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Suppose I have a 480 VAC line, feeding a rectifier and filter caps. Obviously this is not something to be trifled with while live.

Now, suppose there are blocking diodes coming off the DC bus, as shown.

schematic

simulate this circuit – Schematic created using CircuitLab

In theory, in a perfect world, no current can reach the non-energized side of those diodes. Does that mean that side of the diodes is safe to work on without de-energizing the entire system? If not, why not? Are there specific safety standards this would violate?

EDIT: If you're interested in the broader context of this question, I asked a related question about ethics at the engineering exchange.

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    \$\begingroup\$ In a perfect world you wouldn't need your tongue as a voltage sensor. In a not so perfect world, like the one in your circuit, I wouldn't even think of going near there with my tongue. \$\endgroup\$
    – jippie
    Apr 18, 2015 at 5:01
  • \$\begingroup\$ Also keep in mind that, since it's A/C, neither of the outputs is ever likely to measure 0.00V, relative to the earth. Touching either of them may hurt/kill if you're, otherwise, grounded. Touching both, well... you won't have time to worry about it. \$\endgroup\$
    – Jon
    Apr 18, 2015 at 9:34
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    \$\begingroup\$ Diodes can and sometimes do fail short-circuit. \$\endgroup\$
    – user_1818839
    Apr 18, 2015 at 9:43
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    \$\begingroup\$ If you add a ground, an AC source, name and value everything, CircuitLab will simulate this. The time-domain graph shows the no-load voltage settling at tongue-incompatible voltages, despite the diodes. \$\endgroup\$
    – Jon
    Apr 18, 2015 at 10:09
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    \$\begingroup\$ Just so nobody wonders, I know this is unsafe. I am trying to convince someone else! \$\endgroup\$
    – Stephen Collings
    Apr 18, 2015 at 13:09

3 Answers 3

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It's a good idea to take this one step at a time.

In theory, in a perfect world, no current can reach the non-energized side of those diodes.

Well, not strictly true. If you look up the data sheet for the venerable 1N4007, rated for 1000 volts, you'll see that the maximum reverse current is 5 uA at 25 C, and 50 uA at high temps. Let's use the lower number.

Next, let's establish the parameters of a Standard Tongue. I stuck the probes of my trusty Fluke 77 on my tongue, and got a resistance of about 10 kohms (which shows, once again, that I'm not in the running for the title of Silver-Tongued Devil).

5 uA into 10kohms gives about .05 volts.

Furthermore, at 1 MHz the junction capacitance is less than 5 pF for voltages more than 10 volts. Assuming a line frequency of 60 Hz, and enough load to produce a 10% ripple on the cap, the impedance of the two diodes is about 177 Mohm. This will produce an AC voltage of about 4 mv.

So as long as the diodes are in spec, and there are no other current paths available, yes, you can lick it all day long.

But that's not what you asked. What you asked is,

Does that mean that side of the diodes is safe to work on without de-energizing the entire system?

And the answer to that is, if I may quote Russell McMahon,

No NO NO NO NO !!!!!!!!!

Next question:

If not, why not?

Because counting on everything working to spec when the price of a mistake is death does not, in most circles, count as safe. Remember the part when I weasel-worded, and said "and there are no other current paths available"? That's a really important bit of weasel-wording. Trust me on this. The ability of electrical systems to produce unexpected current paths is mind-boggling. In a properly designed, constructed and maintained system, these things are rare. Counting on a system to adhere to these criteria is really, really, dumb.

Are there specific safety standards this would violate?

Assuming you're American, 29CFR 1910.335 (Safeguards for personnel protection.) would be a good place to start. I'm not sure exactly which paragraph "contacting an energized 680 volt electrical source with the tongue" outrages, but it's probably in violation of at least a half-dozen.

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In a perfect world ... Shudder!

In a real world ...
No No No !!!

No NO NO NO NO !!!!!!!!!

Capacitive coupling alone would almost certainly make you VERY much regret the impact on your tongue. If your brain survived to have regrets . Diode reverse leakage would add to that.**
Note that Schottky diodes (unlikely to be used here) have baddish reverse leakage at 27 °C and horrendously bad leakage at, say, 70 °C.

In the real world I'd not touch that outlet with a barge pole (unless it was a certified 11 kV proof barge pole).

Plus Murphy loves to see things break when they cannot possibly [tm] do so.

**-De-rerendered the stylistic and undesired edit:
If your brain survived to have regrets, capacitive coupling alone would almost certainly make you VERY much regret the impact on your tongue. Diode reverse leakage would add to that.

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    \$\begingroup\$ What if it was a certified 10.9 kV proof barge pole? \$\endgroup\$
    – user253751
    Apr 18, 2015 at 12:42
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    \$\begingroup\$ Of course it's right to highlight that this is not safe, but it would still be nice to actually quantify what's supposed to happen, in a realistic implementation of the OP's setting. I.e., if we're not talking about a tongue but about a cheap-ish yet sensitive electronic component with about the same impedance as a tongue. \$\endgroup\$
    – leftaroundabout
    Apr 18, 2015 at 12:52
  • \$\begingroup\$ @leftaroundabout - You can determine that by reading my answer and applying Ohms law and a bit of reasoning. You have about 600 VDC behind two reversed diodes. There is ripple or may be as one assumes that the DC bus MAY be loaded. Failure to assume such fast tracks you to the Darwin Awards shortlist. Any AC will be conducted via any diode capacitance. Data sheets will tell you how much but 100's of pF may be assumed.You'd feel it. The diodes have leakage current You can look it up. TYpically 10 uA max at 25C and 50 uA at 125C. You'd feel it. If there is a small amount of capacitance ... \$\endgroup\$
    – Russell McMahon ♦
    Apr 18, 2015 at 13:13
  • \$\begingroup\$ ... at Vout it will charge to 600 VDC or so. THe carpet shuffling spark maker hurt with one person shuffling - with 5 it was awesome. You could see the spark jump off a pointed finger to eg filing cabinet. One staff member was sensitive enough to the shock as to flee in close to terror from a shuffling line. At 600 V into 10's of pF you probably won't see the spark. Touch much electronic stuff and it would die. Put a big cap on the outlet and let it sit awhile and it could kill you. \$\endgroup\$
    – Russell McMahon ♦
    Apr 18, 2015 at 13:17
  • \$\begingroup\$ @Peter M - de-typoing, improving rendering of temperature notation and similar is appreciated. But stylistic changes to my mode of expression that are clearly not typos would preferably not be contemplated, let alone implemented :-). \$\endgroup\$
    – Russell McMahon ♦
    Apr 18, 2015 at 21:45
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No, that would not be safe. First, even with the diodes and open circuit, you will still measure some large voltage, depending on the meter's input impedance. Second, if you put a load there, current will flow, and depend on the diode's reverse leakage current. As far as safety standards, no idea, I don't work on those types of systems, but my bad idea detector immediately goes off looking at the circuit.

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