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Thread: Earthing equipment questions

  1. #21
    Diamond Member Justloadit's Avatar
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    Switch mode power supplies, have a tendency to create an induced voltage in metal surrounding the PSU. The high switching frequencies cause by the capacitive effects in the internal transformers are usually the cause of this. You will note this tingling feeling when you connect a laptop which is being charged to your desktop PC. The reason for this is that the laptop PSU does not have an earth, because it is supplied with a 2 wire chord. The secondary side negative usually becomes the earth for the laptop, and the electronics negative is usually 100odd Volts to earth. This is what you may be experiencing. This was more prevalent in the days that we used RS232 to transfer data between P.C.s, one could see the spark when the connectors were inserted, and the casings touched.

    This is known as "touch current". Do a google search to get some articles on it.

    Theoretically there should be no problem connecting earth to the chassis, however not knowing your machine, I would not say that it will not cause any failures.
    If you are connecting the PC to the unit, then the PC usually creates the earth path via the comms cable. Once the PC is connected check to see if there is still this elevated voltage.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar pumping, Solar Geyser & Solar Security lighting solutions - www.microsolve.co.za

  2. #22
    Diamond Member AndyD's Avatar
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    I thought it might be because they left it optional for a second CPC to be run and connected to an exposed termination point but you say that wasn't the case with this machine so I'm completely clueless why it would have been left floating.

    The ghost voltage thing is a common issue and it confuses experienced sparkies sometimes. If you ever come across an AC voltage that's an arbitrary value just retest using an old analogue tester with a needle if you have one, if not keep look out for one that's a recognized brand at the car boot sales/gumtree, you can pick them up for cheap and they have their uses.

    Now you're on a roll you should check your other machines for safe earthing just for peace of mind. It's the kind of problem you don't notice until something goes wrong one day and you get a proper shock.
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  3. #23
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    Here is a scenario that I had that that I also couldn't resolve. I was working on a model train DCC controller. The controller was connected to the PC via a usb interface. I connected the negative scope probe clip to the one output rail of the USB interface and when I touched the other rail of the interface the contoller went up in smoke. Everything is really good quality and nothing has ever given any problems on their own. It was just this particular combination that fried the controller. The scope is a 60MHz DSO

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    Silver Member bones's Avatar
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    Quote Originally Posted by adrianh View Post
    @bones - Thank you for your truly interesting take on the workings of a modern laser cutter. May I be so bold as to ask you where you obtained your information? I really need to brush up on my poor understanding of such matters.
    got mine second hand that is how
    they explained it to me and what
    i can tell it works so i guess they
    where right

  5. #25
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    Quote Originally Posted by bones View Post
    got mine second hand that is how
    they explained it to me and what
    i can tell it works so i guess they
    where right
    They were wrong

  6. #26
    Diamond Member Justloadit's Avatar
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    Quote Originally Posted by AndyD View Post
    The ghost voltage thing is a common issue and it confuses experienced sparkies sometimes. If you ever come across an AC voltage that's an arbitrary value just retest using an old analogue tester with a needle if you have one, if not keep look out for one that's a recognized brand at the car boot sales/gumtree, you can pick them up for cheap and they have their uses.
    In the case you do not have an analogue meter, use a 220V pilot light connected to your meter probes when you do the measurement. If there is a high voltage, then the light should glow, and your meter will show the voltage
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar pumping, Solar Geyser & Solar Security lighting solutions - www.microsolve.co.za

  7. #27
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    Quote Originally Posted by Justloadit View Post
    In the case you do not have an analogue meter, use a 220V pilot light connected to your meter probes when you do the measurement. If there is a high voltage, then the light should glow, and your meter will show the voltage
    I'll remember this.

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