High-voltage failsafe?

[GreenGiant]

Also Electricians in the U.S. are not supposed to work on live circuits according to OSHA, the only times they are allowed to is if shutting off power would kill life support systems, safety alarm systems, hazardous area ventilation, etc.
If they must work on live circuits they are supposed to be granted reasonable safety through use of insulating clothing/glove and things.

The only way to be truly safe when working on ANYTHING that plugs into the mains (110,120,220,230 etc) is that it is either unplugged or the breaker is off AND the circuit has been discharge (done through discharge rods/bleeder resistors etc)

 

[JMW]

I don't follow the difference between 120 VAC @15 amps and 30 or even 100 Amps. It only takes 100 ma or so to be lethal. An increase in voltage will facilitate the insulation breakdown of skin with possible lethal results. The current availability over 500 ma is irrelevant. There are documented cases of electrocution using 24 VDC, as it was a boat, the actual voltage was probably closer to 27 volts.
Please remember there are people viewing who are new to electronics and electricity. They may not be as "resistant" to electricity as you, and suffer the consequences. How did you like that pun?

Regards,

Jim W

 

[dbooksta]

Ohm's law: R = I * V. Current kills, but the current that can reach the heart is reduced by the resistance of the body. In my experience 15A at 120V doesn't deliver close to 100mA at the heart through dry skin. Whatever the resistance of a dry human is obviously you can reach a lethal current by either cranking up the source amperage or voltage, and eventually you'll hit that. I'm wary of car and boat batteries because they can deliver 600A, even if it's just 12V or 24V: an order of magnitude more potential current at the heart than one gets working on a household circuit!

 

[KrisBlueNZ]

Ohm's law: R = I * V. Current kills, but the current that can reach the heart is reduced by the resistance of the body. In my experience 15A at 120V doesn't deliver close to 100mA at the heart through dry skin. Whatever the resistance of a dry human is obviously you can reach a lethal current by either cranking up the source amperage or voltage, and eventually you'll hit that. I'm wary of car and boat batteries because they can deliver 600A, even if it's just 12V or 24V: an order of magnitude more potential current at the heart than one gets working on a household circuit!

You do not understand the relationship between voltage, resistance, and current.

 

[JMW]

Ohm's law: R = I * V. Current kills, but the current that can reach the heart is reduced by the resistance of the body. In my experience 15A at 120V doesn't deliver close to 100mA at the heart through dry skin. Whatever the resistance of a dry human is obviously you can reach a lethal current by either cranking up the source amperage or voltage, and eventually you'll hit that. I'm wary of car and boat batteries because they can deliver 600A, even if it's just 12V or 24V: an order of magnitude more potential current at the heart than one gets working on a household circuit!

Can you 'splain how you "crank up the source amperage" while maintaining a constant voltage and resistance?

 

[davenn]

Ohm's law: R = I * V. Current kills, but the current that can reach the heart is reduced by the resistance of the body. In my experience 15A at 120V doesn't deliver close to 100mA at the heart through dry skin. Whatever the resistance of a dry human is obviously you can reach a lethal current by either cranking up the source amperage or voltage, and eventually you'll hit that. I'm wary of car and boat batteries because they can deliver 600A, even if it's just 12V or 24V: an order of magnitude more potential current at the heart than one gets working on a household circuit!

Thats just plain wrong

and if you live by that theory and still play around with mains voltages .... your life on planet earth may well be cut short

you really are a contender for a Darwin award

Dave

 

[dbooksta]

I don't see how all these ad hominem responses benefit anyone. Is something I wrote incorrect? Can you explain what and why -- especially when the "theory" I'm citing is Ohm's law?

 

[BobK]

You stated that 15 Amps is safe. That is incorrect.

Bob

 

[(*steve*)]

I don't see how all these ad hominem responses benefit anyone. Is something I wrote incorrect? Can you explain what and why -- especially when the "theory" I'm citing is Ohm's law?

It's because there's so much wrong with what you say that we tend to dismiss it as the ranting of someone who is probably not going to read a sensible explanation.

But, let's assume we're wrong.

Yes, your skin has a high resistance. So the current which will flow for a given voltage is quite low. Let's assume a resistance of 100,000 ohms across your hands.

For 100 volts, that is only 0.001A -- 1mA. That's about the threshold of what you can feel. It is well below the 10mA which causes muscles to contract powerfully.

However, inside our skin is a nice salty liquid. If the resistance of the skin is compromised (sweat will do it) then the resistance can fall to around 1000 ohms.

For 100 volts, that gives you a shock of around 0.1A (100mA). That is well into the lethal range, and since the path of least resistance is through your blood, a substantial part of that will go where your blood goes -- and that's to your heart.

So what you have is a situation where you might only just feel the shock, but it you begin to sweat, that shock may become lethal. Indeed, one person with a higher resistance skin may not even feel the shock. Another sweaty person may try the same thing and not survive to learn from their mistake.

To make things worse, there are physiological reactions which can lower the resistance, and the fact that 120VAC has a peak voltage of almost 170V makes it *more* dangerous.

If you're interested in a reference, this is pretty good.

When safety is concerned, we're pretty much of the opinion that you should be conservative, take advice, and not just make **** up.

 

[(*steve*)]

especially when the "theory" I'm citing is Ohm's law?

Misquoting something you don't understand doesn't make your arguments look better.

Just sayn...

 

[dbooksta]

When safety is concerned, we're pretty much of the opinion that you should be conservative, take advice, and not just make **** up.

Yes, now we're on the right track. So can anyone offer helpful answers to the original questions? To refresh, I'm essentially asking two things:

1. Are there criteria and/or practices that can render a high-voltage circuit of arbitrary construction safe?

2. Are there criteria and/or practices that enable one to safely work on a high-voltage household circuit that is energized?

 

[davenn]

answers for both those questions would be found in your local (whatever country) power regulations

seek out your local govt and or power authority. They are sure to have all in print

Dave

 

[dbooksta]

I'm familiar with our National Electric Code, and I can't find any useful guidance in it pertaining to practices and procedures for working on household appliances and non-utility power circuits.

 

[davenn]

well legally you shouldnt be working on them unless you are a licenced/certified electrician/electronics technician

The rules are there for your safety and for those that may use the gear that you have been working on

Dave

 

[(*steve*)]

well legally you shouldnt be working on them

Not in all countries...

dbooksta, what sort of stuff do you want to work on. If it's mains, then go by the regulations. Other situations may call for more stringent conditions. Some (say, low voltage DC) may call for less.

From what you say ("household appliances and non-utility power circuits") the electrical regulations should apply -- at least if I'm reading you correctly.

 

[JMW]

In general, the answer to your question is "no". The proper procedure is to first follow the manufactures suggested rules. On the front page of modern appliances is a big warning sign. Follow that precisely. Second, your employer will have a set of guidelines. In the USofA the NEC along with OSHA also have their set of rules. These are usually copyrighted and you usually have to pay for them. Unless I'm intimately familiar with the equipment, I never use "common sense" as a guide, one never knows what is lurking behind door number XXX. Don't believe me, check out the voltage available on a disposable flash camera.
Think for a moment about what you are asking: A device, that can detect a lethal "current" ( the rest of us would try and detect voltage) 100% of the time, under all circumstances, regardless of the environment or the users ability. If I've missed anything please let me know. As to where to look for such a device, I suggest the planet Avatar, 4th tree from the right, next to the blue rock.

 

[JMW]

Upon further reflection there might be a way. Find a completely discharged unit, one that is never been used. Weigh this device. Next take your UUT (Unit Under Test) and weigh it, any additional weight can be attributed to stored electrons. IIRC 6.02x10 23 electrons passing a point in 1 second = 1 joule, basically 1 amp. As electrons are 1/1890 of an Hydrogen atom. For each Joule of stored energy you should see an increase of 1/1890 grams. Hopefully someone will check my premise and point out the errors.

 

[dbooksta]

Not in all countries...

dbooksta, what sort of stuff do you want to work on. If it's mains, then go by the regulations. Other situations may call for more stringent conditions. Some (say, low voltage DC) may call for less.

From what you say ("household appliances and non-utility power circuits") the electrical regulations should apply -- at least if I'm reading you correctly.

My current project is to build and test electrostatic precipitators. My first problem is coming up with a device to put a static voltage on an isolated conductor in the kV range. So I'll be taking apart old devices that I know have high-voltage transformers and using them on my workbench. Some may be line-powered, others might be battery-powered.

 

[dbooksta]

In general, the answer to your question is "no". The proper procedure is to first follow the manufactures suggested rules. On the front page of modern appliances is a big warning sign. Follow that precisely.

Yes, manufacturers uniformly say "Stop! This product should only be serviced by qualified personnel." Of course it's always safe to not do anything, but that won't get me very far.

Unless I'm intimately familiar with the equipment, I never use "common sense" as a guide, one never knows what is lurking behind door number XXX.

OK, maybe the right answer is "Don't open any electronics unless you have a complete circuit diagram and are familiar with the layout." I'll take that as a tentative best answer so far.

Think for a moment about what you are asking: A device, that can detect a lethal "current" ( the rest of us would try and detect voltage) 100% of the time, under all circumstances, regardless of the environment or the users ability. If I've missed anything please let me know.

That's not what I am asking. I want to know if we can define environmental parameters and user practices that will allow one to safely work on circuits of unknown design. I.e., I'm asking if we can define minimal but sufficient criteria for the circuits, environment, and user behavior to prevent serious damage to life or property.

The first question was something like this: I open up some household device so you have access to all of its electronic components. How can you render it safe to touch? If you can't definitively do so with an arbitrary device, then if I allow you to impose some observable criteria on the circuit (e.g., "it doesn't contain the components A, B, or C;" or "it only contains components X, Y, and/or Z") can you do so?

The second question then extends the first: I'm going to reconnect the device to a power source. (As I said previously my goal right now happens to be to get a high-voltage supply to put an electrostatic charge on an isolated conductor.) Now are there rules I can follow that are sufficient to prevent me from getting hurt? E.g.,

1. Keep one hand in your back pocket
2. Don't bridge capacitor leads with a conductor
3. Don't touch any point of the circuit with a probe or connector not insulated against 100kV

 

[GreenGiant]

The first question was something like this: I open up some household device so you have access to all of its electronic components. How can you render it safe to touch? If you can't definitively do so with an arbitrary device, then if I allow you to impose some observable criteria on the circuit (e.g., "it doesn't contain the components A, B, or C;" or "it only contains components X, Y, and/or Z") can you do so?

The second question then extends the first: I'm going to reconnect the device to a power source. (As I said previously my goal right now happens to be to get a high-voltage supply to put an electrostatic charge on an isolated conductor.) Now are there rules I can follow that are sufficient to prevent me from getting hurt? E.g.,

1. Keep one hand in your back pocket
2. Don't bridge capacitor leads with a conductor
3. Don't touch any point of the circuit with a probe or connector not insulated against 100kV

Unfortunately there isnt a way, even with one hand in your pocket, not bridging caps and touching with only an insulated probe there is still huge risk. You can still get burns, electrocuted, etc. The ONLY safe way is complete disconnection from the mains, if you need to take voltage measurements use clamps onto where you want to test (sometimes this means soldering on a small test lead somewhere) then plug it in and take your measurements. Yes this is much more time consuming but at least it is safe...er.

Now just a heads up for you, after you develop your high voltage supply even if the current supply is very low, it is going to offer a MUCH more dangerous situation, so take as many precautions as you can. There may be over current protection but there are still MASSIVE risks, an electrician I used to work with refuses to work on anything more than 120 without double protection (breaker shut off and grounding rod on the feed side in case it gets turned on) his brother died from a 240V panel he was working on, even though there was an instant .5A fuse before where he was shocked. I know this is different and you may only be dealing with 10's of milliamps or less, but an instant draw can exceed an amp even with the best of protection.