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Thread: When does DC voltage become dangerous?

  1. #91
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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by BobBarker View Post
    Also, just for general information... If you attach a full-wave bridge rectifier to your ac line (or transformer)... you WILL NOT get 120v RMS... You WILL get a rectified 169v (you multiply the input by 1.41 to get your output)
    That is not correct, you will still have the exactly same RMS voltage minus the drop in the rectifier.
    But add a capacitor and you get the a DC voltage with a RMS value close to the peak voltage of the original AC voltage (i.e. 1.41 times more voltage).
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  2. #92

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    I've definitely gotten a charge out of this thread, which has sparked several observations.

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  3. #93
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    Default Re: When does DC voltage become dangerous?

    Just wondering, if I'd touch + and - of, let's say, 1000V DC with two of fingers on same hand, would the damage be limited to hand, or current will in some way go through whole body?
    Sorry if this has been asked here, just too much posts to check all of them.

  4. #94
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    Default Re: When does DC voltage become dangerous?

    99% of the time, you will only lose those fingers and the meat in between... but that 1% is still there looking to kill you. so basically, don't go around touching 1000v DC or AC systems unless you really don't value your life.
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  5. #95
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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by flashflood View Post
    I've definitely gotten a charge out of this thread, which has sparked several observations.

    Resistance is futile.
    Protect the family Joules.
    Stay current to maximize your potential.
    Watts your problem? More power to you!

    Oy -- Thank God it's Faraday.
    (I know, I know -- revolting!)
    I appoint you to be in charge of the daily humor.
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  6. #96
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    Default Re: When does DC voltage become dangerous?

    The most damage would be done where the current leaves your body. You need to isolate yourself from any ground. Use isolated tools. People in my line of work die from -48V unfused power. One guy was sweaty and bald bent down to get his wrench was touching a bus and his head hit the frame(which is bonded to ground). That room still smells funky and I get the chills when I work in there.

    Amperage is what does the damage.

    Voltage is just the pressure behind the current. Voltage needs to be high enough and your body's resistance needs to be low enough for the current to flow through you. Without any amperage though it is just like a static shock from the carpet. High voltage but harmless. Just fun to tease the cat.
    Last edited by ToyTank; 04-20-2012 at 11:02 PM.

  7. #97
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    Default Re: When does DC voltage become dangerous?

    To add a theoretical example to Mr Happy's post. Lets say you had an old 350 Cubic Inch V-8 Chevy engine. A few decades ago, the starter motors for these engines pulled about 220 Amps at 12 Volts - OR 2,640 Watts of power were used to accomplish the work of turning over the engine if the starter was in good shape. Fairly thick cables are needed to move 220 Amps of current. IIRC something like #1's or aut or 2/0. In theory, (lots of other complications would occur) if you could find a 24 volt starter for that same engine, the starter would pull only 110 Amps at 24 Volts but you still are using the same 2,640 Watts of power (24 x's 110 = 2,640 Watts) to accomplish the same work. So higher voltage allows you to move less current while maintaining the same power level. Saves a lot of copper!

    I wouldn't say that "Amperage is what does the damage". How many times have we touched a leaking 40KV spark plug wire only to quickly pull your arm away and have your elbow strike something and hurt like heck, if not draw blood.
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  8. #98
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    Default Re: When does DC voltage become dangerous?

    Back at school they taught me that from 40V, DC voltage can become deadly.
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  9. #99

    Default Re: When does DC voltage become dangerous?

    http://www.darwinawards.com/darwin/darwin1999-50.html

    anything 9v and over is potentially lethal in the right circumstances

  10. #100

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    Quote Originally Posted by InHisName View Post
    I appoint you to be in charge of the daily humor.
    Balancing your d(humor)/dt will be a daily delicate balancing act.
    I know you can do it, as long as you don't get overcharged.
    I am honored by this induction.

  11. #101
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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by BVH View Post
    I wouldn't say that "Amperage is what does the damage". How many times have we touched a leaking 40KV spark plug wire only to quickly pull your arm away and have your elbow strike something and hurt like heck, if not draw blood.
    The current itself did no damage, it was your nerves reacting causing the reflex. If that had been a 200 amp you'd lost your arm.

    I stand by my statement that amperage is what does the damage, but yes you need enough voltage to overcome your resistance and complete the circuit.

    Current = Voltage / resistance. I can't find what the average resistance of a human is though. I'm sure there would be variation among people and individuals sweaty and not etc. But if your resistance is higher than the voltage than current= Zero and there is no circuit. Wikipedia... electric shock

    A person can feel at least 1 mA (rms) of AC at 60 Hz, while at least 5 mA for DC. The current may, if it is high enough, cause tissue damage or fibrillation which leads to cardiac arrest. 60 mA of AC (rms, 60 Hz) or 300–500 mA of DC can cause fibrillation.[2][3]

    Body resistance


    The voltage necessary for electrocution depends on the current through the body and the duration of the current. Ohm's law states that the current drawn depends on the resistance of the body. The resistance of human skin varies from person to person and fluctuates between different times of day. The NIOSH states "Under dry conditions, the resistance offered by the human body may be as high as 100,000 Ohms. Wet or broken skin may drop the body's resistance to 1,000 Ohms," adding that "high-voltage electrical energy quickly breaks down human skin, reducing the human body's resistance to 500 Ohms."[9]
    The International Electrotechnical Commission gives the following values for the total body impedance of a hand to hand circuit for dry skin, large contact areas, 50 Hz AC currents (the columns contain the distribution of the impedance in the population percentile; for example at 100 V 50% of the population had an impedance of 1875Ω or less):[1
    0]


    End Quote

    conductors/fuses are rated for amperage not by voltage or wattage.

    Voltage is rated by the insulating properties of the sheath.

    I always like the water analogy. For lower resistance and more current you need bigger ID pipes or wires. If you want to push the water/electricity further you need higher PSI/voltage and thicker walls on your pipe/wire. Over simplified but I still like it.
    You need enough voltage to get the amperage to pass through your body for the amps to be dangerous
    Last edited by ToyTank; 04-21-2012 at 01:23 PM.

  12. #102
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    Thumbs up Re: When does DC voltage become dangerous?

    Quote Originally Posted by Apollo Cree View Post

    Common US household 120VAC wiring.

    You have a "neutral" wire connected to earth ground somewhere. It is always 0V.

    There is a "hot" wire.

    The voltage on the hot wire goes from

    0V
    +170V
    0V
    -170V

    Over a time period of 1/60 second.

    If you run it through a full-wave rectifier, you will get a peak voltage of 170V. If you put a capacitor on it to get DC, you'll get 170V.

    It's called "120V" because that's the effective power in terms of a resistive load like an incandescent light bulb. The 120V refers to RMS (Root Mean Square) voltage. A 120V RMS AC voltage will provide the same amount of power to an incandescent bulb as a 120 V DC supply.

    The maximum voltage present on a US household circuit is 170V. Even though the voltage goes from +170 to -170, there's no way to connect yourself between the + and - parts of the waveform because they occur at different times.

    The common US household 240 VAC circuit has one neutral wire and two hot wires. The hot wires are 180 degrees out of phase. When hot wire A is at +170, hot wire B is at -170 V and vice versa. A 240V load will be wired between the two hot wires and will see + and - 340V peak voltage for an effective 240 VAC RMS voltage.

    If you touch one conductor of a standard US household 240 VAC circuit, you only see 120 VAC. You have to touch both hot wires to see 240 VAC.
    I had to create an account on CPForums just so I could post this comment: THANK YOU Apollo Cree for your fantastic explanation of household AC in the US. I am very interested in electrical / electronics and I've done a lot of reading on the subject over the past few years. Your comment is one of the clearest descriptions of that specific topic I have ever seen. I don't know if you are an educator by profession, but you certainly have a gift for clear communication. Cheers!

  13. #103

    Default Re: When does DC voltage become dangerous?

    Yeah, it's not the voltage, but the current across your heart that will kill you. Think about scuffing your feet on the carpet and touching a door knob .... that's 20,000 volts or so, but barely any current. I've touched a car 12V battery and can feel the tingle from 12VDC ... several amps roaming around inside. At school some kid had a electrical plug in the 115AC socket and two bare wires. A bunch of us took turns grabbing both wires and shocking the crap out of ourselves. I hung on for about 2 seconds and then let go, it was intense!!! So figure 115VAC and 15amps .... BUT that's AC voltage, it's DC that you have to worry about.

    We've all tested a 9VDC battery against our tongue, right? Gets a little zappy if it's a fresh alkaline!

    Too, as someone mentioned, it depends on your body's chemistry and resistance, how sweaty your palms are and all that jazz. Stay safe!

  14. #104
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    Default Re: When does DC voltage become dangerous?

    The thing about most shocks from static is that they are typically of very little energy.

    There might effectively be a high instantaneous current due to the high voltage, but because there's very little energy stored in the first place, the current flows for next to no time, and no damage is done.
    Find a system which could store static electricity of the same voltage but with much more energy stored, and you really wouldn't want to touch it.

  15. #105
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    Default Re: When does DC voltage become dangerous?

    My first electronics class instructor once said all it takes is 30V and 1/10th of an amp to kill you.

    Doesn't seem like much with flashlight batteries that can put out 30A pulses these days.

    I realize we're talking DC, but I had an uncle who was sorta famous for being able to hold onto the wires from a 120V AC wall outlet, although I don't think he was taking it across the chest, but I never witnessed it myself.
    Last edited by magellan; 10-31-2015 at 06:35 PM.
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  16. #106
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    Default Re: When does DC voltage become dangerous?

    In my youth, I accidentally took 12kv 30mA thru my hands. Scared the ever loving hell outta me. Left a bruise on my chest the size of a soft ball and 2 cauterized holes, one on each hand. I lived...... Idk how close to not living I was tho.
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    Default Re: When does DC voltage become dangerous?

    -----
    Last edited by SemiMan; 12-01-2015 at 11:02 PM.

  18. #108
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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by SemiMan View Post
    In general things get "dangerous" at 30V wet, 60V dry.


    Fritz, you were probably pretty lucky, but you would not be the first to get hit with something in that order .... like old TV flybacks, etc.

    It's not just current (or the voltage needed to generate that current given contact points, skin surface resistance, etc.) but what happens w.r.t. your heart. Usually it's extended contact stopping the heart and a significant jolt stopping the heart that causes electrocution. I used to work with electricians that said 277/347 was scarier than 480/600V at times, because 480/600V would seriously hurt .... likely when it threw you across the room. On the other hand, touch 277/347 and it is hard to let go. Often when you do touch something "high voltage", the muscle contractions are severe enough to "throw" you away from the contact. If you happened to "grab" it though, then likely you are toast, as you may not be able to open your hand.

    I am often working around 277VAC, and 500VDC, but in a lab environment. Isolation transformers are a great tool ... rubber gloves too ... and Crocs.
    That's exactly what I was. Lucky.

    Was a NST. The juice went right thru the insulation into my pointer finger on my right hand and into my left palm. Still have the scars.
    Scariest thing was I couldn't let go. Was juiced for approx 10-15s..... Until I could get up enough.... "mental energy" to throw the wires down and at which point I literally jumped across the room, sweating and panting. Unplugged the NST, wrapped the cord around it and put it in the basement until I found a buyer for it. Lol
    I've long since learned.
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    Default Re: When does DC voltage become dangerous?

    -----
    Last edited by SemiMan; 12-01-2015 at 11:01 PM.

  20. #110
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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by SemiMan View Post
    Was that a new neon sign transformer or one of the newer (past 20 years) electronic ones. The new ones are high frequency which ends up being not as dangerous.

    Old magnetic ones are quite dangerous.
    It was one of the really old, iron core, tar potted, ceramic terminal units that breaks several bones if you ever dropped it on your foot.

    I have a few of the newer ones, they have a spark gap that triggers a safety system so are almost useless for Jacob's ladders and the like.
    I also have some of the old iron core as well.... Now that I know more about the safety aspects.
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  21. #111

    Default Re: When does DC voltage become dangerous?

    When I was a kid, I learned how to make an electromagnet by winding bell wire around a nail and connecting the two ends to a dry cell battery. 30 years later, I thought it would be fun to show my son the same thing. I didn't have a dry cell battery, but I did have a nickel metal hydride D cell battery on hand. I figured I'd just hold the wires in place with my thumb and index finger. OUCH! Instantly boiled the skin. I hadn't accounted for the vastly lower internal resistance of NiMH. It's only 1.2 volts, but with almost no resistance, a huge amount of current.

  22. #112
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    Default Re: When does DC voltage become dangerous?

    best post here! ASSume nothing and treat each device as lethal.under the right conditions your heart can be stopped by a watch battery.
    and those that think led circuits are safe are in for a rude awakening!some led backlit tv's have 300+ volt dc strings.led bulbs in 240v areas can have 380v dc in them.same in 120 areas where the bulb uses a doubler.
    some folks who should know better are the worst for doing stupid things.i was in a factory fixing a motor drive unit when the "i know everything"jerk of a maintainence man of the place argued with me when i told him a machine was live.he turned off the wrong switch and i had noticed that.he argued till i tripped the cabinet interlock switch so he could see it power up.
    same guy poking his fingers in stuff asking "whats this do" while i am testing stuff.he pointed at an led in the cap bank.this is a 600v dc bus with huge caps.i told him that light means the cap bank is charged and he would be dead before he hit the floor if he touched it.he left.
    Quote Originally Posted by Apollo Cree View Post
    Will those of you who don't understand electrical safety stop posting things like "XYZ" is safe?



    You are scaring this old engineer.

    It's one thing to post something incorrect and cause someone to buy the wrong flashlight. It's much worse to post incorrect info and kill someone.
    i can be more often found at http://budgetlightforum.com



  23. #113
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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by snakebite View Post
    best post here! ASSume nothing and treat each device as lethal.under the right conditions your heart can be stopped by a watch battery.
    and those that think led circuits are safe are in for a rude awakening!some led backlit tv's have 300+ volt dc strings.led bulbs in 240v areas can have 380v dc in them.same in 120 areas where the bulb uses a doubler.
    some folks who should know better are the worst for doing stupid things.i was in a factory fixing a motor drive unit when the "i know everything"jerk of a maintainence man of the place argued with me when i told him a machine was live.he turned off the wrong switch and i had noticed that.he argued till i tripped the cabinet interlock switch so he could see it power up.
    same guy poking his fingers in stuff asking "whats this do" while i am testing stuff.he pointed at an led in the cap bank.this is a 600v dc bus with huge caps.i told him that light means the cap bank is charged and he would be dead before he hit the floor if he touched it.he left.
    I guess mister Dillwadd never heard of Lock out Tag Out?

    I wasn't involved in the Maintenance department, but worked in testing of materials and introduction into service. I never too for granted that a circuit was dead until I verified. Once verified, MY LOTO lock went on.

    I had a shop worker attempt tightening a hydraulic line on a test machine at was energized with 4000 psi of Skydrol. The tube broke and injected his hand with an ounce or two of Skydrol after ripping a gaping hole in his hand.

    Energy in any form can kill you when released quickly.

  24. #114
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    Default Re: When does DC voltage become dangerous?

    This is factual info, I don't suggest trying it to satisfy your curiosity. While in the USAF a few biker buddies and I went on a country ride and hit several bars to sate our thirst, As time and liquid made it southward we had to make a whizz stop. We made sure no traffic or farmers were present and proceeded to drain our surplus on to the fence that kept the cattle in, (if you observe white insulators on the top wire DO NOT- I repeat DO NOT whizz on that fence!! it uses 48 or higher voltages to keep the cattle contained. we wetted our pants, boots and the other bikers when we made a connection with the power wire. one time will suffice for you to know to NEVER whizz on a farmers fence ever again.
    will work for peanuts

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