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I am experimenting with large currents from ultra capacitor discharge.

For example with 500 A (at 2.8 V) you get a very impressive demonstration of the magnetic field of a straight conductor using compass needles or iron chips (compare: http://iopscience.iop.org/article/10.1088/0143-0807/31/1/L03/pdf).

Another example is the Thomson ring experiment http://www.rose-hulman.edu/~moloney/Ph425/0143-0807_33_6_1625JumpingRing.pdf where you may get up to 9000 A for a very short time.

Suppose all voltages used are below 60 V. What do you need to consider about safety in this case?

Here is what I think:

  • Since the voltage is too low, there should be no danger from current through the human body.
  • There might be a danger from sparks and lightening when if there are contact problems.
    • This may be dangerous because of UV light
    • and because of sparks hitting directly the eye
  • Also the there might be heat problems which makes thinks to vaporize which you may inhale
  • A capacitor discharge generates an EMP which may affect pacemakers for example

I am not sure if I mentioned all possible dangers concerning this. My question is:

  • Under which conditions (minimal current, discharge time...) which danger will become relevant
  • What to do to make it safe
Julia
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    60V is high enough to be a potential hazard. Even car batteries, at no more than 12V, are to be treated with care, because they can output a lot of energy very quickly into whatever they're connected to. This is the sort of thing that, if you have to ask, you really shouldn't be doing it. – Hearth Apr 26 '17 at 12:37
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    High current can generate very high voltage and substantial energy with just a bit of inductance when switched so you can't ignore that. Burn hazards and possible blindness from molten metal and eye or cancer-causing UV skin damage (you can get a bad 'sunburn' from a low voltage arc). Even a car battery can cause loss of a finger if you accidentally weld it across a conductive ring. Try to find University health and safety guidelines or corporate ones. Some of us have written them. – Spehro Pefhany Apr 26 '17 at 12:48
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    Don't forget to add "have fire extinguishers at hand" and "buy some insurance" to your safety checklist. – Enric Blanco Apr 26 '17 at 14:12
  • @SpehroPefhany Can you quantify your point about the inductance? – Julia Apr 26 '17 at 14:33
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    I think Spehro's point is V=L*di/dt. If you have some L and switch the current very rapidly (high di/dt) you can get very large V. So the basic inductance formula quantifies the point nicely. And all cables and wires have some L. – John D Apr 26 '17 at 14:47
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    To make things safe, all electronic circuitry must be suitably enclosed. All electrical connections must be bolted or crimped. All components, particularly capacitors used to generate high current pulsed must be carefully evaluated for the intended duty. The extent of magnetic propagation of magnetic fields needs to be calculated and evaluated. –  Apr 26 '17 at 14:52
  • As @JohnD says V = L * di/dt and energy = \$L \cdot i^2\$. – Spehro Pefhany Apr 26 '17 at 14:53
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    Things I can think of that aren't in your list, mostly collected from YouTube videos of risk analysis by people who like to blow things up: Accessible fire extinguisher; fire resistant carpeted flooring if you're working with things that may spray molten metal (sounds counter-intuitive but carpet prevents molten bits from bouncing into unnoticed/inaccessible places); never work alone always have a buddy; beware of things that create x-rays; have a remote location where you can cut power from if you need to escape; clear the room of tripping hazards; keep the area clean and tidy. – Jason C Apr 27 '17 at 04:37
  • Around here (Netherlands) you'd need to follow safety procedures as mentioned in NEN EN 50110 and the included NEN 3140 for such installations. I'm not sure what the German counterpart would be, but the 50110 follows European standards IIRC. IEC 60204 might be relevant as well (also a lot easier to find). – Mast Apr 29 '17 at 13:20
  • @Felthry Wouldn't a more helpful answer be, "High DC currents have noteworthy/non-obvious dangers. Since you are asking, you should probably hire an electrical engineer..."? What is the point of discouraging people to ask about safety?!? –  Apr 30 '17 at 09:10

3 Answers3

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If we consider this model of a high energy switching circuit we can simulate the induced voltage on a nearby conductor. This provides a simple simulation of the type of electromagnetic interference that would be coupled into nearby conductors or electronic devices.

schematic

simulate this circuit – Schematic created using CircuitLab

As any switch opens, or you simply tap wires together to briefly conduct that 1,000 amps, when you have 1u (1 micron, 10,000 Angstroms, or 1/25th of a mil) separation of the wires, the 3 volt potential causes an arc.

The 100pF across 1micron separation (3mm by 4mm ---- heavy wire --- contact) will resonant with the 1uH (~~ 1 meter) wire in your high-current path. Fring will be 15MHz. What is the dI/dT of 1,000 amps ringing at 15Mhz?

100,000 MegaAmps/second.

Place a wire 4" from the high current, that wire formed into 4" by 4" loop; expect 2,000 volts across the ends of that 4" loop.

J...
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analogsystemsrf
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  • Nice, but I guess that the 200 V will be there only a very short amount of time (how much?). Then the deposited energy will be the relevant parameter. How much will the energy be if I touch the contacts of your 200 V wire? – Julia Apr 26 '17 at 16:10
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    I had a typo; the dI/dT is 100,000 amps/uSec or 100Billion (10^+11) amps/second; thus Vloop is 2,000 volts; note the Vinduce formula --- if totally accurate --- needs some integration and/or natural_logs. However, for ratios as I used, with Distance and Loop edges approx. the same, there is little error. So expect 2,000 volts. – analogsystemsrf Apr 26 '17 at 16:19
  • Ok, then 2000 V, but same question about time scale and energy. – Julia Apr 26 '17 at 18:26
  • ((2000V*1000A)+ 1Human) *T= Deadhuman for all values of T>0 seconds – JRE Apr 26 '17 at 19:30
  • @JRE: Consider a simple parallel plate capacitor (for demonstration experiments) with 15cm plate Diameter and 0,5 cm plate distance. You can charge it with 5000 V and touch it. Nothing dangerous will happen. In your example the current through the body would be roughly 2000V/1000Ohm = 2A (assuming a resistance of 1000 Ohm), not 1000 A. In my capacitor example it would be 5A initially, but very rapidly decaying. So time scale and energy indeed matters in this case. – Julia Apr 27 '17 at 05:20
  • 2A will still kill you. – JRE Apr 27 '17 at 05:22
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    @JRE: As I said, it depends on the time scale and then on the energy. I often touched a demonstration parallel plate capacitor (low energy!) or a whimshurst machine (30 kV), nothing happened (however I calculated the energy content before using it). – Julia Apr 27 '17 at 05:25
  • Using energy stored in Inductor of 0.5 * L * I^2, its 0.5 joules. – analogsystemsrf Apr 27 '17 at 12:39
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    The OP wanted to know how to be safe. This answer illustrates the risk. – analogsystemsrf Apr 27 '17 at 14:08
  • @analogsystemsrf Yeah, I finally got that - my first skim was cursory, admittedly. The edit adds a bit of executive summary to make it clear where you're going with this. +1 – J... Apr 27 '17 at 19:14
  • Wow. Wouldn’t that be sufficient to kill a person by means of currents generated via magnetic induction inside the body? – Demi Apr 29 '17 at 18:27
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    If the ringing frequency is 15MHz, the skin depth (where 63% attenuation occurs) is 17 microns. But at 2,000 volts versus 20 milliVolts (neural potentials), you need 100,000:1 attenuation; at 8.6 dB per skindepth (neper) you need 100dB (100,00:1) / 8.6 dB or 12 skin depths or 200microns. Seems like the scalp tissue will keep the energy out of the neurons. But your mileage may vary on these numbers. – analogsystemsrf May 03 '17 at 03:51
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Some random thoughts... Accidental shorts on high current sources can cause a very loud bang, equivalent to the noise from a handgun. In a small room it will stun you from the loud noise and cause hearing problems for a few hours (personal experience with a 1 Farad cap bank @ 150V that vaporized a circuit board). On low voltage shorts, I wouldn't expect debilitating bangs.

Jewelry (rings, bracelets, necklaces, ...) can severely burn you if said jewelry gets across the cap bank. I've had to pry a colleague's ring off a terminal strip. He still has the scar from the burn. Fortunately, the supply was fused.

Molten bits can fly off. Make sure you wear eye protection.

High currents can damage the internals of capacitors causing spot heating at high resistance points.

qrk
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Be careful! I don't know why your question wasn't answered proprerly. I was shocked to see that "500A" which my eyebrows jump sky high wasn't addressed.

I'm not an expert, so regard my answer as a reference for further research, and a head for others to answer.

While voltage is a determinant in whether current passes through a resistance (such as your body, though it's more an insulator than a resistor - which translates to variable resistance dependent on voltage), it's still important to be wary of the high current.

Generally (at relatively high voltage of roughly 50 and above), if current is 1-10 milliamps it is a little shock, and a bit of a cause for concern. Above 10 millimaps, and you've got yourself a real cause for concern (you can see why I was concerned for that 500A now).

However, this is just a rule of thumb! Other factors that play a role, like your body resistance - which decreases with increased voltage/current/frequency.

The last bit - frequency - could be the difference between living and not on the same voltage/current configuration.

I don't understand it myself, but the higher you go in frequency, the more painful it is, DC or AC.

To read more: light explanation

Nizar K
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    Current kills, that it true. It is also true that it takes voltage to push the current through your body. At low voltage, it is impossible for a high current to flow through your body. – JRE Jul 08 '21 at 18:15
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    Welcome to EE.SE. The OP said "500 A (**at 2.8 V**)" which you seem to have missed. 2.8 V will not drive any significant current through the human body. "*Usually voltage isn't a big factor when it comes to safety.*" Voltage is the major factor. It determines how much current will flow through a particular resistance. It is the reason why high voltage devices have warning labels. The danger of high currents is the risk of burns in under-sized conductors which may reach high temperatures. Remember you can [edit] your post to fix and improve it. – Transistor Jul 08 '21 at 18:22
  • You might let us in on what voltage you are working with. What are you experimenting with. What is the facility like. Do you have the proper training and resources? Do you know what an arc shield is? Will you have somebody else present? Do you have the proper safety equipment. These are a few questions you should be able yes. You need to be comfortable before starting. You might edit your question to include as much as information as you can such as answers to some of these questions, – Gil Jul 09 '21 at 00:51
  • I'm not trained at all. I just wanted to let a fellow in on the scale of things - very roughly. Thus, the "not an expert". If it motivated people to answer, I'm elated!!! I want to learn too. – Nizar K Jul 09 '21 at 17:41
  • I would also mention that the body has variable resistance depending on the voltage and current. For example, it's usually in the megaohms, but if you touch high voltage/high current - it'll be almost 0 resistance. You are more of an insulator than a resistor, to my understanding. Hope that also helps :). – Nizar K Jul 09 '21 at 17:49
  • Also, I'd like to remind the more veteran of you that there's also an edit button in the comments to remove your passive aggressiveness. A barrage of questions aimed at discredibility is fine when it's reasonable, but saying the least of the requirements is to conduct these experiments yourself is preferrable yet unnecessary. While my goal is to aid another learner, I fail to see yours -other than pompousness, of course. While I did make the claim: "voltage isn't a big factor", I certainly didn't disclude it, but I will make an edit clarifying my already clear position on this matter. – Nizar K Jul 09 '21 at 18:25
  • The edit improves your answer, thanks, Nizar. Tip: comments can only be edited for five minutes after posting. After that the only option is deletion. – Transistor Jul 09 '21 at 18:39
  • Unless there's something inductive involved, and then you could have a considerable kick. Inductors resist changes in *current* (like caps resist changes in voltage) and will flow more voltage (caps: current) to try to sustain the current. So the voltage *can* go right through the roof! – Harper - Reinstate Monica Jul 09 '21 at 19:38