Re: Human electrical resistance

[donheff]

Re: Human electrical resistance

Our photovoltaic panels only generate 24V. But if I didn't treat that with the same paranoia I used to reserve for 440 VAC, the current would launch me across the street like a fried sausage before I realized that I should have disconnected something.

I know thais is way, way off topic but a bunch of you are engineers and Nord's staement just reminded me of something that has always bothered me but I have never gotten a good explanation about. The issue of amps and voltage in basic electronics always confused the heck out of me. V=A/O tells me that 24 volts is not going to deliver a significant amperage through a human being but I understand that in some circumstances it can. Is it basically that I drasticaly overestimate the resistance of the human body? I.e is it that pretty much any old source that is capable of delivering high amperage will drive a boat load through us? And the reason most "high voltage" sources (like the 10,000 V sign transformer I got zapped by as a kid) just can't deliver mcuh in the way of amps so the voltage drops in low load situations? How much resistance does a body have?

 

[DRiP Guy]

Re: Human electrical resistance

I know thais is way, way off topic but a bunch of you are engineers and Nord's staement just reminded me of something that has always bothered me but I have never gotten a good explanation about. The issue of amps and voltage in basic electronics always confused the heck out of me. V=A/O tells me that 24 volts is not going to deliver a significant amperage through a human being but I understand that in some circumstances it can. Is it basically that I drasticaly overestimate the resistance of the human body? I.e is it that pretty much any old source that is capable of delivering high amperage will drive a boat load through us? And the reason most "high voltage" sources (like the 10,000 V sign transformer I got zapped by as a kid) just can't deliver mcuh in the way of amps so the voltage drops in low load situations? How much resistance does a body have?

Resistance of dry skin is in the Megohms. If we say wall outlets are around 100V, that limits current through the body to milliamps, AND it will be distributed broadly through the salt water bag that your organs look like electrically. Not concentrated in heart area. I have seen electricians (don't try this!) with calloused fingers who can touch a terminal and tell you "oh that's dead", "oh thats single phase 110", "oh that's high voltage". Again, don't try it.

The best way to think of electricity is good old Ohm's law E= I*R, where E = voltage (pressure of water in the neighborhood water tower), I = current (the Everglades is a river of grass -- not much voltage, but a heck of a lot of current) and R = resistance (the diameter of the hose, or a kink in the hose).


The key to understanding physiological response to shock is is two things:
1) when anything breaks the skin (indwelling catheter), the skin's resistance is bypassed, and any current is concentrated in that area - both usually bad
and
2) the heart is a lot more sensitive to AC than to DC (60HZ is about as bad as it can get, in fact)

So if you bypass the skin's resistance, and you concentrate the current (a needle, a probe, and that current is AC, it can cause fibrillation at current densities below your normal level of even feeling!

Next week we cover hole flow in solid state semi-conductors!

 

[Nords]

Re: Human electrical resistance

V=A/O tells me that 24 volts is not going to deliver a significant amperage through a human being but I understand that in some circumstances it can. Is it basically that I drasticaly overestimate the resistance of the human body? I.e is it that pretty much any old source that is capable of delivering high amperage will drive a boat load through us? And the reason most "high voltage" sources (like the 10,000 V sign transformer I got zapped by as a kid) just can't deliver mcuh in the way of amps so the voltage drops in low load situations? How much resistance does a body have?

Body resistance changes so much under different conditions that it's difficult to say. The resistance of the electron's "path to ground" is another variable. That dry-skin resistance will change dramatically if you've been sweating and you're standing in a mud puddle. And is that voltage a DC voltage or an AC voltage?

Naval Ships Technical Manuals used to sidestep the issue of human resistance by requiring anything over 30V to be considered as hazardous. On an "average" human being that'll result in a current as little as a couple hundred milliamps, but it can still disrupt cardiac function or brain activity. For example if you reach up and grab a wire while you're sweating in that mud puddle, encouraging electrons to flow at a rate of 100 mA, as DRiP says you'll probably survive long enough to say "Ouch!" and you may even retain enough muscle control to let go. If however you grab a wire in each hand and the 100 mA current travels across your chest, or if the metal frame of your eyeglasses is grounded to the panel you're working inside, then you won't be contributing a statement to the mishap investigation. Even being alive after a shock is no guarantee of survival-- cardiac function can be disrupted enough to later cause fibrillation or even arrest. One of the first things a Navy electrician gets after surviving an electrocution is an ECG.

People familiar with that 30V rule tend to make assumptions that can get them into trouble. You're right that the hazard is proportional to the amperage traveling through the body, not the voltage of the source. But working with a photovoltaic panel should be no problem since they're usualy 12-24 volts, right? Sorry. A 75-watt panel operating on 12 volts will pump out over six amps. The panel has very little resistance and develops one heck of an electron flow. (That's its job.) If you become that panel's only path to ground then you're probably not going to survive the experience. Most survivors were lucky to not be the source's only path to ground.

So even after the circuit is shut off & de-energized (as far as I can tell), I work one-handed and wear sneakers.

 

[DRiP Guy]

Re: Human electrical resistance

... over six amps.

..I work one-handed and wear sneakers.

6 amps - yikes!

Excellent points. I guess good habits die hard, too -- to this day, I also follow the one hand and no jewelry rules that I learned as a teenager, even when working with 12V auto systems. Overkill? Not if you've seen a wedding band melted to a finger by shorting through a handwrench to an auto battery terminal... I found it is far easier to just try to follow safe conventions rather than even worry about which are safe voltages...

 

[thetucsonkid]

Re: Human electrical resistance

As Drip Guy said, dry skin has very high electrical resistance. While it only takes about 10 milliamps to do significant damage, at 24 volts the amount of current that passes through dry skin is not high enough to hurt you. OSHA only requires Lockout/Tagout of voltages over 50V. However, being a safety conscious electrical engineer, I require my electricians to Lockout/tagout every circuit and to check 3 times, once with a voltage detector, once with a meter, and the third a short to ground.

 

[Martha]

Re: Human electrical resistance

Man you guys are nerds. But I love you anyway.

 

[donheff]

Re: Human electrical resistance

I guess my confusion results from brain damage from electric shock. Here is how it started. I always understood that voltage = amperes over ohms (or whatever the heck it was). Then when I was a kid I made a Jacob's ladder (one of those two rod things with the electric arc traveling up the rods ala Dr Frankenstein) using a 10,000 volt sign transformer to generate the spark. When the rods got a little too wide the spark stopped forming at the bottom. Without thinking I reached out and squeezed them tighter I was surprised that the 10,000V shock felt about the same as a 110V shock. Several people told me that the sign transformer can't deliver enough amperage to shock you any worse than a 110 circuit -- which can deliver a lot of amperage.

Soooo, if the above is true, my conclusion is that the sign transformer must only be able to generate the full 10,000V when the resistance is pretty high. If you put a low resistance load on the transformer its voltage must drop or it would deliver a boatload of amperage. Am I correct in my assumptions? Similarly, I am comfortable testing a 9V transistor battery with my tongue but I would hesitate to lick a car battery. Am I correct to worry? If so, does that mean the transistor battery does not generate the full 9V pressure in the environment of my salty, wet tongue?

 

[donheff]

Re: Human electrical resistance

Addendum. Lest my cred as a nerd be questioned because of my confusion about basic electricity, I need to reassure everyone that I am conversant about weird stuff that has no practical value. For example, after a few drinks on New Year's eve I explained to doubting friends why quantum interference proves that there are parallel universes.

 

[DRiP Guy]

Re: Human electrical resistance

I guess my confusion results from brain damage from electric shock. Here is how it started. I always understood that voltage = amperes over ohms (or whatever the heck it was). Then when I was a kid I made a Jacob's ladder (one of those two rod things with the electric arc traveling up the rods ala Dr Frankenstein) using a 10,000 volt sign transformer to generate the spark. When the rods got a little too wide the spark stopped forming at the bottom. Without thinking I reached out and squeezed them tighter I was surprised that the 10,000V shock felt about the same as a 110V shock. Several people told me that the sign transformer can't deliver enough amperage to shock you any worse than a 110 circuit -- which can deliver a lot of amperage.

Soooo, if the above is true, my conclusion is that the sign transformer must only be able to generate the full 10,000V when the resistance is pretty high. If you put a low resistance load on the transformer its voltage must drop or it would deliver a boatload of amperage. Am I correct in my assumptions? Similarly, I am comfortable testing a 9V transistor battery with my tongue but I would hesitate to lick a car battery. Am I correct to worry? If so, does that mean the transistor battery does not generate the full 9V pressure in the environment of my salty, wet tongue?

Your assumption that systems must make trade-offs as to whether they are optimized for high voltage OR for high current is generally correct. That is why the second formula P=I*E is a good one to recall; static electricity (e-field) is in the thousands of volts DC, but it cannot sustain delivering much current for very long. The Van de Graaff generator is a fun tool to explore with, as well, to seal these concepts in the mind.

http://amasci.com/emotor/voltmeas.html

 

[ERD50]

Re: Human electrical resistance

I was surprised that the 10,000V shock felt about the same as a 110V shock. Several people told me that the sign transformer can't deliver enough amperage to shock you any worse than a 110 circuit -- which can deliver a lot of amperage.

Soooo, if the above is true, my conclusion is that the sign transformer must only be able to generate the full 10,000V when the resistance is pretty high. If you put a low resistance load on the transformer its voltage must drop or it would deliver a boatload of amperage. Am I correct in my assumptions?

Correct. That xfmr produced high voltage but very little current capability (total power was probably still fairly low. Power = E*I; E was high, I was low. Another techno way to look at it is the xfmr has a high 'internal resistance' (a tight 'kink in the hose', if you will). When you try to draw a lot of current (water) from it, the voltage (pressure) drops, because of that high internal resistance (kink).

If so, does that mean the transistor battery does not generate the full 9V pressure in the environment of my salty, wet tongue?

I'm pretty sure that either 9 or 12V just is not high enough to produce much current, even across a salty, wet tongue - the resistance is still too high. Enough to feel it, but not enough to harm you. You could prove/disprove this by putting a volt meter on the battery while putting on your tongue (sounds like fun!!??) - if the voltage drops from the 'load' - then the battery cannot put out enough current for that 'load'.

I felt a fairly strong shock from 12V once - I found I had a sliver of steel wool in my finger that came into contact with the 12V -that jolt went right into my nerves, I guess - probably surprised me more than anything.

The danger in a high current - low voltage source (like a 12V car battery, or solar panel), as mentioned above, is that the current is high enough that any metal that shorts across it gets turned into an instant heating element. Enough to burn right through a human appendage. Watches, rings, etc are a real danger when working on a car - physical and electrical. I always joke about 'divorcing' my wife when I work on the car.

-ERD50

 

[sailor]

Re: Human electrical resistance

But working with a photovoltaic panel should be no problem since they're usualy 12-24 volts, right? Sorry. A 75-watt panel operating on 12 volts will pump out over six amps. The panel has very little resistance and develops one heck of an electron flow. (That's its job.) If you become that panel's only path to ground then you're probably not going to survive the experience. Most survivors were lucky to not be the source's only path to ground.

With all dues respect Sir - I disagree.
The solar panels actually have relatively high internal resistance and in 12 or 24V variety (and not connected in series) are typically not a source of real concern. A car starting battery on order of magnitude lower internal resistance and there is not a lot of stories of somebody being terminally electrocuted by a car battery. Welded a screwdriver across the terminal - yes, maybe even got a burn, yes, but not really electrocuted.
Your 75W PV panel has probably a resistance around 2 ohms, a car battery might have 20milli ohms ( if around 500 CCA).

Sailor,
who when he was a young tadpole finished Electronics High School (it wasn't really named like this, but there is no US equivalent, I guess a combination of a vocational school with a college prep) and had more than his share of shocks, from relatively mild 48V DC telco power supplies to the most unpleasant ones during repairing live 25kV color CRT.

PS: Thanks for listing Navy manual specs. Interesting how the numbers do differ between countries. In gross EU bureaucracy 50 milliamps is considered "potentially lethal" as is anything above 48V, even for the equipment intended for hazardous condition (like coal mines) and sea duty.

PS2: If not for the new NEC and some not very well documented "opinions", which unfortuately became a "de facto" law in US, you would not need to worry about being shocekd by PV panels, because you could have it left floating vs. grounding one side. But this is a topic for another rant

 

[Nords]

Re: Human electrical resistance

PS2: If not for the new NEC and some not very well documented "opinions", which unfortuately became a "de facto" law in US, you would not need to worry about being shocekd by PV panels, because you could have it left floating vs. grounding one side. But this is a topic for another rant

Submarine electrical systems are ungrounded by design, and frequently grounded by all manner of problems that hopefully are caught before a second ground occurs and the arcing & sparking begins. Don't get me started...

Man you guys are nerds. But I love you anyway.

That's what they all say until the circuit breakers start popping. But I think that chicks dig guys wearing rubber!

 

[wabmester]

Re: Human electrical resistance

Question for you EE-types:

I bought an UPS rated for 1000VA and 670W max load. I did a run-down test with a 60W lamp, and the low-battery alarm sounded after 2 hours 15 mins.

That's just a bit over 1 Amp-hour, right? Was I naive to expect 8 Amp-hours, or did I get a dud?

 

[Nords]

Re: Human electrical resistance

Question for you EE-types:

I bought an UPS rated for 1000VA and 670W max load. I did a run-down test with a 60W lamp, and the low-battery alarm sounded after 2 hours 15 mins.

That's just a bit over 1 Amp-hour, right? Was I naive to expect 8 Amp-hours, or did I get a dud?

Assuming a 0.8 AC power factor, 60W = (0.65A)(115V)(0.80)

(EDIT: I just learned that the characters forming the mathematical expression "eight right paren" also generate a shades-wearing smiley emoticon. 8) Stupid BBS code.)

(0.65A)*(2.25 hr) = 1.47 A-hr.

I guess you could check your calculations by plugging your Kill-A-Watt into the UPS and then plugging your lamp into that. And then you could verify it with a compact fluorescent!

 

[DRiP Guy]

Re: Human electrical resistance

Question for you EE-types:

I bought an UPS rated for 1000VA and 670W max load. I did a run-down test with a 60W lamp, and the low-battery alarm sounded after 2 hours 15 mins.

That's just a bit over 1 Amp-hour, right? Was I naive to expect 8 Amp-hours, or did I get a dud?

It doesn't quite work that way. The UPS had a DC to AC inverter to run, it has a battery with it's own load response... the VA rating is straight volts*amps, and the way you get the 670W is assuming a power factor of ~0.67... .sometimes the output of a computer-intended UPS is *not* very close to sinusoidal... a typical UPS from office depot is designed to give you 15 minutes or so of power to the switcher on your PC to close out programs and shut down orderly, not to provide reliable egress incandescent lighting, etc. There may be a time rating on your unit. That will be a better indicator of when it will 'run down' than will it's rated ability to handle peak power.

Your ordinary "seven amp-hour" 12 volt SLA brick may or may not be able to deliver as much power as you'd expect from that rating, even in the lower-current two-battery 24 volt configuration that a lot of UPSes use. Don't expect more than a couple of years of life out of bargain-priced SLA batteries in a cheap UPS.

http://www.dansdata.com/diyups.htm


And if you are interested in power factor, or why 1000 VA isn't always (ever) 1000 Watts:
http://www.microconsultants.com/tips/pwrfact/pfarticl.htm

 

[wabmester]

Re: Human electrical resistance

It's an APC Smart-UPS 1000VA, which is supposedly the type with the "good" inverter. I bought it because I can adjust input voltage sensitivty, which I think will allow me to recharge it via a generator.

I checked the battery specs, and the battery seems to be rated for 8 amp-hours:

specs

So, even with the inverter and the AC power factor, does it seem reasonable that it would only run a 60W bulb for 2 hours?

 

[DRiP Guy]

Re: Human electrical resistance

Actually, well yeah.

Do you know at what DC voltage in your UPS shuts down and stops inverting?

Like I said, even with a nice lead acid battery (and that looks a lot better than a lot I've seen in UPS duty), the UPS is not perfectly efficient, and it will shut down for low voltage at some trip point while the battery could actually still provide reasonable power out (but at reduced DC voltage)

IF you want a way to satisfy yourself you have not gotten a dog replacement battery, I think I had a link on testing UPS batteries...

Valve-regulated batteries are easily overstressed by a full battery rundown, so you should test them more often.

While conducting a discharge test, you should continuously check the battery cell voltages manually or with a cell monitor. When the battery string is no longer able to provide designed discharge time (e.g., 15 min), or 80% of the rated capacity, then it's time to consider full battery replacement.

http://powerquality.com/mag/power_maintain_test_ups/

http://www.riello-ups.co.uk/riello/htm/battest.htm

 

[ERD50]

Re: Human electrical resistance

I checked the battery specs, and the battery seems to be rated for 8 amp-hours:

So, even with the inverter and the AC power factor, does it seem reasonable that it would only run a 60W bulb for 2 hours?

Yes, simple answer (I hope):

According to the specs you link, the 12V battery is good for 8 amp-hours.

Now, the inverter steps that 12V up to 120V. Ten times the voltage will be one/tenth the current for the same power. So, roughly, 8 Amp-hours at 12V will be about 0.8 Amp-hours at 120V (round numbers, ignoring efficiencies), which is about what you are seeing.

They usually spec a UPS at X time at rated power, that is what you need to look at, not the battery spec.

EDIT/ADD:

I bought an UPS rated for 1000VA and 670W max load.

So, ~ 2hours @ 60W will be about 11 minutes at the max load of 670W.



-ERD50

 

[wabmester]

Re: Human electrical resistance

Now, the inverter steps that 12V up to 120V. Ten times the voltage will be one/tenth the current for the same power. So, roughly, 8 Amp-hours at 12V will be about 0.8 Amp-hours at 120V (round numbers, ignoring efficiencies), which is about what you are seeing.

D'oh. Yes, I should have been thinking in terms of volt-amp-hours. Apparently, there are 4 cells. So, in theory, that would give me something like 384 V-A-H.

But, after reading the full specs, it looks like the battery is rated for 336 V-A-H. At 100%-efficient 120V *DC*, that would give me over 5 hours for a 60W bulb.

But the UPS is rated for 30 mins at half-load (335W), and 8.8 mins at full load (670W). That doesn't scale linearly, but it seems like 2 hours for a 60W bulb is about right.

 

[Toejam]

Re: Human electrical resistance

When I saw the topic for this thread, all I could think about was the 28 year old man in Gig Harbor, WA who took his dog for a walk in the evening shortly after the big windstorm which occurred in mid-December here in the Pacific Northwest. The man accidentally stepped on a downed "live" wire from the storm, and both he and his dog were electrocuted. So sad.

During the windstorm, there was a run on generators. I couldn't even find one to rent. I had no power for 4 days and it was friggin cold. During that time, I wore 3 layers of clothes and kept a stocking cap on my head when I went to sleep. Very uncomfortable. A couple of weeks after the storm, I bought a 4,000 watt generator in case of another power outage.

 

[DRiP Guy]

Re: Human electrical resistance

When I saw the topic for this thread, all I could think about was the 28 year old man in Gig Harbor, WA who took his dog for a walk in the evening shortly after the big windstorm which occurred in mid-December here in the Pacific Northwest. The man accidentally stepped on a downed "live" wire from the storm, and both he and his dog were electrocuted. So sad.

During the windstorm, there was a run on generators. I couldn't even find one to rent. I had no power for 4 days and it was friggin cold. During that time, I wore 3 layers of clothes and kept a stocking cap on my head when I went to sleep. Very uncomfortable. A couple of weeks after the storm, I bought a 4,000 watt generator in case of another power outage.

I lived in S Fl for 15 years through Andrew and all other hurricanes without a generator. No problem. But after three weeks with no power year before last, I now have a generator, keep lots of fuel cans handy in garage, and also have several fiberglass mat lead acid batteries I keep topped off along with a DC to AC inverter for backup if the generator fails. I keep low power fluorescents for light & a tiny LCD TV.

 

[TromboneAl]

I just stuck my kill-a-watt meter up my butt, but I can't read it.

 

[REWahoo]

I just stuck my kill-a-watt meter up my butt, but I can't read it.

Try inserting it with the meter side facing out.

 

[wabmester]

Re: Human electrical resistance

Ialso have several fiberglass mat lead acid batteries I keep topped off along with a DC to AC inverter for backup if the generator fails.

So, you made your own UPS. How many batteries, and is the efficiency any better than off-the-shelf UPSes?

 

[DRiP Guy]

Re: Human electrical resistance

So, you made your own UPS. How many batteries, and is the efficiency any better than off-the-shelf UPSes?

I have three spare automotive batteries (I'm a car nut with three cars, too) - two Optimas and a conventional wet lead acid.

The inverter, I fear is likely to be woefully inefficient if I checked, since I got it at AutoZone - so it is intended to be an A/C source for your car/camper. I also have a Black and Decker Electromate 400 which is a combo jump-starter/air pump/DC supply/worklamp/inverter AC source (I am not kidding <laugh> !)

Since I had the inverter before the generator, and during my power outage, I learned to get by with swapping the auto batteries into and out of my truck to keep them all topped off. Got by okay, but the generator is the way to go!