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I am working on an electroadhesive for a project this year, and I need to find some sources that I can cite for the safety of the high voltages involved.

The high voltage converter that I was going to use is a 555 timer driven transformer that can supply a couple microamps of a couple kilovolts that wouldn't even approach the amount of current required to risk the safety of a human. Keep in mind that the circuit is intentionally left open, and the only current flowing is through discharge and losses of the high voltage electrodes.

Here's a simple summary of the risk, if a 1700 pF capacitor discharges at 2000 V, there is a very rough estimation of 2000 V at 1 amp for tens of microseconds. The stray charges on the surface seem to be the primary danger. I believe the human body can go down to 200 Ohms given very optimized conditions such as being in a conductive fluid and having low impedance, and even this gives a reasonable result.

(1700C/V)E-12 = Capacitance, 200 ohms = Resistance of a body, 2000 V = Voltage

So the amount of static charges on one "plate" of our capacitor is (1700E-12/2000) at 2000 V. According to ohm's law, 2000 V over 200 ohms allows 10 A of current to flow. 10 A = 10 C/sec, so to solve for time, we divide our charges by the rate of flow (might be a crude estimation if there is no continuous voltage source but I think this is still an okay estimation). The time to discharge should be 1700E-13 or 1.7E-10 seconds.

Feel free to correct my math, as I am just getting into electrical engineering, but even if it was right there is no way to prove that this is safe besides considering it one pulse of high voltage AC or something, given what I have found.

Keep in mind in case it helps finding a source about safety, that there are only ((10 A * 2000 V) * 1.7E-10) Joules being transferred to your body. Canceling zeros and simplifying leaves us with (2E5 * 1.7E-10)J, or 3.4E-5 Joules. According to the first source linked below, 50 J is maybe a characteristic of dangerous voltages, but this still doesn't formally state anything about the safety of a short DC pulse.

Isn't this the same thing that happens when you get shocked by rubbing a carpet or touching a doorknob? Despite the common occurrence of getting shocked by high voltage over short periods of time, I can't find any sources, and I need to cite them in order to have formal proof that this is safe. If anyone can find some information on this or fix my calculations that would be awesome. Thanks.

Here is what I have found on my own so far:

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    \$\begingroup\$ just a general remark on safe design: If you, instead of an (ancient, bad at driving outputs sharply, power hungry, imprecise and hard to control) timer IC used a flyback voltage converter controller IC, you would be getting a design by just following the example from the datasheet that has voltage feedback and possibly to probably cycle-by-cylce current limiting. That's two security features right there that you're missing by doing this the 1960's way. \$\endgroup\$ Commented Nov 20 at 14:05
  • \$\begingroup\$ I'm trying to find a reliable source stating that a discharge of less than 0.25 J is safe regardless of voltage, but so far I'm only finding random not-so-reliable documents and things that state it in less clear terms. I hope someone can find a good source for that. \$\endgroup\$ Commented Nov 20 at 20:27
  • \$\begingroup\$ @Sophie Swett The IEC TS 60479-1 Effects of current on human beings and livestock - Part 1: General aspects, might be a good starting point. My notes say that they have plots of current versus duration, and values of resistances across the body, but I don’t have access to them anymore. \$\endgroup\$ Commented Nov 22 at 0:31

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The high voltage converter that I was going to use is a 555 timer driven transformer that can supply a couple microamps of a couple kilovolts

If a couple of microamps is all that is needed to be supplied then, put 1 MΩ\$^1\$ in series with the output terminal. At 2 μA the volt drop would be 2 volts and nothing compared to the open-circuit voltage of a couple of kV.

This means that the maximum discharge current into a low-impedance human being is just a milliamp or so i.e. the added output resistor limits the current rather than the resistance of the human body.

Of course there will still be a little stray capacitance on the raw output line but, it's going to be a few picofarads and safe as walking across a carpet and touching a doorknob (as you alluded).

I will also add that we have a model of the human body (notably called the human body model) that is used to determine how much charge can flow into an earthed/grounded sensitive circuit should the body get charged up (carpet walking e.g.). That model (aka the HBM) is 100 pF in series with 1500 Ω (not 200 Ω). We make generators that mimic this to test the susceptibility of our circuits to ESD so, this might be a more appropriate route to consider.

enter image description here

HBM image from here.

\$^1\$ The 1 MΩ resistor needs to be voltage rated to withstand 2 kV. Most resistors are rated less than 200 volts but, there are a few rated for 2 kV or, if you can only find ones rated for 1 kV you can use two 470 kΩ resistors in series. Make sure you apply sufficient clearance/creepage between resistor terminals. Maybe 6 mm is using IPC2221 table 6-1 as your guideline.

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  • \$\begingroup\$ you might mention that the resistor has to be rated for the 2 kV, which general purpose resistor are not. Either use a HV resistor, or a lot of equal GP resistors in series to get the safe voltage withstanding. \$\endgroup\$ Commented Nov 20 at 14:39
  • \$\begingroup\$ @Neil_UK your comment saved me from stating it but, I suppose I ought to drop a few words into the answer. \$\endgroup\$ Commented Nov 20 at 14:45
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For military equipment, protection against accidental contact for anything above 30 V (DC or rms) is a requirement MIL-HDBK-454C (General Guidelines for Electronic Equipment):

MIL-HDBK-454C section 4.5.3

Above 500 V, interlocks are required on the equipment. The 30 V limit is echoed in MIL-STD-1472H (Human engineering):

MIL-STD-1472H section 5.7.8

That being said, according to MIL-HDBK-454C current is the more important factor when it comes to shock:

MIL-HDBK-454C section 5.2.1 MIL-HDBK-454C table 1-i

Like @Andy aka mentioned, limiting current from a high voltage source is important when it comes to safety.

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  • \$\begingroup\$ I'm not sure if these standards are relevant for high-impedance power sources. Brushing my hair produces voltages in the thousands of volts, but that doesn't mean that my hairbrush needs guarding with non-bypassable interlocks. \$\endgroup\$ Commented Nov 20 at 20:23
  • \$\begingroup\$ @Sophie Swett True, however your hairbrush isn’t a piece of equipment that contains a 2 kV power supply. The OP is attempting to convince someone that a piece of equipment with a high voltage electrode is safe to touch. The military standards say no, but they are also very strict since they make no allowances for current limiting or duration. \$\endgroup\$ Commented Nov 21 at 22:36

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