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recently I have been hacking few psu and have to deal with some high voltage caps. It sort of scared me, but thinking about it, what is the chance of it really killing me.

  1. Is it safe to touch one terminal of a charged and opened 120v cap? I mean the other terminal is opened, and connected to nothing, so there is no current flow, so it should be safe.

  2. Touching 2 terminals of a charged cap with one of your finger will give you a shock in your finger, but since no current go through your heart, it will not kill you. am I correct?

  3. The only scenario a charged cap could kill you is when you touch one terminal with one hand, and other terminal with other hand, and the shock go through your heart. but still, what is the chance of that tiny 120v killing you if you are a health person. I think doctors use high voltage to save people.

Is a tiny 120v charged cap really that intimidating?

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marked as duplicate by PeterJ, Passerby, Leon Heller, Ricardo, Dave Tweed Aug 26 '15 at 11:52

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • \$\begingroup\$ Note that "it's not the voltage but the current". A voltage over about 50 volts can kill you (much lower if the skin is wet). At issue is how much current flows and for how long. @Mats puts some parameters on this. \$\endgroup\$ – Hot Licks Aug 26 '15 at 11:50
  • \$\begingroup\$ note that one of the terminals may be connected to ground and the case, so the two hand thing is pretty easy to do by accident if you are holding the device with one hand and poking around with the other. source: experience. \$\endgroup\$ – John Meacham Jun 28 '17 at 5:14
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You are right that 3) is the most risky situation, and I (and I think most other contributers here) have experienced 2) (not with that sissy 120V you americans use, but with the european 220V that real men use...) and survived with only a tinkling sensation, and in theory 1) should be harmless.

But that depends on the situation being exactly what you think it is. And we never make mistakes, do we? There never are any unexpected leakage paths, especially not across you skin because you were sweathing from concentration?

I can cross the street in front of my house with my eyes closed for 10 times and I will (probably) survive unharmed. That doesn't realy make it a good idea to do it an 11'th time...

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    \$\begingroup\$ (not with that sissy 120V you americans use, but with the european 220V that real men use...)... :-| \$\endgroup\$ – WedaPashi Aug 26 '15 at 8:24
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    \$\begingroup\$ +1. I once had the pleasure to short a disposable camera flash cap. Not funny. \$\endgroup\$ – Vladimir Cravero Aug 26 '15 at 9:06
  • \$\begingroup\$ @VladimirCravero I once cooked a small piece of bacon by running the charge from a camera flash cap across it. But that, IIRC, was at 500V, not 120V, which makes such things easier. \$\endgroup\$ – Jules Aug 26 '15 at 10:17
  • \$\begingroup\$ @Jules now that's definitely something worth trying. \$\endgroup\$ – Vladimir Cravero Aug 26 '15 at 10:18
  • \$\begingroup\$ @VladimirCravero Once I had the pleasure of pluggin a cut plug in the socket. All I remember was that everything was white and I temporarly lost my (h)earing. It was 220V. The plug wasn't connected to anything, but I was 5 and decided to try it. \$\endgroup\$ – Ismael Miguel Aug 26 '15 at 11:27
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It also depends on the size (charge) of the cap, not only the voltage.

In PSU and other equipment used to provide a smooth DC output from an AC source, large (high farad) caps are used. Best is to try to drain them a little bit slow with a resistor, not touching the terminals with bare hands (use a non conductive pliers or similar).

Other place with high voltage caps are in cameras, the flash mechanism use 100-200µF caps charged with 200-300 volt.

An defibrillator can give up to 360 Joules of energy (http://www.resuscitationcentral.com/defibrillation/biphasic-waveform/)

The energy stored in a cap of 4700µF and 230 Volt (found in one of my PSU) would be around 125 Joule according to the calculation on this page (http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capeng.html)

So, I recommend not to go with question #3 on the terminals. The energy (~60 Jouls for 120 volt) might not kill you, but it will be very unpleasant.

Note: The top voltage of an cap is often 1.41 times the voltage rating due to the top voltage in the AC-system. 120 an 230 is the average.

Note2: Caps can be filled with even higher voltage if an voltage boosting circuit is present. Like the one in the camera flash (driven by a 3,7 volt battery) (http://electronics.howstuffworks.com/camera-flash3.htm)

Note3: Long ago when I went to school and learned electronics, a practical joke was to charge a small cap with say 100 volt and throw it to someone with a -'hey, catch'.

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When dealing with killer caps the figure you want to talk about is energy.

For a capacitor you can calculate stored energy as:

$$ E=\frac{1}{2}CV^2 $$

An input cap in an ATX supply is in the range of the mF, while voltage is approximately 170VDC. That adds up to 144J. And yes, 144J is enough to kill you, since voltage is above 50V that is the minimum to start serious conduction in the human body.

When working with (possibly) dangerous equipment always keep your left hand behind your back, and wear rubber footwear.

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  • \$\begingroup\$ Energy is another aspect of capacitor danger, but without sufficient voltage it is very unlikely that a capacitor can kill you by inducing a current through your body. But it sure can do harm when shorted. IIRC there was a question about this recently. \$\endgroup\$ – Wouter van Ooijen Aug 26 '15 at 10:22
  • \$\begingroup\$ yeah that is (marginally) covered by the 50V thing \$\endgroup\$ – Vladimir Cravero Aug 26 '15 at 10:28
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    \$\begingroup\$ Sort of. The thing is that that a cap will only deliver current for short amount of time. And the current needed to cause fibrilation in under 1s is much larger than for longer exposures. Also the current from a cap drop exponentially with time. From bme.ccny.cuny.edu/faculty/mbikson/BiksonMSafeVoltageReview.pdf: "The threshold values for ventricular fibrillation, the “main cause of death by electrical shock”, were 40 mA for >3s exposure, 50 mA at 1 s exposure, and 500 mA for 0.1 s exposure (left hand-to-foot/feet)." \$\endgroup\$ – Fizz Nov 5 '15 at 8:55
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If you've got a charged cap, it will be most likely be connected to some circuits... Or in the progress of unassembling it, you'll be touching it - perhaps with some other instruments.

Its easy to construct a path to connect 2 different parts of your body with the 2 pins of the capacitator in this way...

So the short answer would be "yes", i think.

(And most of the time, you might be lucky.)

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It's useful to do a bit of risk analysis.

Purely electrically, discharging a 120 V capacitor through your finger is very unlikely to immediately kill you, but likely to give you a burn mark. Discharging it through the body carries a greater risk, but realistically, you will probably be startled but fine. Keep in mind that universal power supplies will often boost the voltage to about 300 V in its first stage, to have a smooth, well defined DC voltage to work with. The power stored in the capacitor will relatively quickly be dissipated through the DC/DC converter and the capacitor's voltage will stabilize at the DC/DC converter cutoff voltage, and will then very slowly discharge through whatever leakage path are in place. The purely electrical risk analysis will be the same.

The bigger risk in this case is actually your reaction during the shock. Startled as you are, you may do any number of more or less dangerous movements. Maybe you will jump backwards and hit your head. Maybe you will charge your arm toward the exposed 230 V mains wire you have lying around the bench. Maybe you will pull out the oscilloscope from the shelf and have it fall over your head. You get the idea.

You can practically treat the capacitor, on its own, as being non-lethal. The more pertinent issue is to develop a careful more attitude to safety than thinking "this setup is safe, so I don't need to worry." What if you didn't turn off the power switch this time? What if you left that temporary test wire in place and forgot about it? What if you're making an assumption about the circuit that is not true, or the circuit is broken and doesn't work as expected - perhaps being the reason why you're debugging it in the first place.

You have everything to gain and nothing to lose from embracing a paranoid attitude about electrical safety. Always measure twice, every time. Never assume that a circuit is safe or discharged. Remember, accidents happen when you don't expect them.

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