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Thread: DC Current Safety Threshold

  1. #11
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    Quote Originally Posted by K8MHZ View Post
    Those patients have afib and the DC is in measured pulses of around 5ms. That is totally different from the effects of steady current DC on a healthy person.
    The DC supplied by a defibrillator to those patients having a fib to revive them have several thousands volts applied across the heart compared with steady state DC. In spite of that, it can save lives, while the latter can kill.

  2. #12
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    Quote Originally Posted by K8MHZ View Post
    Actually, it's the opposite. DC through the heart is more dangerous than AC.
    Actually, it depends on the frequency. 50-60 Hz is the most dangerous, requiring about half as much current as DC to inflict the same harm. 400 Hz is about equally dangerous as DC. and increasing frequencies are increasingly less dangerous.

    Quote Originally Posted by K8MHZ View Post
    Can you show me a single incident where a person was electrocuted by a 12 volt car battery?
    No, but there's a possibility that it has happened with 24 volts. Unfortunately, overlapping jurisdictions and limited resources prevented it from being investigated.

    It went down like this:
    - Late at night, a woman went to the garage to coax a guy to quit tinkering with his beloved extreme car stereos and come to bed. She found him wedged in the trunk and experiencing tetanic muscle spasms, and called for help.
    - Paramedics arrived and declined to begin treatment due to the clear & present electrical danger. They called for additional help.
    - Firefighters arrived and cut cables until the tetany ceased.
    - Paramedics examined the guy and deemed him "not a viable candidate for resuscitation".
    - The coroner ruled it "accidental". There being no sign of foul play, nobody investigated anything, despite the possibility that it might have been a precedent-setting electrical-trauma incident. 24 volts isn't usually fatal, but being wedged into a confined space might have resulted in electrical contact for an extended period of time.

    Or there could have been a 120-volt drop light in the trunk with him. Like I said, nobody investigated anything.

  3. #13
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    Quote Originally Posted by drcampbell View Post
    Actually, it depends on the frequency. 50-60 Hz is the most dangerous, requiring about half as much current as DC to inflict the same harm. 400 Hz is about equally dangerous as DC. and increasing frequencies are increasingly less dangerous.
    May be high frequency current flows along skin due to 'skin effect' not passing through internal organs, making it less dangerous?

  4. #14
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    Wrong test equipment?

    Most meters have a fuse for 10 amps on the current range, I have never seen one that will carry 40 amps. The fuse on the meter should of blown before the lead heated up. Is some part of the story is missing?
    A cowboy may get thrown , but they always get up and walk forward.

  5. #15
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    Quote Originally Posted by just the cowboy View Post
    Most meters have a fuse for 10 amps on the current range, I have never seen one that will carry 40 amps. The fuse on the meter should of blown before the lead heated up. Is some part of the story is missing?
    I wondered about that too...
    We wouldn't normally measure direct current directly with a meter in series. In the old days we'd have a shunt. Now it is mostly Hall effect transducers.
    Si hoc legere scis nimium eruditionis habes.

  6. #16
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    Edison invented the electric chair in order to prove to the public how much more dangerous AC was (in his mind) than DC. But the thing is, Edison's demonstrations, first on an elephant, then on condemned prisoners, never included a side by side comparison to a DC powered electric chair. The effects might have been different, the results likely the same. But that wasn't his point...

    The only electrocution I have ever witnessed was on a DC bus of a large drive at a steel mill. I've witnessed, and experienced, several AC shocks, resulting in no deaths or even serious injuries. I'm not saying AC is safer, nor DC. It's all about the extenuating circumstances, which are so varied and complex that nobody is qualified to pass judgement in advance as to what is or is not 100% safe. So no matter what, both should be taken seriously.
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  7. #17
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    Quote Originally Posted by Jraef View Post
    I'm not saying AC is safer, nor DC. It's all about the extenuating circumstances, which are so varied and complex that nobody is qualified to pass judgement in advance as to what is or is not 100% safe. So no matter what, both should be taken seriously.


    Dead is dead.
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    Derek

  8. #18
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    Quote Originally Posted by Jraef View Post
    The only electrocution I have ever witnessed was on a DC bus of a large drive at a steel mill. I've witnessed, and experienced, several AC shocks, resulting in no deaths or even serious injuries. I'm not saying AC is safer, nor DC. It's all about the extenuating circumstances, which are so varied and complex that nobody is qualified to pass judgement in advance as to what is or is not 100% safe. So no matter what, both should be taken seriously.
    I've had to sit through various electrical safety seminars (including those from Eaton and Cooper Controls). The lowest voltage that might cause a fatality was said to be 70V dc or ac. However, it is as you say, dependent on actual circumstances.

    Somewhat related to your steel mill example............
    We did an upgrade of a rolling mill that previously had a Ward Leonard system with a floating DC bus. This was replaced with SCR dc drives. The existing dc motors were retained.

    A technician used to regularly inspect the brushgear and check spring tension - by hand while the motors were running!
    With the old floating system, the risk was minimal. He tried it with the new SCR system - just the once..........
    Si hoc legere scis nimium eruditionis habes.

  9. #19
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    Quote Originally Posted by Besoeker View Post
    I've had to sit through various electrical safety seminars (including those from Eaton and Cooper Controls). The lowest voltage that might cause a fatality was said to be 70V dc or ac. However, it is as you say, dependent on actual circumstances.

    Somewhat related to your steel mill example............
    We did an upgrade of a rolling mill that previously had a Ward Leonard system with a floating DC bus. This was replaced with SCR dc drives. The existing dc motors were retained.

    A technician used to regularly inspect the brushgear and check spring tension - by hand while the motors were running!
    With the old floating system, the risk was minimal. He tried it with the new SCR system - just the once..........
    Ouch.

    My guy was doing the commissioning of some big ABB DC rolling mill drives and was taking measurements with the doors open. He was squatting on his haunches and for some reason lost his balance, leaned in and put his forearm across one DC bus. Then what we THINK he did (it was all so fast) was a reflex to push off with his other hand, but made contact with the other bus. So he locked on and we had to push him off with a board, but by then it was too late. He was still alive for a few minutes waiting for the ambulance, but didn't make it to the hospital.
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  10. #20
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    Quote Originally Posted by Besoeker View Post
    I wondered about that too...
    We wouldn't normally measure direct current directly with a meter in series. In the old days we'd have a shunt. Now it is mostly Hall effect transducers.
    The researcher was not measuring current, he was using the test leads as jumpers between the power supply and the test probes. There was no fuse installed, only exposed terminals.

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