AC or DC Shock: Which is worse? OUCH! [RE-wrenches]

Wed Jul 21 23:06:41 PDT 2004

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Mark, Nick, et al,
 I have worked for years with very high voltage DC circuit in my years 
as a service guy for big mainframe Uninterruptible Power Systems. I have 
been shocked many times by both AC and DC, upwards of 600 
volts--sometimes AC,  (both 50 and 60 HZ) and sometimes DC. it is never 
pleasant, and I have felt my heart go "whew, that was close" more than a 
couple times. I guess I have always had an angel riding on my shoulder.  
Or it could be the "training" I gave to my nervous system when my 
brother and I, in the early 50's, at about 5 and 7 years old, would have 
contests to see who could hang onto the electric fence the longer. 
Sometimes he won, sometimes me, with the record being around 5 minutes. 
Maybe I learded from this not to hang on, and developed a fast jerk 
reflex? I really don't know. 

But working on power systems with multiple power inputs, even though I 
always follow the "kill the power first, but always act as though it is 
hot" rule, I have been bit more than I care to remember--maybe a couple 
dozen times. the problem with batteries is they are always hot.   
Capacitors can carry quite a jolt, too.  And these were not always small 
amperage shocks, either. I guess it depends more on your heart and your 
reaction than anything. I have also know people who were hit by lighting 
and survived, and were shook up for a few days afterwards. This is DV 
voltage in the magnitude of 200,000 or more volts, with huge amperage, 
too.  
 This wasn't a shock, but one time I was in a bank finishing up a 
battery change-out on a 50 KW UPS with a nominal 360 volt bank with 30 x 
12 volt, 120 amp hr. gell batteries in series. As they were new, and 
before completing the last connection, I measured them at 420 VDC. The 
way the whole battery cabinet was arranged (with no possibilty of doing 
it differently) was that the positve and negative terminals were both at 
the top of the stack, only about 2 or 3 inchs apart. I wore rubber 
gloves, eye goggles, and a heavy long sleeve denim shirt. As I was 
tightening the last bolt with my 1/2 inch  end wrench with a rubberized 
high voltage coating, it must have had  a thin spot and it arced. Boy, 
did it arc! Even though I was holding the wrench so that my hand would 
completely surround the wrench and not allow it to touch the other 
terminal, it somehow did, and flashed like a bolt of lighting.   It 
vaporzed about half of the round socket end of my wrench, as well as the 
positive battery terminal (the entire lead post) of the top-most 
battery. I did not get a shock, but I did fall back off my mini-step 
ladder, and I had melted lead splatter in my full beard (which kept my 
face from getting burned. I figure it was easily something around 420 V 
x 200 A (instaneous) = 84 KVA.
 The smoke was also a big phenomenon, but I was able to tell the 
security guard in time to disable the fire sprinkler system so it didn't 
soak all the people and all the money. 
 I know I was being as careful as ever working at that bank, and I had 
been doing this kind of work for many years. But still, things happen, 
and the more you  are exposed to danger, the higher your odds are that 
Murphy's law will show up. 
 I much prefer working on 48 volt PV sysgtems to those big behemoth 
UPS's from the mainframe computer days. But when you have a big PV 
system, 120 VDC is  still a good way to go.
 I hope someone gets a chuckle (and learns the lesson I learned the hard 
way) out of all this.
  As for Nextek, it almost makes sense, except for the fact that when a 
DC lighting ballast burns out, you can't get a replacement at your local 
electric wholesale house. But I see no difference in running high 
voltage DC vs. 240 VAC or higher.   
Regards,
Robert Warren

Mark Robinson wrote:
> 
> 	I've been doing some Nextek-related research. Several people have 
> resisted the idea of running DC power lines through offices because of 
> the increased hazard of DC shock. I've been finding all kinds of 
> information about whether an AC shock is worse than a DC shock. 
> Unfortunately, it's heavily contradictory. There hasn't been a lot of 
> testing done. Subjects are hard to find. (Any Volunteers?)
> 
> 	As an electrician, I was always under the assumption that you 'can't 
> let go' of DC. This is challenged in several studies, one claiming:
> 
> ...AC is said to be four to five times more dangerous than DC. For one 
> thing, AC causes more severe muscular contractions(
> http://pchem.scs.uiuc.edu/pchemlab/electric.htm). Reportedly it's the 
> frequency that causes contractions.... And 60 Hz is the worst. 
> 
> Personally, I've been knocked off a ladder by both!
> 
> http://www.fishock.com/service/shock_new.html claims that the AC/DC
> component is not significant.
> 
> A Comparison chart at 
> http://www.allaboutcircuits.com/vol_1/chpt_3/4.html
> shows that DC is safer than 60Hz, and 10Hz is even better.
> 
> Another site (http://www.codecheck.com/ecution.htm) claims that there 
> are
> more horror stories about DC shock because more victims have lived to 
> tell
> them.
> 
> If anyone has any references or thoughts to add... I'd appreciate it.
> 

What will we do if there is another fuel shortage? 
Did you know that there is a pollution free, completely
renewable fuel which you can use right now in your own car? 
Don't replace the engine, replace the fuel.
Check out my website on making your own fuel: 
http://running_on_alcohol.tripod.com 
robertwarren at mail.com

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