Getting shocked from my workbench equipment!

Discussion in 'General Electronics Chat' started by BlackCow, Aug 1, 2011.

  1. BlackCow

    Thread Starter Active Member

    May 11, 2009
    65
    1
    I noticed I got a zap from my Oscilloscope and from my serial terminal whenever I touched bare metal on them. Both of these pieces of equipment are quite old.

    Here is where I feel very stupid... I used a cheater two prong to three prong converter (I live in the USA) that was not polarised. I plugged my bench power strip into this. It very well could have been upside down :(

    I threw it out and replaced the two pronged outlet with a three pronged outlet, however because this house was wired in the 1950s there is no ground wire for the third prong, but at least I know hot is hot and neutral is neutral.

    After doing this I took my multimeter and stuck one probe to the neutral side of the outlet and the other probe to the metal case of the oscope and still I measured around 55 to 60 volts!!! I also measured the same amount of voltage on my serial terminal as well!

    So then I took the oscope to a three pronged outlet that I know for sure was grounded properly (my father wired it himself when he installed the wall) and I did not read any voltage from the case.

    What is weird is that I never used to get a shock from anything until recently, perhaps before I wasn't grounded enough to get one?

    I guess the proper thing to do is to give my outlet a proper earth ground, but even without one I don't think I should be getting shocks from my equipment if it is plugged in the right way! (Hot to Hot and Neutral to Neutral).

    Do you think when both pieces of equipment were plugged in upside down that may have broke something which would result in shocks :confused:

    I really don't want to have to throw out my oscilloscope or my terminal!
     
  2. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
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    There can be a number of causes of this issue. Based on what you described, my first suspicion is that both the scope and the terminal may have relatively large leakage currents. This is not unknown for older equipment, as insulation degradation, poor/bad design, or manufacturing defects can lead to or exacerbate leakage. You probably don't have the equipment and knowledge to safely test the leakage currents, so I'd recommend you take it somewhere and have it done. A good appliance repair or electronics repair shop should be able to test the devices for you for a nominal charge. Finding and fixing the defect, if any, will be more expensive.

    One thing you can do is install a GFI outlet. It will work fine without being grounded and would help protect you from such shocks. You might want to consult with an electrician, as there can be lots of regulations for such things. However, a simple $20 GFI adapter (like the ShockBuster) could certainly be used. Note: this is a band-aid and does NOT fix the fundamental problem (which e.g. may be a leakage current problem), so finding and fixing the root cause is desired.

    One of the problems with measuring like you did with your multimeter (I assume it was the typical DMM) is that the high impedance input can be "fooled" by "phantom" voltages that are caused by capacitive coupling. That's another possible explanation for what you observed.

    Because you may not have the knowledge and equipment to figure out the root cause, the best approach may be to hire an electrician. Make sure you get one that knows what leakage currents are and how to troubleshoot them.
     
  3. PatM

    Active Member

    Dec 31, 2010
    81
    72
    If the house was wired in the 50's it most likely would have used Bx cable.
    http://homerenovations.about.com/b/2007/05/09/what-is-bx-or-type-ac-wiring.htm
    While probably not up to code by modern standards, the outer sheath and the bare wire inside (if properly wired) would provide the ground to the metal box and thus to the 3rd prong of the outlet when the outlet is screwed down to the box.
    Get a cheap tester like this to find out if the outlet is wired correctly.
    http://www.google.com/products/cata...48544694&ei=1kE3Tt_PGovaNrHo7K0C&ved=0CBAQ8wI
     
    Last edited: Aug 1, 2011
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,766
    928
    If the copper grounding rod has been in the dirt for 60 something years, it is possible that the poster simply needs to have a new grounding rod installed??
     
  5. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    Some machines have intentional leaks to case. The early Fender guitar amplifiers had a "grounding" switch that connected the case to the power line with a capacitor in one of the switch positions. My opinion of a stupid move, but Fender wasn't the only one.

    My Phillps PM3226 scope has a .1uf cap from both sides of the power line to the case. This capacitive voltage divider puts the case at 60 V RMS whenever the bond wire is not connected to earth.

    Point is, the leakage might be intentional.
     
    Last edited: Aug 1, 2011
  6. BlackCow

    Thread Starter Active Member

    May 11, 2009
    65
    1
    Thanks for all the help! I'm just wondering, is it possible that plugging both devices in the wrong way broke them? It just seems weird that I never had this problem before and now both of them zapped me!

    I can at least wire ground to a cold water pipe, I did find out that a proper ground fixed it.
     
  7. PeeSeeBee

    Member

    Jun 17, 2011
    43
    7
    As #12 pointed out, most mains equipment has a filter between the mains input & PSU. Part of that filter consists of a capacitor across the Live to chassis, & Neutral to chassis. The capacitors form a potential divider across the mains, putting 60V on the chassis.

    If the chassis is not earthed but you are, then you will feel it every time you touch it. Perhaps in the past you may have had something else connected to the scope to ground it, such as a coax lead on one of the inputs connected to something that was earthed.
     
  8. gwalker

    New Member

    Aug 2, 2011
    2
    0
    grounding (actually bonding ) your equipment to a cold water pipe is very dodgy , if any of the piping system has been replaced with a section of plastic pipe , or there are corroded (thus a high resistance path )sections you may electrocute yourself or someone else in the shower or bathtub !I agree with the poster that suggested checking the integrity of your main ground rod and conductor and the section of wiring to that particular outlet .It sounds like the neutral has come adrift from the ground somewhere possibly in the panel .An electrician should check this out .A GFCI outlet will be safe but will of course trip on that kind of leakage .
     
  9. BlackCow

    Thread Starter Active Member

    May 11, 2009
    65
    1
    I got a socket tester. When plugged into the power strip it shows that hot and neutral are reversed!!!

    When plugged into the wall that the power strip is plugged into it just shows the open ground (which is expected)...

    I have no idea why its doing that but it looks like I need to stay the hell away from that power strip!
     
    Last edited: Aug 2, 2011
  10. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Or just open it up and fix it if you can (some power strips are made such that this is possible).
     
  11. BlackCow

    Thread Starter Active Member

    May 11, 2009
    65
    1
    Yeah it is open-able... I just don't understand how it got like that in the first place! :eek:
     
  12. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    One of the most important things in repairing machines is that you have to look at them to figure out "how it got like that".

    You are about to meet step one: Open it up and look at it.
     
  13. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Precisely. Yesterday my wife wanted a hose bib I installed about 20 years ago moved over a couple of meters (it is connected to a bib on our house and runs under our deck via a chunk of rubber hose). "Oh, this is a real simple task!" she said, coming up with ideas of just running a new chunk of pipe. But I wanted to reroute the hose to avoid kinking the hose, which meant drilling a new 1-1/8" hole at the other end of a redwood board. So it all had to come apart. I took up 4 or 5 of the long deck boards and we found that many of the supports under the deck have rotted over the last couple of decades. So what turned into a "simple" task has now become lots of work, simply by opening it up and looking at it. :eek:
     
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