High Voltage Danger? When?

[RussMartin]

I'm going to underscore what Darrel has already stated--always treat electricity with respect!

My knowledge of history isn't all that great, either, but I believe the AC versus DC issue began with Edison versus Nikola Tesla; Westinghouse came in a little later.

On one hand, I routinely test 9-volt batteries by touching the terminals to the end of my tongue. In high school, I knew a TV repairman who could tell whether the AC line voltage was right by touching two bare wires with one hand, and he died of cancer at age 81. And a former co-worker of mine, about four years ago, survived 28,000 volts DC (television transmitter) but had some nasty burns and many skin grafts after. He's also suffered some subsequent neurological problems. (If you think I mean, "He's not as sharp as he used to be," you're right.)

When I was a kid, I learned the "one hand in your pocket" rule, and I still use it. (When you're working with a live circuit, keep one hand in your pocket.) All my close encounters with electricity have been with one hand--including about 1000 watts of RF across the palm of my right hand (and a burn). The scariest shock I ever got was from 90 volts of AC telephone ringer--again, just my right hand, but it wanted to grab and hold on.

And, of course, whenever possible, work on the circuit or equipment with the power off!

My understanding has always been that current kills, not voltage--otherwise we'd all keel over and croak when we get that static shock from a doorknob on a really dry day. The big difference, supposedly, is that low-frequency AC disrupts the electrical rhythm of the human heart, so it is much more dangerous at comparatively lower voltages; high-voltage DC just cooks you.

Don't be a resistor! (Very bad pun . . . )

[Luciano]

Hi,

FOR ALUMINUM ELECTROLYTIC CAPACITORS, CAUTION:

Personnel injury or property damage may result from a capacitor explosion or the expulsion
of electrolyte due to mechanical or electrical disruption of the capacitor. Contact with
liquid or vapor is to be avoided. In the event of contact with the skin or eyes, or accidental
oral ingestion, action may be taken as follows:

Emergency treatment is as follows:

Skin Contact: May result in irritation.
Flush and wash thoroughly with soap and water immediately.

Eye Contact: Contact lenses must be removed at once.
Immediately flush the open eye(s) with large amounts of water and seek immediate medical attention.

Oral Ingestion: As an interim emergency measure, administer warm water or milk, induce vomiting and
seek immediate medical attention.

Vapors: Avoid inhalation of vapors. Ventilate area thoroughly and go to fresh air.

* * * * *

Often risk takers are not aware of the danger they put themselves in:



Best regards,

Luciano

[Darrel Taylor]

Good old YouTube.

Blowing up capacitors.
Seems to be one of the favorite pass times there.

http://www.youtube.com/results?searc...plod+capacitor

_

[walkura]

Like all others said before its always wise to be carefull and if possible work without power applied .
However you will have to test you're projects so there will always be moments you will work under voltage .
Battery's (and capacitors) store energy which can take hours to charge but depending on what type of battery it is can discharge in seconds .
Don't let youreself be fooled by a nickel cadnium penlight for instance ,you might not expect it .
But we measured peaks over 200 Ampere from a AA Nicad battery .
Since that day i got a lot more carefull with throwing walkman battery's in my pockets .
What i mean to say is this low voltage doesnt mean no danger ,the danger just lies more in the burnwounds .
With battery's in series however 120 volts & up battery's can be very dangerous .
Like before the peakcurrents delivered can be very very high .
Simple car battery's are well capable of delivering 200 Ampere or more (you will have a voltage drop though) .
10 carbattery's in series delivering 120 (138) volt imagine youreself dropping a screwdriver .
I would put my money on the battery that screwdriver goes up in smoke .
Long ago at work i once made a wiring error with 12 gel battery's in series (12 volt 7Ah) a 6mm wire just dissapeared partly ,i was lucky black hands, black face wire & accu destroyed but i wasnt hurted .
Just be carefull with batterypowered projects .
Use fuses were possible and if unfused is needed switch on remotely (saves youre eyebrows).

Goodluck

[therian]

I usually sense 100 volts floating in air with my oscilloscope, it amps what dangerous not volts

[Luciano]

Hi,

The current flowing through your body is determined by
the electrical resistance of the body and the voltage
applied.

Best regards,

Luciano

* * * *

Amount of Body Resistance.

Your body resistance varies greatly in different parts of your body.
A value of 1500 ohms is commonly used as the resistance between major
extremities of an average human body: hand to hand, or hand to foot.

Let’s use Ohm’s Law to figure how much current would flow through
your body if you accidentally grabbed a wire carrying 120 volts
alternating current (vac).

Ohm’s Law for figuring current is I = E/R.

Let E = 120 VAC— The voltage you grabbed
Let R = 1500 Ohms—Your (average) body resistance

Now let’s compute it:

I = 120/1500 = .080 amperes (80 milliamperes).

I = 80 milliamperes. So if you grabbed a 120-vac wire, 80 milliamperes
of current would flow through your body.

=============================================
Human reaction (at 60Hz)

1.1 milliamperes: PERCEPTION - A slight tingling sensation.

10 milliamperes: CAN'T LET GO - Arm and hand muscles close involuntarily. (120 lb. person).

16 milliamperes: CAN'T LET GO (175 lb. person).

18 milliamperes: CAN'T BREATHE - Paralysis of the chest muscles.

65 milliamperes: HEART FIBRILLATION - Rapid irregular contractions of the heart muscles. (Could be fatal).
=============================================

Now use the above table to determine the effect of 80 milliamperes
of electric shock. You can see that you may not be around long enough
to grab any more wires. You grabbed 80 milliamps of current!
That’s 15 milliamps beyond what could be fatal. It’s also 70 milliamps
beyond the can’t- let-go threshold, and 62 milliamps beyond what
is needed to cause you to stop breathing. It’s important to remember that
the 1500 ohms is just an average value. Body resistance varies from
person to person and may often be LESS than 1500 ohms. When your
skin is moist, your body resistance could be as low as 300 ohms. Also, breaks
in your skin at the point of contact reduce your skin resistance to nearly
zero. Skin resistance is only important when you’re handling voltages of
less than 240 volts. If you get shocked by more than 240 volts, the voltage
arc will burn through your skin and leave deep third-degree bums where it
enters your body.

Time of Current Flow.

The longer you’re being shocked, the more chance there is for your heart to
begin fibrillation. Fibrillation is the shocking of your heart into a useless
flutter. Most people who die from electric shock die from fibrillation.
Fibrillation in a normal adult is unlikely if the current in milliamperes is
less than 116/t, where t is the shock duration in seconds. The longer you
are shocked, the less current is needed to cause heart fibrillation. Here are
some examples of shock current levels and durations that would cause fibrillation:

21 milliamperes for 30 seconds
44 milliamperes for 7 seconds

* * *

[versatronics]

How much electricity will kill me?
This is an often asked question. How many Volts? How many Amps? To answer this we need to look at kinds of damage an electric current can cause.

The first and least likely one is tissue damage as a result of burning. Ignoring other electrical effects such as microwave cooking and high frequency transmitter outputs, and looking just at direct electrical contact, burning damage is only likely from very high voltage sources in excess of 1000V for example Eskom power lines and Transnet overhead rail power systems, and as we are not likely to come into contact with these, we will just look at mains borne 220V/380V systems.

This leaves us the most likely cause of death which is the failure of the heart muscle through fibrillation and this will occur when a (generally accepted) current of between 60mA and 1A passes through the heart. The heart's electrical system gets disrupted and it stops pumping. So you might say it's the current that kills you, but wait, to get such a current to pass through the heart we have to satisfy Ohm's law which tells us that to get a current through a resistance (the body and skin resistance) we need a certain voltage. (current = voltage ÷ resistance).

This leads us to the source of the variability: What is the resistance of the body?
If you were to stick a drip needle in each arm and measure the resistance between them, you could get a resistance of as little as 500Ω. To get a current of 60mA to pass through this you would need only 30V! This is why electro-medical equipment has to be so carefully designed. Your skin resistance varies greatly with the dampness, sweat and salinity, and can vary from 1000Ω to 30,000 Ω.
At 1000Ω you would need a voltage of only 60V to be lethal, on the other hand with very dry clean skin at 30,000Ω a voltage of 220V would only produce a tingling 7mA and this is why many folks have had shocks which did them no harm. So at 220V you need a body resistance less than 3700Ω to kill you. Of more concern at higher voltages greater than 500V, skin puncturing can occur, causing a now much lower resistance and potential fatalities.

So we can now answer the question "Is it the current or is it the voltage that kills you?"
Certainly it is the current, but for it to reach a lethal level we need the voltage to drive it. To be safe, treat anything over 24V with caution, and anything over 50V with a great deal of respect. Anything over 100V is certainly able to kill you.

Graham Lambert