3
$\begingroup$

Suppose that I am familiar with the safety procedures for working on 20 A 120 VAC 60 Hz household lines.

What should I be prepared for before working on high voltage DC circuits (say, 800 volts) as is commonly found on the anodes in tube transmitters?

$\endgroup$
  • 1
    $\begingroup$ To those answering "what the voltage/current can do to you" I might remind you that the question is about "what should I be prepared for" -- not getting shocked is painless, regardless of the voltage. If you take appropriate precautions and safety measures, you won't get shocked -- what are those? $\endgroup$ – Zeiss Ikon Jul 23 at 23:20
  • $\begingroup$ @Zeiss Ikon The protocols I would use for this exact case would be distance. Maximize distance from anything conductive that is or possibly is carrying current. Wear flexible rubber gloves if avalible. If not avalible then buy rubber gloves at a store and take electrical tape putting 2 layers around and over The fingers and all the way up to the hand. Usually with general HV use 1 hand. In this case use 2. With 1 as the main hand and the other as a backup. And that is what is necessary. $\endgroup$ – Scientist Smith YT Jul 26 at 17:27
  • $\begingroup$ @Zeiss Ikon Other things such as Fire extinguisers, poking sticks and tinkering toys are a luxury. They are not necessary to carry out this task. If you follow the safety procedures all the way through and not take dumb shortcuts then you won't need a fire extinguisher or whatever else. Including but not limited to, very thick rubber gloves, rubber mats, ridiculous insulation methods, power off the whole time, and so on. This is exactly what I would do if I was carrying out work on 800 VDC equipment. As long as there are no deviations in the procedure nothing will go wrong. $\endgroup$ – Scientist Smith YT Jul 26 at 17:32
  • 1
    $\begingroup$ @ScientistSmithYT Have you ever tried to work in a pair of Playtex gloves wrapped in electrical tape? I'd be barely ahead of a set of Apollo moon gloves. I wear nitrile examining gloves at work, but I'm under no illusion they'll protect from electrical contact -- they're to keep my hands from being permanently black. $\endgroup$ – Zeiss Ikon Jul 26 at 19:20
  • $\begingroup$ @Zeiss Ikon In Fact yes I have. It works very well if done right. $\endgroup$ – Scientist Smith YT Jul 29 at 17:24
3
$\begingroup$

800 volts is far more likely to burn you. Just doubling the voltage from 120 to 240 will quadruple the power heating your flesh.

This is just basic Ohm's Law: [P=E*I]

DC is one nasty customer:

It is easy to get complacent after spending a youth and a career working with docile, harmless 5-24 volts DC, or well-behaved 100 - 240 V AC voltages because of its frequent zero crossings.

DC in that same range is a mean drunk. You may have been in very old houses and felt switches that had a definitive SNAP when switched on or off. Those are throwbacks to when house power was DC, and they snap the contacts quite wide, to assure an arc is snuffed. Above that, you need magnetic or pneumatic "blowouts" designed to pull the arc up into an arc chute to blow it out. ,

... Look at the DC ratings for contactors and relays. You will see very different voltage ratings for DC than AC.

As a result, the various regulations treat higher voltage DC differently from low voltage, and allowable maximums are typically in the 30-50 volt range


And if RF is present, the burn will be even worse.


A friend of mine was severely burned by 240V when his hand was momentarily inside a breaker panel, and it took several months to heal and for the redness to go away.

Also, I watched an old electrician show off by testing for the presence of 120 volts at the ends of two wires using his old and dry thumb and forefinger. He would have been burned if he was touching 800 volts, either AC or DC.

$\endgroup$
  • $\begingroup$ How did this answer get 1 upvote. It doesn't even answer the question. It just gives his opinion on the danger level. He is using basic ohms law. The law used for AC isn't basic ohms law. There are two ohms laws, AC and DC ohms law.. It only gives an opinion that doesn't answer the question asked and only talks around the question. Who upvoted this question? $\endgroup$ – Scientist Smith YT Jul 25 at 17:57
  • $\begingroup$ @ScientistSmithYT Fair enough; maybe you can edit your answer taking into account DC vs. AC. $\endgroup$ – Mike Waters Jul 25 at 18:51
  • $\begingroup$ I'll do that when I can. $\endgroup$ – Scientist Smith YT Jul 25 at 23:46
1
$\begingroup$

First of all, a voltage of 800VDC can be very dangerous if you happen to be touching anything at ground potential. An 800V potential is REALLY eager to pass some current down to ground, compared to a 120V potential.

Also, because of Ohm's Law, if you were to touch something at a potential 6.66 higher (800V against 120V), you will get 6.66 times the current flowing through you, and we all know that it's current that kills!

Static electricity can be in the order of tens of thousands of volts, and can surprise us when we touch it - because it has no current (or such a ridiculously low current that we only feel it as a slight jolt). But if you touch 800VDC coming from a power supply that can give several milliamps you are going to be in real trouble.

To say a 120VAC circuit with a 20A breaker on it is more dangerous than an 800VDC power supply, is itself a very dangerous statement. Usually, domestic appliances are protected by a circuit breaker that will detect any current flowing to ground, and will trip the breaker very quickly before you do too much damage to yourself. And if they are not, you will still get a fraction of the current flowing through your body compared to what you would get from the 800VDC PSU.

I have had a shock from mains voltage (240V AC) when I was a child, which threw me down a flight of stairs from the jolt. And I had a shock from the anode cap of a (thankfully black-and-white) television's CRT. That was WAY more painful (and to be fair, was several kilovolts, albeit at much lower current) - and I was extremely lucky that my dad had taught me that when you work inside a TV, you always work with one hand behind your back. The last thing you need is any current (even a few mA can be fatal) going between your arms and across your heart.

$\endgroup$
  • 1
    $\begingroup$ If you have 240VAC, it's 50 Hz. There's a big difference in hazard between 50 Hz and 60 Hz, as 60 Hz can trigger v-fib at very low currents. $\endgroup$ – Zeiss Ikon Jul 23 at 14:41
  • 1
    $\begingroup$ I still contend that 240V is potentially (see what I did there?) more dangerous generally, because the doubling of the voltage represents a higher risk of death than the 20% increase in frequency $\endgroup$ – Scott Earle Jul 23 at 15:17
  • $\begingroup$ It's not higher frequency that's more dangerous -- RF (in a wire) isn't any more hazardous than DC. It's specifically 60 Hz that's prone to trigger v-fib. $\endgroup$ – Zeiss Ikon Jul 23 at 15:57
  • 1
    $\begingroup$ From your link "A.C. is said to be four to five times more dangerous than D.C", so that means 120V AC would be as dangerous as 480-600V DC? Then my original statement still stands $\endgroup$ – Scott Earle Jul 24 at 2:49
  • 1
    $\begingroup$ Last actual experiments on this might date from Edison's battle with Westinghouse. $\endgroup$ – Zeiss Ikon Jul 24 at 19:28
-2
$\begingroup$

Online references confirm my long standing understanding that 30 mA across the chest at 60 Hz is likely to be fatal, and state that it generally takes more than ten times that current of DC to have a similar effect:

Either AC or DC currents can cause fibrillation of the heart at high enough levels. This typically takes place at 30 mA of AC (rms, 60 Hz) or 300 – 500 mA of DC.

This is the basis behind my understanding that, in general, 120 VAC 60 Hz is more dangerous than 800 VDC, even from a large capacitor. Assuming similar skin and body resistance, it would take approximately 1200 VDC (ten times the RMS voltage of 120 VAC) to produce the same risk of fibrillation -- and it takes tens of times that again to quickly produce fatal levels of burns because the body is mostly water, and it takes a tremendous amount of energy to heat water compared to most other substances.

Not to say 800V from a capacitor can't kill you -- just that it's significantly less likely to do so than 120VAC at 60 Hz.

Generally, then, I'd need to avoid offering cross-chest paths to ground (the standard precaution has been "keep one hand in your pocket" for nearly a century), use insulated tools when I must make an adjustment with power on, check that energy storing capacitors (like the filter caps in the power supply) have drained through their bleed resistors before working with power down, and as an added precaution, manually short capacitors (preferably through a suitable resistance -- high wattage 1k ohm resistor, for instance) if I can't avoid touching their terminals or conductors connected to them.

Most of these are similar if not identical to the precautions I take daily in repairing 120V power tools that must often be tested while partly disassembled.

$\endgroup$
-2
$\begingroup$

I'll answer your questions along the way in order of how they are asked. As well as giving some background information on this misunderstood topic. The 800 VDC is less dangerous on certain circumstances. Let me give examples of where it is dangerous and where it isn't. The most dangerous is when the output current is usually greater than 1.5 amps for your exact voltage. By output I am meaning the amperage that is avalible. Not the amperage that is going through your body. Now that seems like a lot. But given you're skin is dry and even if you touched both ends on purpose, it would feel weird and possibly possibly hurt a little. But it won't kill you on the first try. But if your hands are wet, well lets just make sure no one with wet hands touches something with that much amperage and voltage output.

One of the things that is very misunderstood by many is that 1 volt can kill or that amperage kills no matter what the voltage is. That is entirely wrong. Very very wrong. I'm sure a couple people reading and editing this will know this is true. And some won't and will try to edit it to say something different. The non dangerous conditions are if the amperage is 1 amp or below with the 800 VDC given. But this condition is strictly only for dry hands. Wet hands are a no no. The only time wet hands would equal the danger level of dry hands is if the amperage was 200 mA and below. It will hurt a little more than dry hands. And this is why I will say 50 mA is the best. But max is 200 mA.

For your exact info that was given I will say to wear some class 1 electrician gloves. (With a radio you may not have very good flexibility with the electrician gloves, so thick rubber harbor freight gloves will be sufficient enough. If not then wrap 1 or 2 layers of electrical tape over the fingers of contact on the glove) Class 2 is nice to have, and covers up to 12.5 kVAC. AC is not equal to DC when it comes to insulation. For example, I had a 10 kV DC insulator. And 2.320 kV made the insulator breakdown and catch fire. So the lesson here is DC and AC insulation is never ever the same. So make sure you are using the right type and classification of electrician gloves. If you need more info on electrician gloves try to contact a HV linemen or a master electrician in your area. For the 800 VDC if you do not have the money or resources to get the class 1 electrician gloves, harbor freight or any local hardware store might have some thicker rubber gloves. Use those and a chicken stick. That will also work in certain circumstances. But I personally don't recommend it unless you can insulate the equipment or whatever it is good enough. A chicken stick is usually some insulator (PVC pipe) with a conductor on the end. Depending on what you need done, I would maybe use a plastic coat hanger and break off a straight end.

I'm just covering most of the variables but not all of them. Again if you can specify what kind of equipment or if its a circuit board or something I can help more.

I thought I would also add a couple corrections or suggestions. The outlet frequency you gave leads me to believe you're on the U.S power grid. Outlet power can be said as (in your case) 20 Amp 120 or 125 VAC 50-60 Hz. U.S power grids don't have just one set frequency. They tend to usually be in the 50-60 range with + or - 5 Hz. So they really are around 45-65 Hz. But it is usually said as 50-60 Hz.

If you could look on the device and post the input and output info. Such as the output and input voltage, amperage, wattage, VA or KVA. As well as the operating frequency. I can then make a better judgement on what info I give. Those that want to edit this, please feel free to if I have missed any information that is important or that is a side note.

Other Important Information:

Your hands can easily be 100,000 ohms and at max 120,000 ohms generally speaking. By generally speaking I am covering humid climates up to 75 percent humidity and down to 3 percent humidity. 3 percent is as dry as Utah air is. 75 percent would be like California, Florida, Oregon and even Maui Hawaii. If you feel your hands becoming moist the resistance can drop to 50,000 ohms. In some cases if completely soaked will drop down to a very minimum of 100 ohms. But this is the resistance from the left arm to the right. Measurements for feet to hands will have a greater resistance. The values I have given are not based on formulas, these values are based on a real life experiment I did. The feet to hands resistance according to the data I've already gathered with my other data says the resistance will naturally be greater. But I haven't done the feet to hands resistance test myself. So I dont have real values for that yet.

$\endgroup$
  • $\begingroup$ You might want to make a distinction between "available amps" (which is what I think you're referencing) and "delivered amps" for which 200 mA is 200 mA, wet hands or dry. You might also consider that making adjustments inside a radio chassis with Class 1 or Class 2 gloves on is, well, pretty unlikely. $\endgroup$ – Zeiss Ikon Jul 24 at 12:37
  • 1
    $\begingroup$ Seems to me that, Ohm's law being what it is, wet or dry hands are strongly determinant on what current you get at a given voltage. A 9V battery doesn't produce enough current on dry skin to detect, but bridge the terminals with your tongue and you get quite the tingle (fortunately, the cells in an alkaline are limited to a fraction of an amp -- a rechargeable might hurt more). $\endgroup$ – Zeiss Ikon Jul 25 at 18:12
  • $\begingroup$ @ZeissIkon That's correct. But how about if we all assume dry hands? $\endgroup$ – Mike Waters Jul 25 at 19:07
  • $\begingroup$ @MikeWaters How dry are my hands going to be while doing power-on adjustments in my SB-102 after reading all these answers? $\endgroup$ – Zeiss Ikon Jul 25 at 19:24
  • $\begingroup$ @Mike Waters Your hands can easily be 100,000 ohms and at max 120,000 ohms generally speaking. By generally speaking I am covering humid climates up to 75 percent humidity and down to 3 percent humidity. 3 percent is as dry as Utah air is. 75 percent would be like California, Florida, Oregon and even Maui Hawaii. If you feel your hands becoming moist the resistance can drop to 50,000 ohms. In some cases if completely soaked will drop down to a very minimum of 100 ohms. But this is the resistance from the left arm to the right. Measurements for feet to hands will have a greater resistance. $\endgroup$ – Scientist Smith YT Jul 25 at 23:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.