When does DC voltage become dangerous?

[shadowjk]

ack there I saw how the banning of light bulbs has taken effect. That is so crazy...)

I thought only the high power bulbs were banned, that shouldn't make a differe..... oh wait, sorry, this is CPF.

 

[LuxLuthor]

Theoretically then, there should be no problem touching a 9V transistor battery to your tongue. First up to the plate?

 

[wyager]

That's a bad thing to assume...

Most large flat panel monitors aren't lit by EL panels. I was surprised to draw an arc across two terminals in one, then found it was lit by two long fluorescent tubes with several hundred volts at 50kHz. Even EL panels run at 100 volts or more. The only true low voltage displays are LED displays, or ones backlit with LEDs like my modified TV at home.

I once worked on laser bar code scanners years ago - they ran on 10kV at 1ma. Yikes! Now they use 3 volt diodes...

Correct. They used a mini Helium-neon laser that runs in the tens of KVs range. Modern diodes almost never run above 6v, and those red diodes never above 3.3v. I've heard that 60V will cause electrical breakdown of dry skin (it becomes conductive). Wet skin, I don't know. I know the medical industry will use saltwater covered electrodes to significantly reduce the required voltage for muscle tests. Also, voltage will never kill you. You could have a billion volts pass through you and be fine, as long as the current is lower. and @PMM, I believe that the AC wave of a 120V household plug extends to 120V either direction, so it's really 240V. They call it 120 because if you used a full-wave rectifier it would come out to be 120. It goes from wire A@0V, wire B@120V to wire A@120V, wire B@0V. Only 2 wires are needed.

Theoretically then, there should be no problem touching a 9V transistor battery to your tongue. First up to the plate?

No. Your tongue is saturated with water, and has no insulating layer. Skin is a decent insulator.
will

 

[Quension]

and @PMM, I believe that the AC wave of a 120V household plug extends to 120V either direction, so it's really 240V. They call it 120 because if you used a full-wave rectifier it would come out to be 120. It goes from wire A@0V, wire B@120V to wire A@120V, wire B@0V. Only 2 wires are needed.

If you're speaking of US 120V household electricity, that isn't correct. It's hard to tell since you're directing your comment to someone in the UK who never mentioned 120V domestic, and I don't know where you are located...

 

[Mr Happy]

Also, voltage will never kill you. You could have a billion volts pass through you and be fine, as long as the current is lower.

Volts don't pass through things, current passes through things. Volts sit across things. As it happens, if you had a billion volts sitting across you then several million amps would be flowing through you and you would be vaporized to atoms.

Likewise, the AC mains is called 120 V because if you connect it to a resistor (or person) it behaves like 120 V (not 240 V). Even though the voltage is alternating, you can never get +120 and -120 at the same time (though you can get a peak voltage of 170 V).

 

[TorchBoy]

I believe that the AC wave of a 120V household plug extends to 120V either direction, so it's really 240V. They call it 120 because if you used a full-wave rectifier it would come out to be 120. It goes from wire A@0V, wire B@120V to wire A@120V, wire B@0V. Only 2 wires are needed.

It's called 120 V because that's (approximately) what its RMS value is. The presence of 240 V AC in the United States I think comes from two phases which have a potential difference of 240 V between them. In other words, they're 180° out of phase. Using only one phase, you have a potential difference between 120 V AC and 0 V AC (ground). Using the two phases your potential difference is between 120 V AC and -120 V AC (not sure about the notation for negative AC).

A 9V battery on the tongue feels fuzzy, and might even be painful if applied to the tip of the tongue. Several of them in series can cause instant burns (as seen on YouTube).

 

[Apollo Cree]

You could have a billion volts pass through you and be fine, as long as the current is lower.

NO.

You could have a device that generates 100,000 volts open circuit that is current limited to, for instance, 1 mA. When you touch it, the voltage of the device will drop to some much lower voltage, perhaps 100V.

You still need to be careful. Even if the device will only generate 1 mA long term, there will always be some capacitance in the device and things it's connected to. You will get a higher current when you first touch the device until the capacitance in the circuit is discharged.

This capacitance is, for instance, the reason you get a small zap when you walk across a carpet and touch a doorknob. It's lethal voltage and current, but it only lasts a tiny fraction of a second. If you have a high voltage, low current device and it gets connected to something with a lot of capacitance, it CAN kill you.

I believe that the AC wave of a 120V household plug extends to 120V either direction, so it's really 240V. They call it 120 because if you used a full-wave rectifier it would come out to be 120. It goes from wire A@0V, wire B@120V to wire A@120V, wire B@0V. Only 2 wires are needed.

NO!!! absolutely wrong.

Common US household 120VAC wiring.

You have a "neutral" wire connected to earth ground somewhere. It is always 0V.

There is a "hot" wire.

The voltage on the hot wire goes from

0V
+170V
0V
-170V

Over a time period of 1/60 second.

If you run it through a full-wave rectifier, you will get a peak voltage of 170V. If you put a capacitor on it to get DC, you'll get 170V.

It's called "120V" because that's the effective power in terms of a resistive load like an incandescent light bulb. The 120V refers to RMS (Root Mean Square) voltage. A 120V RMS AC voltage will provide the same amount of power to an incandescent bulb as a 120 V DC supply.

The maximum voltage present on a US household circuit is 170V. Even though the voltage goes from +170 to -170, there's no way to connect yourself between the + and - parts of the waveform because they occur at different times.

The common US household 240 VAC circuit has one neutral wire and two hot wires. The hot wires are 180 degrees out of phase. When hot wire A is at +170, hot wire B is at -170 V and vice versa. A 240V load will be wired between the two hot wires and will see + and - 340V peak voltage for an effective 240 VAC RMS voltage.

If you touch one conductor of a standard US household 240 VAC circuit, you only see 120 VAC. You have to touch both hot wires to see 240 VAC.

 

[Apollo Cree]

Will those of you who don't understand electrical safety stop posting things like "XYZ" is safe?



You are scaring this old engineer.

It's one thing to post something incorrect and cause someone to buy the wrong flashlight. It's much worse to post incorrect info and kill someone.

 

[Apollo Cree]

Theoretically then, there should be no problem touching a 9V transistor battery to your tongue. First up to the plate?

Been there, done that, got the T-shirt.

I've always stopped doing it really quickly. I don't recommend the experience to anyone.

 

[wyager]

If you're speaking of US 120V household electricity, that isn't correct. It's hard to tell since you're directing your comment to someone in the UK who never mentioned 120V domestic, and I don't know where you are located...

Sorry, I'm here in the USA. I have no idea about the UK system other than it's referred to as 240V.

Volts don't pass through things, current passes through things. Volts sit across things. As it happens, if you had a billion volts sitting across you then several million amps would be flowing through you and you would be vaporized to atoms.

Likewise, the AC mains is called 120 V because if you connect it to a resistor (or person) it behaves like 120 V (not 240 V). Even though the voltage is alternating, you can never get +120 and -120 at the same time (though you can get a peak voltage of 170 V).

Sorry, I gave a pretty bad explanation. If you had a billion volts "sitting across" you and a trillion ohm resistor in series then the amperage flowing through you should be safe.... less than 1mA, right? 1000000000/1000000000000 or more=.001 amps or less. And yeah, that's what I was trying to say about AC. It deviates so that the maximum voltage difference at any one time is <170V. I mistakenly wrote 120V.

You still need to be careful. Even if the device will only generate 1 mA long term, there will always be some capacitance in the device and things it's connected to. You will get a higher current when you first touch the device until the capacitance in the circuit is discharged.

I was talking about a theoretical circuit-I'm aware that current will build up in something left alone.

Common US household 120VAC wiring.
You have a "neutral" wire connected to earth ground somewhere. It is always 0V.
There is a "hot" wire.
The voltage on the hot wire goes from
0V
+170V
0V
-170V
Over a time period of 1/60 second.
If you run it through a full-wave rectifier, you will get a peak voltage of 170V. If you put a capacitor on it to get DC, you'll get 170V.
It's called "120V" because that's the effective power in terms of a resistive load like an incandescent light bulb. The 120V refers to RMS (Root Mean Square) voltage. A 120V RMS AC voltage will provide the same amount of power to an incandescent bulb as a 120 V DC supply.
The maximum voltage present on a US household circuit is 170V. Even though the voltage goes from +170 to -170, there's no way to connect yourself between the + and - parts of the waveform because they occur at different times.
The common US household 240 VAC circuit has one neutral wire and two hot wires. The hot wires are 180 degrees out of phase. When hot wire A is at +170, hot wire B is at -170 V and vice versa. A 240V load will be wired between the two hot wires and will see + and - 340V peak voltage for an effective 240 VAC RMS voltage.
If you touch one conductor of a standard US household 240 VAC circuit, you only see 120 VAC. You have to touch both hot wires to see 240 VAC.

Ah, I see. Sorry about that, I thought each wire alternated. I was aware, however, that you could not get the max positive and negative voltage at the same time. And I have to do some more research on RMS, it's still fuzzy for me but thanks for the help!

Been there, done that, got the T-shirt.

I've always stopped doing it really quickly. I don't recommend the experience to anyone.

this is probably just me, but I love the experience! Probably a bad idea, but when I was a little kid I touched my tongue so much with the 9V battery I got a big brown mark in the shape of battery terminals and a line between them my parents didn't let me have any batteries for a while after that....

will

 

[Apollo Cree]

Sorry, I gave a pretty bad explanation. If you had a billion volts "sitting across" you and a trillion ohm resistor in series then the amperage flowing through you should be safe.... less than 1mA, right? 1000000000/1000000000000 or more=.001 amps or less.

999,999,999 V would be sitting across the resistor and 1V would be sitting across you.

If you're standing under a 384 KV power line, 383,999.9 volts are standing across the air between you and the powerline overhead and 0.1 V is standing between your head and your feet.

Also, when you first reach up and touch the bare terminal on the billion volt resistor, the capacitance between the wire and the ground might carry enough joules to electrocute you.

BTW, I'd like to see a billion volt 1 trillion ohm resistor. It would need to be 500 feet long or the voltage would simply arc between the terminals through the air.

 

[wyager]

999,999,999 V would be sitting across the resistor and 1V would be sitting across you.

If you're standing under a 384 KV power line, 383,999.9 volts are standing across the air between you and the powerline overhead and 0.1 V is standing between your head and your feet.

Also, when you first reach up and touch the bare terminal on the billion volt resistor, the capacitance between the wire and the ground might carry enough joules to electrocute you.

BTW, I'd like to see a billion volt 1 trillion ohm resistor. It would need to be 500 feet long or the voltage would simply arc between the terminals through the air.

My DMM says I'm about 18MΩ.... so 18MΩ + 1 Trillion Ω = 1000018000000, and 18000000/1000018000000=1.7999676 × 10^-5, and 1000000000V*1.7999676 × 10^-5=17999.676V. So there would still be significant voltage going through me, right? And once again, theoretical circuit... no capacitance, no dielectric breakdown. Also the resistor would have to be about 850M long in dry air, if there were dielectric breakdown.

will

 

[jtr1962]

This capacitance is, for instance, the reason you get a small zap when you walk across a carpet and touch a doorknob. It's lethal voltage and current, but it only lasts a tiny fraction of a second. If you have a high voltage, low current device and it gets connected to something with a lot of capacitance, it CAN kill you.

That's exactly what I meant in an earlier post when I said it's really the total number of joules which kill you, not voltage or current. As a general rule, always be cautious around large capacitors, especially those charged to ~50 V or more ( although as I mentioned in my earlier post a large enough 12V capacitor can still store enough energy to kill you, given a low impedance path through your body ).

 

[Apollo Cree]

That's exactly what I meant in an earlier post when I said it's really the total number of joules which kill you, not voltage or current. As a general rule, always be cautious around large capacitors, especially those charged to ~50 V or more ( although as I mentioned in my earlier post a large enough 12V capacitor can still store enough energy to kill you, given a low impedance path through your body ).

No, "joules" is still absolutely wrong in terms of what it takes to kill you. High current, short duration without that many joules can kill you. Low current, long duration with lots of joules may be entirely safe.

 

[griff]

touch a 9v battery to your tongue

 

[Apollo Cree]

My DMM says I'm about 18MΩ.... so 18MΩ + 1 Trillion Ω = 1000018000000, and 18000000/1000018000000=1.7999676 × 10^-5, and 1000000000V*1.7999676 × 10^-5=17999.676V. So there would still be significant voltage going through me, right? And once again, theoretical circuit... no capacitance, no dielectric breakdown. Also the resistor would have to be about 850M long in dry air, if there were dielectric breakdown.

will

There would be no voltage "going through" you. Voltage exists across you, not through you. It's like saying there's 6 feet of height "flowing though" your body from your head down to your feet.

In theory, the current would only be 1 mA flowing through you. If your body's resistance was really 18MΩ, you'd have 18 KV across your body, but only 1 mA so you wouldn't be electrocuted.

However, if you body resistance was really 18MΩ, you wouldn't be able to electrocute yourself with household current. 120 V would only produce 0.006 mA and you probably wouldn't even feel it.

Measuring the body's resistance is very tricky. It varies so widely with varying conditions that measuring it with a home voltmeter is almost meaningless. I think the human body is also not really a "pure" resistor and the measured resistance varies with the applied voltage.

 

[TorchBoy]

I said it's really the total number of joules which kill you, not voltage or current.

Everything I can recall on the matter says it is current that kills, and maybe not very much of it. I'll side with Apollo Cree on that.

 

[Mr Happy]

Everything I can recall on the matter says it is current that kills, and maybe not very much of it. I'll side with Apollo Cree on that.

It's actually sort of complicated. Your heart is like an oscillator, with waves of electricity flowing across it in a regular cycle. Think of it like a pendulum. If you push a pendulum at the right time during its swing it just swings a bit higher and carries on. But if you push a pendulum at the wrong time you can stop it dead so it's hardly swinging at all.

So it is with your heart. If an electric shock from outside the body hits the heart at the right time in the cycle nothing may happen. But if it hits the heart at the wrong time it can kick the heart out of its normal cycle, leaving it effectively stopped. Whether or not this happens depends on when the shock arrives and how long it lasts for.

The defibrillators that they have in hospitals deliver a carefully measured shock to the heart of a size and duration to kick it back into its normal rhythm. It's like giving a pendulum a big push in the right direction to get it swinging again.

But if there is one thing to take away from this thread, it is that electricity is dangerous. There are unpredictable circumstances that mean no voltage is guaranteed safe, and no current is guaranteed harmless. If you want to avoid the risk of death, don't get casual when working with electrical equipment, and don't assume that it won't happen to you.

 

[LuxLuthor]

No. Your tongue is saturated with water, and has no insulating layer. Skin is a decent insulator.
will

Oh come one. Don't be a wuss. Apollo Cree did it, and got a free t-shirt.

 

[Apollo Cree]

Oh come one. Don't be a wuss. Apollo Cree did it, and got a free t-shirt.

Yup!

By the way, notice they got the apostrophe wrong. :devil:

Yes, it was free. I only had to pay $199 processing to Billy Mays.