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Thread: I got zapped

  1. #1
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    Default I got zapped

    I am in the process of converting a 40Watt laser that used a Moshi motherboard to DSP using a Ruida 6442XS.

    I have many switches on the top panel and I was testing the setup yesterday. The first switch is daisy chained to the second and the third and the fourth. these switches control the power, laser, water and air individually. I also have a master power off switch that breaks the input 240 volts to the whole system. The switch is on one input line only as one side of the switch is push to make and the other side is push to break.

    So, there I was looking at the laser's PSU and I turned off all the switches and the master switch, grabbed the connector to pull out the PSU's plug and I got zapped by 240V.

    When I stopped swearing, I thought to myself, "hang on, I shouldn't have got zapped, even if I didn't have everything turned off the house circuit should have kicked in immediately and saved me from any pain".

    I whipped out my trusty ammeter and tested the 240V plug. I got nothing across the live and neutral plug pins, but 240 across the live and earth. I immediately unplugged everything under the house and measured things again with the same results. Later, I measured again and I then got 240 across the live and neutral and 240 across live and earth. I checked many power points throughout the house and got similar readings.

    I phoned an electrician to come and look at it but he at first told me to phone the power company and later said (after I tested the safety switches) that the system seemed to be OK and put it down to 'electrickery'. He is obviously a friend of Rodney's.

    Can anyone explain the problem and what might be the solution?


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    Bob Willson
    The term 'grammar nazi' was invented to make people, who don't know their grammar, feel OK about being uneducated.

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  3. #2
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    Default

    How old is your house?
    I believe there was a period where one or more of the power companies deliberately connected neutral and earth

    how long have you lived there?
    and have you done any wiring work that could have resulted in a live to earth connection. One place to look would be a light fitting.

    do you have solar power?
    could there be a dodgy connection at th meter board?
    regards from Alberta, Canada

    ian

  4. #3
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    Default

    Neutral and earth are, or should be, connected at the switchbox. Lookup MEN - Multiple Earthed Neutral.

  5. #4
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    Hi,
    Sounds like the switch is in the neutral instead of the active leg.
    That is why it pays to pull the plug and not just switch off when working on circuits.
    Regards
    Hugh

    Enough is enough, more than enough is too much.

  6. #5
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    Default

    Also what meter are you using to measure the voltage? A lot of digital meters don't add any load and you can read an induced voltage in those lines.

  7. #6
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    Quote Originally Posted by A Duke View Post
    Hi,
    Sounds like the switch is in the neutral instead of the active leg.
    That is why it pays to pull the plug and not just switch off when working on circuits.
    Regards

    That is what I reckon too.
    Cliff.
    If you find a post of mine that is missing a pic that you'd like to see, let me know & I'll see if I can find a copy.

  8. #7
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    Quote Originally Posted by Bob Willson View Post
    I shouldn't have got zapped, even if I didn't have everything turned off the house circuit should have kicked in immediately and saved me from any pain
    You need to be careful with your terminology, particularly when talking about the lethal voltages you're playing with. The phrase "the house circuit should have kicked in" makes no sense. What you might have meant is that the RCD on the circuit should have tripped, and you would have confirmed that the circuit in question was actually protected by an RCD, and you would have tested the RCD to confirm it was working. If all that was the case, there's a serious problem with your wiring that needs urgent attention.

    I whipped out my trusty ammeter and tested the 240V plug. I got nothing across the live and neutral plug pins, but 240 across the live and earth.
    Again, you need to be careful with your words. You put an ammeter across a mains connection and an explosion will ensue. Secondly, was it the 240V plug you tested, which is typically a male connector with male pins? There should be no voltage anywhere on this as it's the load side. What you might have meant is the mains socket (or power point), unless there's a proprietary connector in the machine.

    Again, you are messing with lethal voltages and have been zapped once so far, so being careful and precise in describing the fault is extremely important.

    In answer to your question, as others have said, neutral and earth should be at the same potential as they are bonded at the switchboard (and at other points upstream) so seeing 240V between active and earth is normal. Again, as others have said, the most likely cause of getting zapped is incorrect polarity - the neutral was being switched, not the active. However, again, if the circuit does indeed have a safety switch (RCD) on it, and it didn't trip, you have a problem.

    One tool that you might want to try, and is much safer than a meter, is a socket tester from Jaycar. It'll confirm the polarity of a power point is correct, and you can use it to test the trip threshold of the RCD on the circuit.

  9. #8
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    Quote Originally Posted by ian View Post
    How old is your house?
    I believe there was a period where one or more of the power companies deliberately connected neutral and earth

    how long have you lived there?
    and have you done any wiring work that could have resulted in a live to earth connection. One place to look would be a light fitting.

    do you have solar power?
    could there be a dodgy connection at th meter board?

    Hello Ian
    The house is about 45 - 50 years old now.
    The whole house was rewired after the floods and it is all in pretty good condition.
    I do have solar power as well, but I see no reason why that should suddenly start acting up; but I will check it out.
    Thanks
    Bob Willson
    The term 'grammar nazi' was invented to make people, who don't know their grammar, feel OK about being uneducated.

  10. #9
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    Hi Geoff
    I looked at the MEN system and it shows what others have stated in their replies. That the earth should indeed be connected to the neutral wire at all times, but not to the active wire.
    It appears that at some point, the active and neutral wires have been switched and that a careful point by point inspection needs to be made to determine where the problem lies.
    The tester mentioned by RustyArc will help me to find this very quickly.
    Bob Willson
    The term 'grammar nazi' was invented to make people, who don't know their grammar, feel OK about being uneducated.

  11. #10
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    Thanks Hugh
    I am certain that this is a major part of the problem.
    I would also have preferred a switch that cut both wires.
    Thanks
    Bob Willson
    The term 'grammar nazi' was invented to make people, who don't know their grammar, feel OK about being uneducated.

  12. #11
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    Quote Originally Posted by benhoskin View Post
    Also what meter are you using to measure the voltage? A lot of digital meters don't add any load and you can read an induced voltage in those lines.

    Hi Ben
    I am using an APPA 93T meter. Not the best available, but price constrains most things in our lives.
    Bob Willson
    The term 'grammar nazi' was invented to make people, who don't know their grammar, feel OK about being uneducated.

  13. #12
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    Quote Originally Posted by Cliff Rogers View Post
    That is what I reckon too.

    It certainly looks like it Cliff.
    Thanks
    Bob Willson
    The term 'grammar nazi' was invented to make people, who don't know their grammar, feel OK about being uneducated.

  14. #13
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    Quote Originally Posted by RustyArc View Post
    What you might have meant is that the RCD on the circuit should have tripped, and you would have confirmed that the circuit in question was actually protected by an RCD, and you would have tested the RCD to confirm it was working. If all that was the case, there's a serious problem with your wiring that needs urgent attention.
    I couldn't think of the correct words at the time Rusty. Age often prevents me being as mentally agile as I once was. RCD it is


    Quote Originally Posted by RustyArc View Post
    Again, you need to be careful with your words. You put an ammeter across a mains connection and an explosion will ensue. Secondly, was it the 240V plug you tested, which is typically a male connector with male pins? There should be no voltage anywhere on this as it's the load side. What you might have meant is the mains socket (or power point), unless there's a proprietary connector in the machine.
    I t was the female end of the extension lead that I was testing. This was after I had pulled it from the machine but while it was still connected to the house wiring[/QUOTE]

    Quote Originally Posted by RustyArc View Post
    In answer to your question, as others have said, neutral and earth should be at the same potential as they are bonded at the switchboard (and at other points upstream) so seeing 240V between active and earth is normal. Again, as others have said, the most likely cause of getting zapped is incorrect polarity - the neutral was being switched, not the active. However, again, if the circuit does indeed have a safety switch (RCD) on it, and it didn't trip, you have a problem.

    One tool that you might want to try, and is much safer than a meter, is a socket tester from Jaycar. It'll confirm the polarity of a power point is correct, and you can use it to test the trip threshold of the RCD on the circuit.
    Thanks for that Rusty. I will get one ASAP
    Bob Willson
    The term 'grammar nazi' was invented to make people, who don't know their grammar, feel OK about being uneducated.

  15. #14
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    I will also try to find a big red switch that cuts both legs of the input power.
    I would have thought that this was the standard way to do it; not have one cut and one not cut.
    Meantime, I will switch the wires that I am cutting. If I had used my voltage detector, this would not have happened.
    Bob Willson
    The term 'grammar nazi' was invented to make people, who don't know their grammar, feel OK about being uneducated.

  16. #15
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    Quote Originally Posted by Bob Willson View Post
    I will also try to find a big red switch that cuts both legs of the input power.
    I would have thought that this was the standard way to do it; not have one cut and one not cut.
    Meantime, I will switch the wires that I am cutting. If I had used my voltage detector, this would not have happened.
    Bob, by switching the wires that are "switched" you are fixing the wrong problem. There should be a consistent connection method throughout your house and shed. Do not change the switching, get the inconsistency fixed.
    Re you getting zapped, the issue there is that you did not unplug the appliance/machine that you were working on, working on a machine that is still plugged in is asking for trouble.

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