Just a quick safety question after creating a small spark

Discussion in 'General Electronics Chat' started by BrainFog, Feb 22, 2011.

  1. BrainFog

    Thread Starter Member

    Jan 24, 2011
    This may be an obvious or daft but it is about safety so I will consider it important enough to ask.

    I am very new to electronics and am still working on my first project and getting through some electronics books. Lots of the parts that I ordered arrived today including a couple of transformers. As part of a quick experiment I decided to make a simple unregulated 12volt power supply; but, and here is where the stupidity comes in, I though it may have been a good idea to test the mains voltage as so many transformers have set specified input voltage rather than a ratio of the primary and secondary windings. Expecting to get a reading of 220-240v I decided to test the mains voltage, all I got was a nice spark. No harm was done, just lost a 10A multimeter fuse and a 3A plug fuse. Can I assume that it is important to never connect a multimeter to the mains? Does a transformer prevent this from happening not only because of low voltage but because of isolation? So it is 99% safe. What is the best way to prevent this in the future?

    A thought occures, was my mistake because I slowly moved the multimeter to the connection from the plug rather than having it connected from the moment I flicked the power switch and by doing this I caused capacitance to build up between the connectors causing the spark before they met?

    I do not want to risk myself or anything I intend to build. Thank you
  2. SgtWookie


    Jul 17, 2007
    It is more likely that you attempted to change ranges on the meter while the probes were connected, or you had the meter set to measure current or Ohms instead of the highest AC voltage range.

    Don't attempt to measure mains current with a multimeter; you will blow the fuse nearly instantly - right after the probe tips get burned.

    Always check for the presence of voltage prior to measuring resistance in Ohms. If there is any voltage present, not only will your readings in Ohms be inaccurate, but you risk blowing the meters' fuse.

    It is safer to use a precision 1 Ohm resistor in series with a current source and measure the voltage drop across the resistor, than to try to measure the current directly. I=E/R, so 1V across 1 Ohm = 1 Ampere. I have a 50 Watt 1 Ohm resistor that I use for such purposes.

    Never change ranges or functions on the meter while the probes are connected. If you inadvertently pass through Ohms or current measurement settings when you meant to measure voltage, you will blow the fuse.

    Always replace the meters' fuse(s) with the exact same type and voltage rating of the original.
  3. BrainFog

    Thread Starter Member

    Jan 24, 2011
    No I did not change the setting at all while connected to anything. I set it to 500v alternating, the one with the wave next to it, and double checked before doing anything.

    By the way when I said power switch I was referring to the one by the plug socket in the wall. Thinking about it I have never seen a switch by the wall socket when they are shown on American TV. Hmmmm
  4. GetDeviceInfo

    AAC Fanatic!

    Jun 7, 2009
    check where your probes are connected on the meter. ampere measurements typically have a seperate terminal. Read up on your meter so that you completely understand which terminal is which.
  5. Adjuster

    Late Member

    Dec 26, 2010
    I think that it is very likely that you have made a mistake of some kind, unless the meter is faulty. Is there a separate socket for the current ranges, or perhaps for the 10A range only? Often a probe connected to such a socket is permanently connected to a shunt, irrespective of the range setting. If you have the probe connected to such an input there will always be a low resistance to common.

    Clearly, you should never measure hazardous voltages unless absolutely necessary. On the other hand, a meter with a 500V AC range should be able to measure a 230V AC line without blowing any fuses.

    Before using the meter again to measure ANY voltages, try to check out its input resistance by using another meter. For a digital meter, the resistance should be in the megohms on any voltage range. Do not use the meter again if the resistance is low, or more damage may be done.
  6. BrainFog

    Thread Starter Member

    Jan 24, 2011
    I did check it and all are connected correctly, For the 500v test I connected it to the 10A socket rather than the 500ma one as instructed by the manual.

    I am more interested in the nature of what causes sparks to occur and their relation to voltage and current and a few other things. That is more on the physics side of things.
  7. debjit625

    Well-Known Member

    Apr 17, 2010
    No,we check the main's voltage and current connected with any load with multimeter,their is no problem but by only using proper range and proper connection.

    As you said you blown the 10A fuse of your meter it seems that your connection was wrong,many meters have a separate plug for measuring higher range of current above mA,I think you was using the 10A plug for measuring main's voltage which you should never do.For measuring voltage use the V/Ω and Common connection plug.

    You see the 10ADC plug down ,I think you used it for measuring main's voltage.


    Good Luck
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  8. BrainFog

    Thread Starter Member

    Jan 24, 2011
    Sorry ignore what I said. I realised it was connected incorrectly now. I arrogantly thought that the colour used to indicate 10A fuse for another setting also meant that I should use it to test 500volts as it was marked in the exact same red. I am feeling a bit embarrassed but at least I have learned from it. :)
  9. BrainFog

    Thread Starter Member

    Jan 24, 2011
    I hit reply and now I see you got it utterly correct debjit625.

    Thank you for your help everyone.