Getting zapped by DC Voltage

Discussion in 'Electrical Forum discussion & Blog' started by speedbump, Dec 19, 2007.

  1. speedbump

    speedbump Previous member

    Messages:
    4,540
    Location:
    Riverview, Fl.
    From another Thread, Jimbo said the above. I know DC Voltage can come get you, but I'm wondering about something.

    I recently got shocked unhooking Coax from a Receiver at my Repeater site for Ham Radio. The Receiver has a small pig tail that the Coax plugs into. I had one piece in each hand and as soon as the two came part, I got nailed. It felt like 115 AC. Another time, I got nailed from our Pump hoist which is a Diesel with two 12 volt batteries, I was hooking up jumper cables to try out a new Electro Magnet we had built to check casing depth in a well. Same thing, I got nailed from a 12 volt battery. I have never before gotten nailed by 12 Volts DC and some say I'm crazy, but trust me, I felt the shock.

    Has anyone got any experience with this sort of thing or can you share your experiences with weird occurrences like these?

    bob...
  2. Ian Gills

    Ian Gills Senior Robin Hood Guy

    Messages:
    2,777
    Location:
    USA
    Not DC, but I got zapped disconnecting a ceiling fan. The power was off, and all the wires were disconnected, but when I had the fan on the floor a capacitor must have discharged and it hit me.

    Ouch, and very weird.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,013
    Location:
    New England
    An antenna could have a huge static charge on it...just like touching the doorknob after shuffling across a carpet. Also, if the transmitter was on, there could be numerous watts of RF energy that likes to jump a gap. Your muscles are more inclined to notice a/c type signals rather than dc.

    I know I've been zapped by high voltage a couple of times...one when I was testing a HV regulator assembly for a radar...I never did find that test lead after it got thrown! I've also seen a guy get zapped by a 40Kv supply...it burned a channel from his elbow to his wrist (inside) and arced out his boots through the nails in the heel. Somebody else got to the emergency shut off before I did on that one, but that guy was never the same...didn't really want to work on radars any more!

    The resistance in your body depends a lot on how sweaty you are...the salty sweat reduces the resistance and you become less of an insulator and more of a conductor.

    Bottom line, you can arc weld with dc, so it has significant potential as does a/c. A/c messes up the bodies ability to exchange messages in the nerves, while dc could burn you. Both are dangerous under adverse conditions.

    Given enough current, you certainly don't want to be the fusible link.
  4. kd

    kd New Member

    Messages:
    207
    My friend unplugged a fluorescent fixture from a ceiling plug. The plug swung down and touched his lip...and shocked him. All wire is like a battery or capacitor-it holds an electric charge which eventually dissipates down to zero. But you can accidentally use your body to draw off the current.
  5. alternety

    alternety Like an engineer

    Messages:
    651
    Location:
    Washington
    Speedbump - the antenna could have been static. If the transmitter was not turned off a bit of RF could do it; it likes to crawl around. The batteries were probably because of what you were testing. Unlike plain old wire, capacitors and inductors do store "electricity". You built a big inductor (electromagnet) and then charged and probably discharged the stored energy with sparking as the cable brushed the contact. Being a Ham you have probably seen the results of the voltage spikes inductors can cause when the field collapses. That is why you have to put those protective diodes on electronic switches used on inductive loads. Taping a heavy duty coil (probably had an iron core) with the power source can cause ringing which can induce a voltage much higher than the initial source.

    For people fooling around inside a TV using a CRT - an example of capacitance storage can be experienced by taking a set that has been off for some time and sticking you finger in the hole (DO NOT DO THIS) on the side of the picture tube with the red wire going to it. The picture is a very high quality capacitor and you can get a spark of 5 or 10,000 volts after many days (months) powered off.

    My high voltage story. Building about 30' wide and 75' long. Contains two 1.5 megawatt transmitters I was maintaining. Antenna at one end, power entrance diagonally across the shack. 3 miles of dipole antenna a minimum of 150' above terrain, on top of a moderate mountain.

    Thunder storm heard in the distance. SOP - disconnect and ground antenna feed with big knife switch. A few minutes later a lightening bolt entered the shack from the grounded antenna feed and left via the service entrance. New addition to SOP - and then leave. There was no lightening outside - just a static build up.

    Except for a few brown marks and some trouble hearing for a while; no damage. But it was impressive.
  6. frenchie

    frenchie Jack of all trades

    This thread is starting to remind me of this video Phil posted at JLC last year...

    http://www.glumbert.com/media/highpower


    Personally, I think you're all nuts. Even 115 scares the crap outta me.
  7. Furd

    Furd Engineer

    Messages:
    446
    Location:
    Wet side of Washington State
    Direct current will pull a much longer arc than a similar AC voltage.

    A few years ago I was attempting to "polarize" an exciter generator in the historical museum I sometimes volunteer at. I was using #4 jumper cables to connect 120 volts DC (single phase full wave rectifier) to the field winding and when I disconnected I pulled an arc that was close to a foot long. I was surprised that I didn't blow the rectifier or the fuse supplying it.

    And yeah, the generator now works!
  8. portiz

    portiz New Member

    Messages:
    5
    AC's actual energy is about 70% of that same DC voltage.
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