DC versus AC safety


by adoado
Tags: safety, versus
adoado
adoado is offline
#1
Nov24-11, 06:51 AM
P: 72
Hey guys,

My understanding of circuit analysis and the like is very minimal, and I have had a certain lack of understanding about something for a long time, and I feel I need to get a solid answer.

I understand the fundamental difference between a voltage or potential difference and a current, but I fail to see 'when' a combination of these becomes dangerous. The general statement I hear is 'DC is safe, AC is dangerous' but why? Is there a blanket current - x Amps, that is dangerous? And if so, why?

Cheers for reading!
Adrian
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mrspeedybob
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#2
Nov24-11, 07:53 AM
P: 686
DC is not safe! I deal with DC power sources in the 300 - 400 volt range on a regular basis. I expect that if I ever become part of the circuit I will die.

In general the DC sources you will come in contact with are low voltage systems like flashlight batteries (1.5 volts) or car batteries (12.6 volts). These voltages will not force enough current through your body to be harmful. In general the AC power sources you will come into contact with are utilities. A 120 volt household circuit will force enough current through your body to harm you. The difference has nothing to do with AC vs. DC, it has everything to do with the voltage.

Voltage by itself is not particularly harmful. I could expose you to 10,000,000 volts and under the right conditions you will experience no ill effects and feel nothing. Current and duration must also be considered. The following chart addresses current...

I was unable to find information on the effects of duration but it obviously plays a roll. If the duration of an electric shock was short enough then there would be just 1 electron involved. 1 electron would need some bodacious voltage behind it to be harmful

Voltage is relevant in that it is the electrical pressure which causes current to flow. Ohms law states that I=V/R where I is current, V is voltage, and R is resistance. If a particular path through your body is 10,000 ohms then 1 volt will result in a current flow of .0001 amperes, that is not enough to even feel. You can also compute the amount of electrical power being dissipated by your body by multiplying the volts by the amperes, in this case it is 0.0001 watts. Now if 100 volts was applied to the same path it would result in a current flow of .01 amperes. According the the chart that's enough to be painful. Power flow in this case would be 1 watt. 10,000 times the first scenario. So you can see that power flow through your body will increase with the square of voltage.
richard w
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#3
Nov24-11, 04:13 PM
P: 1
AC current is much more safer than DC from what I know...... On handling point of view DC could probably be safer.....

adoado
adoado is offline
#4
Nov24-11, 07:20 PM
P: 72

DC versus AC safety


Awesome, thanks a lot for the replies! Using your myspeedybob as a guide, I found this:

http://www.tpub.com/content/et/14086/css/14086_34.htm

It explains pretty much exactly what you said. So I guess as a general rule, assume the resistance of the body is R = 1500 Ω. Then, find the current by the voltage employed, and reference this to that chart?
davenn
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#5
Nov24-11, 07:53 PM
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Quote Quote by richard w View Post
AC current is much more safer than DC from what I know...... On handling point of view DC could probably be safer.....
no, read the previous post by Mr speedybob

in addition, any voltage over ~ 50V AC or DC is considered dangerous, as mr speedy bob alluded to, there is enough voltage to push enough current through you to kill you.

it only takes 30 - 40mA through the heart to put it into fibrilation and that doesnt matter if its AC or DC

Dave
mrspeedybob
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#6
Nov24-11, 09:21 PM
P: 686
Quote Quote by adoado View Post
Awesome, thanks a lot for the replies! Using your myspeedybob as a guide, I found this:

http://www.tpub.com/content/et/14086/css/14086_34.htm

It explains pretty much exactly what you said. So I guess as a general rule, assume the resistance of the body is R = 1500 Ω. Then, find the current by the voltage employed, and reference this to that chart?
The 1500Ω figure is meaningless. Depending on where I touch the leads to my skin my fluke reads anywhere from 500Ω to 90kΩ.
cepheid
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#7
Nov24-11, 09:31 PM
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Quote Quote by mrspeedybob View Post
DC is not safe! I deal with DC power sources in the 300 - 400 volt range on a regular basis. I expect that if I ever become part of the circuit I will die.

In general the DC sources you will come in contact with are low voltage systems like flashlight batteries (1.5 volts) or car batteries (12.6 volts). These voltages will not force enough current through your body to be harmful. In general the AC power sources you will come into contact with are utilities. A 120 volt household circuit will force enough current through your body to harm you. The difference has nothing to do with AC vs. DC, it has everything to do with the voltage.

Voltage by itself is not particularly harmful. I could expose you to 10,000,000 volts and under the right conditions you will experience no ill effects and feel nothing. Current and duration must also be considered. The following chart addresses current...

I was unable to find information on the effects of duration but it obviously plays a roll. If the duration of an electric shock was short enough then there would be just 1 electron involved. 1 electron would need some bodacious voltage behind it to be harmful

Voltage is relevant in that it is the electrical pressure which causes current to flow. Ohms law states that I=V/R where I is current, V is voltage, and R is resistance. If a particular path through your body is 10,000 ohms then 1 volt will result in a current flow of .0001 amperes, that is not enough to even feel. You can also compute the amount of electrical power being dissipated by your body by multiplying the volts by the amperes, in this case it is 0.0001 watts. Now if 100 volts was applied to the same path it would result in a current flow of .01 amperes. According the the chart that's enough to be painful. Power flow in this case would be 1 watt. 10,000 times the first scenario. So you can see that power flow through your body will increase with the square of voltage.
So the numbers on the bottom (in the log scale) are current in mA? What, then are the numbers on the left-hand side of each bar? What do the shaded areas represent?
taterz
taterz is offline
#8
Nov24-11, 10:48 PM
P: 6
Quote Quote by adoado View Post
Hey guys,

My understanding of circuit analysis and the like is very minimal, and I have had a certain lack of understanding about something for a long time, and I feel I need to get a solid answer.

I understand the fundamental difference between a voltage or potential difference and a current, but I fail to see 'when' a combination of these becomes dangerous. The general statement I hear is 'DC is safe, AC is dangerous' but why? Is there a blanket current - x Amps, that is dangerous? And if so, why?

Cheers for reading!
Adrian
I understand that any voltage, AC or DC, being greater than 35 V is dangerous. DC sources are usually low voltage (i.e. battery) so for around the home the DC sources (batteries - less than 12V) are safe but the AC source (house wiring) is 120 VAC and dangerous.
taterz
taterz is offline
#9
Nov24-11, 10:51 PM
P: 6
By the way, I have a Chevy Volt. DC voltage of 1000 volts. I open the hood, pull back the insulator on two of the wires (+ and - on the 1000 volt supply) touch these two wires --- I die. The Chevy technicians that work on the Volt had to go through special training on how to handle the high voltage.


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