Volts vs Amps

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
OK then, WHY are the threshold values always expressed in mA as opposed to volts or watts?

Is it because the authors of those values don't know what they are talking about? If you and Iwire's stance is indeed true, then why hasn't anyone else discovered this a long time ago and changed 'the books'?
Because it's a convenient way to measure it (assuming an ideal current source and a constant human body resistance), that's all. There is also a voltage threshhold (assuming an ideal voltage source and a constant human body resistance) that will kill. Current drives voltage or voltage drives current; you can express it either way and neither is more correct than the other. Ohm's Law.
 
So I have a lot of trouble trying to get my head around the concept of volts vs. amps. I've heard the water pressure vs amount available concept but that doesn't seem to work with many examples. Youtube has good ones about rocks that fall but from different heights but that doesn't explain how a 9V battery tickles your tongue but a 6V truck battery has enough amps to kill you! I was trying to use the concept of the strong body-builder laborer who can't do much work because he gets hungry too quickly, of has no skills. Or the concept of the really hot super sexy babe but she is totally frigid; you know, all volts but no amps. \ Does anyone have good links? about volts/amps I mean?

The answers that helped you best about the difference between a 9V battery and a truck battery were the ones that talked about "internal resistance".

Imagine putting a perfect, superconducting short across the terminals of both batteries. You won't get infinite current even though I = V / R. The 9V battery, the 6V battery, and your cat's fur are all resistors as well as voltage sources. The plates in the truck battery are as big as bus bars. Internal resistance is very low and if you look at videos of shorted vehicle batteries it's scary. Rub a glass rod on your cat and you get tens of thousands of volts, but the fur is an insulator. The 9V battery is in between.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
I am pretty sure you will find voltage thresholds in OSHA but I could be wrong.

However you may want to look at this

http://ecmweb.com/shock-amp-electrocution/small-contact-voltage-exposures-not-lethal-human

Good article, thanks.

Yeah, we kind of overlooked time.

The time/current curve’s validity is backed up by actual mortality information.

Yes, there is mention of a 50 volt value, but that's for surface contact only, the above is the 'bottom line'. 12 volts can be lethal if the conductors are inside the body like an IV needle.
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Sorta retired........
I'm sure many could run with hot chick concept - but would have their posts removed because we are supposed to be nice on this site;)

Well, at least professional...
Nice got tossed a long time ago.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I dunno.

So far, everyone here has been very nice to me.
That's why I prefer moderated forums (or fora for you Latin sticklers). Remember Usenet? Flame on! :rant::rant::rant::rant::rant::rant::rant::rant:
 

Carultch

Senior Member
Location
Massachusetts
OK then, WHY are the threshold values always expressed in mA as opposed to volts or watts?

Is it because the authors of those values don't know what they are talking about? If you and Iwire's stance is indeed true, then why hasn't anyone else discovered this a long time ago and changed 'the books'?

The way that I see it, people always say "it's not the volts that is dangerous, it's the amps". And, while this is oversimplified, I understand that the intention of saying it this way, is to remind you that absolute voltage level itself isn't dangerous. It is only when the voltage can cause a current to flow, that it is dangerous.

In otherwords, think of the "bird on the wire". The bird is in contact with a high voltage relative to ground, but isn't in contact with anything other than that line of higher voltage. It is not absolute voltage at a particular location that does harm. It is a voltage difference causing current to flow. Volts still do directly determine the amps, but it is specifically Volts at point A minus Volts at point B.

An interesting question I thought of, is whether or not the ampere threasholds are valid no matter what the skin condition of resistance is. In otherwords, consider this example:
10 milliamps driven by 1000V through a 100 kOhm person with dry skin
10 milliamps driven by 10 Volts through the same person, but 1 kOhm after going swimming for a while (across the same two points on the body/through the same path in the body)

Are these two shock situations equivalent in danger?

I would expect that the first situation is much more dangerous. I would think that it is really cumulative Watt-seconds that would be a better metric of shock hazard.
 

mivey

Senior Member
And there isn't going to be any voltage unless there are transmission lines, and there aren't going to be any transmission lines if there isn't a generator and there isn't going to be a generator without fuel. So I guess by your and iWire's thinking, fuel is what actually electrocutes people.

I don't care if there wouldn't be current flow if there wasn't voltage potential to allow the flow. It is still and always will be current flow that electrocutes people.
Iwire addressed the rest but if I may pick one nit:

It is current, not current flow. Current is charge flow and you would not say charge flow flow. Just for the record.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
The way that I see it, people always say "it's not the volts that is dangerous, it's the amps". And, while this is oversimplified, I understand that the intention of saying it this way, is to remind you that absolute voltage level itself isn't dangerous. It is only when the voltage can cause a current to flow, that it is dangerous.

In otherwords, think of the "bird on the wire". The bird is in contact with a high voltage relative to ground, but isn't in contact with anything other than that line of higher voltage. It is not absolute voltage at a particular location that does harm. It is a voltage difference causing current to flow. Volts still do directly determine the amps, but it is specifically Volts at point A minus Volts at point B.

An interesting question I thought of, is whether or not the ampere threasholds are valid no matter what the skin condition of resistance is. In otherwords, consider this example:
10 milliamps driven by 1000V through a 100 kOhm person with dry skin
10 milliamps driven by 10 Volts through the same person, but 1 kOhm after going swimming for a while (across the same two points on the body/through the same path in the body)

Are these two shock situations equivalent in danger?

I would expect that the first situation is much more dangerous. I would think that it is really cumulative Watt-seconds that would be a better metric of shock hazard.

The study linked a few posts up is couched in terms of voltage and the wattage it produces rather than current, but almost in passing it mentions that a worst case skin and body resistance of 500 ohms is assumed.
Medical studies have shown that whatever driving voltage is necessary, the effects for a given current path are directly correlated with current and time.
When the resistance is higher the lethal voltage is correspondingly higher.

At some point in voltage you get a punch through effect on skin resistance, and so the damage versus voltage curve is not smooth.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
It we should bury 'amperage' maybe we should bury 'voltage' too?

(Are there any other units we talk about that way? Hmm. Mileage. But that means something different. And no one says kilometerage.)
 

kwired

Electron manager
Location
NE Nebraska
Think about the bird on the wire.

The bird can be at a voltage level of thousands of volts, but he is OK. He can be 1,000's of volts above or below the earth, the next wire over, or the moon for that matter.

He will be just fine until he does something that causes current to flow through his body. Then, no more bird.

All those years being at 4,160 volts without so much as a rash. Then he gets a bit to big for his britches and takes 900 mA from wing to wing.

What killed him? the 900 mA, or the 4,160 volts?

The power of provided by volts and amps together.
which was about 37.5 VA if 900mA @4160 volts.

OK then, WHY are the threshold values always expressed in mA as opposed to volts or watts?

Is it because the authors of those values don't know what they are talking about? If you and Iwire's stance is indeed true, then why hasn't anyone else discovered this a long time ago and changed 'the books'?
My guess is most information is based on 120-277 volts nominal, possibly leaning more toward 120 since that seems to be most common voltage to ground that people get caught up in. Increase voltage and leave all other conditions the same and you will have more current flow, so if conditions were right to kill you at 120 then they also should at 277, 2400, 7200 with no question.

The way that I see it, people always say "it's not the volts that is dangerous, it's the amps". And, while this is oversimplified, I understand that the intention of saying it this way, is to remind you that absolute voltage level itself isn't dangerous. It is only when the voltage can cause a current to flow, that it is dangerous.

In otherwords, think of the "bird on the wire". The bird is in contact with a high voltage relative to ground, but isn't in contact with anything other than that line of higher voltage. It is not absolute voltage at a particular location that does harm. It is a voltage difference causing current to flow. Volts still do directly determine the amps, but it is specifically Volts at point A minus Volts at point B.

An interesting question I thought of, is whether or not the ampere threasholds are valid no matter what the skin condition of resistance is. In otherwords, consider this example:
10 milliamps driven by 1000V through a 100 kOhm person with dry skin
10 milliamps driven by 10 Volts through the same person, but 1 kOhm after going swimming for a while (across the same two points on the body/through the same path in the body)

Are these two shock situations equivalent in danger?

I would expect that the first situation is much more dangerous. I would think that it is really cumulative Watt-seconds that would be a better metric of shock hazard.

Conditions that change your body resistance do have impact. I have worked on doorbell buttons, chimes etc. that operate at 16 VAC and you generally never have to worry about feeling that voltage if it is energized. I remember one time on a new home the masons were washing their brick and were using muratic acid which is common, the wire for the doorbell button was energized and with the acid present (even after rinse there was still enough to make things pretty conductive) 16 volts across those two leads and through just a portion of your hand of the push button circuit made you think you contacted a 120 volt line. Only time I was ever shocked by 16 volt doorbell circuit.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
OK then, WHY are the threshold values always expressed in mA as opposed to volts or watts?

Is it because the authors of those values don't know what they are talking about? If you and Iwire's stance is indeed true, then why hasn't anyone else discovered this a long time ago and changed 'the books'?

IMHO Iwire's basic point, that voltage and current will both be present during an electric shock injury, is quite valid. Unless someone has been rendered superconducting there will of necessity be voltage present whenever there is current flow through a human body. (And if someone has been rendered superconducting, they probably have other problems to deal with!)

There is, however, a useful basis for using _current_ to describe electric shock damage.

In particular, for low level shocks which cause injury by disrupting muscle action, the level of damage appears to be related to the amount of current, not the total power delivered. The voltage needed to deliver a damaging current flow will vary all over the place, depending on the resistance of the specific injury path.

Since the current is the relative constant and the voltage is all over the place, describing the current and ignoring the voltage makes sense...but the voltage must be present!

-Jon
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
It's not the fall that kills ya'; it's the sudden stop at the bottom. :D
 

Carultch

Senior Member
Location
Massachusetts
It we should bury 'amperage' maybe we should bury 'voltage' too?

(Are there any other units we talk about that way? Hmm. Mileage. But that means something different. And no one says kilometerage.)

Language development is a strange thing. Who has the authority to decide when a newly evolved word is a real word, and when it is slang?

The English language is not a dead language like Latin, which means it has exactly the same vocabulary and grammar 100 years ago as it does today.
 
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