarify their statement.
     
  2. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    If you know the human bodies impedance at a certain voltage you can measure leakage current.
    http://www.epanorama.net/documents/safety/voltage_effect.html

    Make something crude base on a
    Figure 3: Human Body Model or Network for IEC60601-1
    http://www.quadtech.com/newsletter/035117.pdf

    Or use what the pros use.
    http://www.simpsonelectric.com/228.htm

    Some info
    http://www.evaluationengineering.com/archive/articles/1199safe.htm

    Leakage current flows from user-accessible parts of the product to ground when the unit is operating. If the current is excessive, it can cause an electrical shock.

    A leakage current tester measures leakage while simulating the impedance of a human body, generally about 1,500 W in parallel with 0.15 µF. The measurement reference point is switchable to the high, neutral, or ground wire of the power cord. Current is measured through the ground connector from the case to one of the lines or from one point to another on the case. Most standards set a limit of 0.50 to 0.75 mA for this current.

    Medical equipment carries a different set of measurement conditions. The simulation resistance is 1,000 W, and the current limit generally is 1.0 mA.


    Definition
    http://www.phihong.com/html/leakage_current.html
     
  3. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    That human body model should be good for different voltages afaik.

    There are several tests but you'd want to hook ground to one end and then touch the other to the metal. The digital multi-meter gets hooked where shown.

    You're actually reading a voltage on the meter that translates to current. I think it should be a straight reading with no math involved but I can't find the information I used to build one 20 years ago.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Don't miss the improtant aspect of this poor grounding. If the outlets are the grounded type with three holes each, then your landlord is responsible for having the apartment properly wired. Failure to do so sunjects you to shock hazard, and him to liability issues.

    Worry less about accurate measurements of leakage voltage off your computer and more about getting a potentially lethal condition taken care of. If there is a renter's assistance association in your city, contact them for moral and legal support.
     
  5. Codesmith

    Thread Starter New Member

    Jul 19, 2006
    5
    0
    Most of my questions about my wiring situation have been answered elsewhere to my satisfaction. The problems are now more practical than technical.

    The outlets were originally 2 prong as is the case for all the appartments in this complex, and the only reason I have three prong outlets near my PC is that a friend installed them for me. If I complain about them not being grounded they will just switch them back to 2 prong outlets, and I can do that myself.

    The grouding wire is connected properly it just doesn't go anywhere.

    Since I am not willing to take the cover off my breaker box or knock holes in the walls and the owners won't make any changes that are not required by law I am probably going to have to move to get properly grounded outlets for my PC.

    ---

    The main reason I posted here is to aid in my understanding of electronics.

    I enjoy learning things I am curious about. Its my main recreational activity. So I will spend a month studying this, a few weeks studing that.

    At the moment I just finished reading a book on home wiring and that got me feeling a little regret that I went with the philosophy/computer science degrees rather than electrical enginnering.

    So I find myself studying electronics.

    Not all the people arguing against the measurement being valid have make idiotic remarks, some of them sound intelligent enought that its possible that they actually know what there are talking about and I just don't get it.

    B]Basically its important to me to know whether I am right or wrong because if I am wrong there is something important that I think I understand, but don't.[/B]

    So if I am making a mistake and I can't measure the amperage passing through my body or the potential between myself and bare metal on my computer's case in the way describes please let me know.

    You don't have to prove it too me step by step, simply saying it won't work because of X, or you obviously don't know about Y will be greatly appreciated.

    Right now I am discovered I need to know more about AC current and impedence and capacitance, which I don't understand as well as I thought.
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Please refrain from exposing yourself to any more unknown voltages. It is not safe to do such a thing.

    As to the accuracy of your readings...

    The voltmeter measures the difference of potential between the probes. In this case, there was a difference of potential of 60V AC between your hand and the case. Note that this was an open-circuit potential.

    The ammeter measures current through the ammeter. 300 milliamps of current were flowing between your hand and the case. (Note that 600 milliamps of current through the human chest cavity will keep the heart from beating.) Note that when using the ammeter, you placed a small resistance in the circuit in place of the large open-circuit resistance. Think about measuring the voltage across an open switch versus measuring current by shunting across the switch with an ammeter.

    Next time you want to try this, please begin by using the ohmmeter! Measure the resistance between your hand and the carpet. Then use a resistor of similar value to continue your experiments. Nobody cares if a resistor gets fried - lots of people would care if you got fried!
     
  7. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    Something else to think about: The NEC code requires that when a two prong outlet is replaced with a three prong outlet, it MUST be replaced with a GFCI receptacle. You may not attempt to ground the new receptacle, nor run any ground wires from it to other receps in the same circuit. GFCI receps come with two labels in the box. One says something along the lines of "GFCI protected", the other "No service ground". The GFCI will protect you against a grounded appliance in this application, but will not protect against ground fault currents (as in the case of hair dryer falling into the bathroom sink).

    My guess is that your freind probably jumpered the neutral to the ground on the new three prong receps, and this is most likely where your stray voltages are coming from.

    erin
     
  8. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    I forgot to mention that you're supposed to use the second "no service ground" label when changing a 2-prong recep to 3-prong GFCI recep.

    erin
     
  9. rukrazy?

    Member

    Mar 5, 2005
    21
    0
    I may be wrong on this but the National electric Code requires GFIs only in areas that have water nearby such as laundry rooms, Bathrooms, and outside outlets.

    Make sure you are on an insulated floor, if you are being exposed to faulty wiring. There are two prongs on older AC. sockets The smaller is always hot. The larger is always ground (or return)
    The reason is that with the smaller blade of the plug it cannot carry as much current as the larger blade. It would blow out because it cannot carry as much current as ground. But if that happens you would have more problems than you can comprehend. That is why circuit panels and fuses are used. Computers are not designed for two wire systems.
    So if the wire is backward then you would have the case at a lethal exposed voltage.

    You had better tell the landlord about the problem in writing. Then you would have a standing if something goes wrong. You can also call (notify)the power company to have them contact the owner. Then if anything goes wrong (such as an electrical fire) He will be Legally Libel.
    Hopefully He will correct the problem.

    If not tell him your breaking your lease because it is a safety electrical hazard.
    PS. you can get grounding indicators at most hardware stores for around $5 to check for correct wiring. They have indicator lights to show what faults in the connections are, such as wrong polarity or ground connections.
    Well worth the investment.
    Don't play around with your life it takes very little current to kill you. Even at 40 volts. It is possible you could have a faulty computer power supply but I doubt that.
     
  10. Codesmith

    Thread Starter New Member

    Jul 19, 2006
    5
    0
    I hadn't already shocked myself accidentally about a dozen times and it was at the threshold of perception, otherwise I would never have risked grounding it to myself. I have an aluminum case with lots of exposed metal. :(

    It would only happen for a day or two at at time and was driving me nuts, until I realized it was only on extreemly humid days and found my PSU is only rated for 85% RH.

    But if I had no clue how strong the current was I would never have done what I did.
     
  11. Codesmith

    Thread Starter New Member

    Jul 19, 2006
    5
    0
    Is there a requirement for the copper grounding wire used in a 2 prong outlet to actually be properly grounded?

    Because the 3 prong outlets were not installed by the landlord, none of the other appartments have them except in the kitchen. The mantinence staff will fix the grounding problem by reverting to 2 prong outlets.

    Anyway I am curious about the best course of action with the outlets.

    Should I reinstall the two prong and use 2-3 prong adapters even though the center screw isn't grounded?

    Should I purchase GFCI outlets and use those but in an ungrounded fashion?

    Should I just leave the 3 prong outlets in even though the grouding wire isnt' grounded?

    Also is there any advantage to connecting the grounding wire to neutral as someone suggested my friend might have done?

    As far as the PSU shocking me goes, two different PSUs and one of my two CRTs also leaked current to ground when the humidity was in the 90's.

    I think maybe I should buy a dehumidifier and run it on extreemly humid days?

    Also does anyone know if turning on the AC reduces humidity?

    Oh and does air conduct more at lets say 70F 90% RH vs 90F 90% Relative humidity?

    ---
    Oh and can I simply calculate my resistance based on the voltage and amperage readings I got?

    --
    BTW I am on a month to month lease but it won't be moving for 3 to 6 months, so I just want to know the best way to connect my PCs until then given the current state of my wiring.

    I also noticed the incomming cold water piple has a lot of white sealant inside one of the fitting. There is a copper wire from the wall to the cold water pipe. I am wondering if all that goo is completely covering the threads of the pipe? I might be able to clamp a copper wire on either side and regournd my outlets. Worth a try?
     
  12. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    NEC 406.3(D)(3)Non-Grounding-Type Receptacles: Where grounding means does not exist in the receptacle enclosure, the installation shall comply with (D)(3)(a), (D)(3)(b) or (D)(3)(c).

    (a) Non-grounding-type receptacle(s) shall be permitted to be replaced with another non-grounding-type receptacle(s).

    (b) A non-grounding-type receptacle(s) shall be permitted to be replaced with a ground-fault circuit-interrupter-type receptacle(s). These receptacles shall be marked "No Equipment Ground". An equipment grounding conductor shall not be connected from the ground-fault circuit-interrupter-type receptacle to any outlet supplied from the ground-fault circuit-interrupter-type receptacle.

    (c) A non-grounding-type receptacle(s) shall be permitted to be replaced with a grounding type receptacle(s) where supplied through a ground-fault circuit-interrupter. Grounding type receptacle(s) supplied through the ground-fault circuit interrupter shall be marked "GFCI Protected" and "No Equipment Ground". An equipment grounding conductor shall not be connected between the grounding-type receptacle(s)

    NEC 210.8(A) Dwelling Units: All 125-volt, single-phase, 15- and 20-ampere receptacles installed in locations specified in (1) throuhg (8) shall have ground-fault circuit-interrupter protection for personnel.

    (1) Bathrooms
    (2)Garages and also secondary buildings that have a floor located at or below grade,...
    (3)Outdoors
    (4)Craw spaces - at or below grade level
    (5)Unfinished basements,...
    (6)Kitchens,...
    (7)Laundry, utility and wetbar sinks where the receptacles are installed with 6ft of the outside edge of the sink
    (8)Boathouses

    Note that GFCI is not required in a laundry unless there is a sink in the laundry room.

    Hope this clears up the deal with replacing two-prong for three-prong receps.
     
  13. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    See the post above regarding the GFCI stuff. DO NOT connect the neutral to the ground. It will lie to a checker (such as rukrazy suggested) and show that the circuit is grounded when in fact it is not. Up until 25 or so years ago, service grounds were not required in residential buildings. I own two apartment buildings (one was built in '75) that do not have have grounds in the apartments. But, the service neutrals were usually bonded to a ground. Jumpering the neutral to the ground may fake out a cheap test instrument, but it does not provide ground protection. Use the GFCI outlets as the Code indicates.

    Yes, turning on the AC will lower the humidy, but no where near the way a dehumidifier will. You can pick up a humidity indicator / monitor at Wal-Mart for about $7 to see if you actually do have a problem, but an average dehumidifier will run upwards of $100.

    The white sealant that you're seeing is probably just that: pipe sealant, to keep water from leaking out of the threads of the pipe fittings. Where a dwelling is supplied by a metal water line (as most older buildings are) the Code requires that the service panel be grounded not only to a grounding electrode (such as a grounding rod) but also to the incoming water main. (Never mind that back then they didn't have the courtesy to run ground wires from the panel to the outlets!) Newer dwellings are usually supplied with PVC water mains, so it would be silly to "ground" the service panel to them.

    To answer your question directly: No, do not run any kind of a ground wire to your receps. Follow the Code posted above.

    Incidentally, it is my experience that if you're receiving shocks from the outer case of an electrical appliance, the appliance is usually what's at fault, assuming that you haven't got stray votages from cross-wiring a recep as discussed earlier.
     
  14. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    As a tennant, you should do nothing more than contact your landlord. Your landlord or their electrician should replace the outlet at the front of the circuit with a GFCI outlet labled "ungrounded" and protect the rest of the circuits with the "line" output of the GFCI - again with appropriate lables.



    I think the white goo on the pipes is to keep them from leaking. IIRC, water pipes are no longer considered a suitable ground and must be supplimented with one or more grounding rods.
     
  15. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    Back in the day (about 30 years ago) water pipes were probably the most widely used grounding electrode around. And at that time, they were considered THE grounding system to have. Eventually they gave way to better (and easier) grounding methods such as grounding rods, concrete encased grounding electrodes, or any other of about 7 NEC approved methods. (See NEC 250.52) A metal water pipe "...in direct contact with earth for 10ft or more..." is still considered an approved grounding method. In fact, it's the first one listed in the NEC. But, thingmaker3 is absolutely correct: by today's Code it MUST be supplemented by another approved grounding means. (250.53(D)(2))

    If your place was built when water pipes were an approved, solitary grounding means, then you're "grandfathered" into being in compliance. That is, until you start making changes or improvements. If you mess with a grounding method or service, you must upgrade the entire service to current Code. Same goes for the individual branch circuits.
     
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