Why will 240 volt condenser kill me but not 7,000 volt electric fence?

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47 replies · 13K views
Averagesupernova said:
It's entirely possible he was wrong. I'll stand on a dry wood surface any day ahead of any concrete. Standing on dry concrete is better than standing in a pool of water of course.
Having shoes on helps too.
 
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phinds said:
Yep. Also, when I was an engineering trainee at NASA while I was getting my undergraduate degree, I used to go into the lab I was assigned to and I would VERY carefully touch 120 volt lines (yeah, I was young and stupid) to perk myself up in the morning. It worked but I wouldn't do it now. The technicians I worked with would roll their eyes and say things like "moronic college boy!"
What? You didn't know about coffee? What's up with that? :wink:
 
timmeister37 said:
What city were you in when you were an engineering trainee at NASA?
Greenbelt, MD. Goddard Space Flight Center.
 
sysprog said:
What? You didn't know about coffee? What's up with that? :wink:
I HATE coffee.
 
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phinds said:
I HATE coffee.
That's a serious sin! :oops:
 
phinds said:
Greenbelt, MD. Goddard Space Flight Center.
I lived in Huntsville, AL most of my life. The reason i asked is that i thought you might have worked at Marshal Space Flight Center on Redstone Arsenal in Huntsville.

I did not even know NASA had facilities in MD. I thought NASA was only at Houston, Huntsville, and Cape Canaveral.
 
sysprog said:
That's a serious sin! :oops:
It leaves more coffee for the rest of us
 
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timmeister37 said:
I lived in Huntsville, AL most of my life. The reason i asked is that i thought you might have worked at Marshal Space Flight Center on Redstone Arsenal in Huntsville.

I did not even know NASA had facilities in MD. I thought NASA was only at Houston, Huntsville, and Cape Canaveral.
They also have a few others. Goddard was the communications center among other things.
 
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timmeister37 said:
It leaves more coffee for the rest of us
Yep. I'm a black coffee junkie. I guess I can forgive @phinds for disliking coffee with that in consideration.
 
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timmeister37 said:
Then why did my HVAC instructor say that the only reason i am still alive after touching the live contacts on the contactor is/was the concrete floor that i was standing on?
I wonder why a professional or even an instructor is not following safety protocols. Were you taught lockout-tagout before going on the job site? Also, when the trades were male dominated there was a safety tip that went like: never put your fingers where you wouldn't put your...

Perhaps he meant: if the wires were live, then the only reason why you are alive...
 
Dr_Nate said:
I wonder why a professional or even an instructor is not following safety protocols. Were you taught lockout-tagout before going on the job site? Also, when the trades were male dominated there was a safety tip that went like: never put your fingers where you wouldn't put your...

Perhaps he meant: if the wires were live, then the only reason why you are alive...
I imagine the instructor’s tone was pretty severe, having seen a student deliberately touch live contacts :oops:

If we were to pick apart the phrasing, the concrete wasn’t the only reason the op was still alive. Decent rubber-soled boots, right hand rather than left...etc. Locking off the power is a good habit, but sometimes you have to work live for diagnostic reasons, or to prevent disruption to critical circuits.

The old TV engineers’ habit of working with your left hand in your back pocket is still relevant. It stops you leaning on earthed metal cabinet doors, risking the dangerous hand-to-hand shock. Gloves are also under-used, probably for historic reasons. There are some excellent modern ones which allow good dexterity while providing some protection rom accidental contact.
 
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timmeister37 said:
Then why did my HVAC instructor say that the only reason i am still alive after touching the live contacts on the contactor is/was the concrete floor that i was standing on?
Surface covering on indoor concrete floors provides excellent electrical resistance.
So he could have been referring to that particular floor, or floors like it.

Bare concrete - variable due to moisture content.

But with anything electrical, if you do not know it is live, assume in all cases that it is.
 
Dr_Nate said:
I wonder why a professional or even an instructor is not following safety protocols. Were you taught lockout-tagout before going on the job site? Also, when the trades were male dominated there was a safety tip that went like: never put your fingers where you wouldn't put your...

The unit was live for diagnostic reasons. I was taught lockout-tagout. HVAC is still heavily male-dominated. It's just no longer unheard of for a woman to work in HVAC. About your rule "never put your fingers where you wouldn't put your..." hahaha

Perhaps he meant: if the wires were live, then the only reason why you are alive...

No. The wires were live for diagnostic reasons.
 
Years ago I measured the output of a fence post charger. A very narrow pulse and widely spaced.
Thus the energy is fairly low and in short burst. There have been instances where the fence owner substituted 120 VAC and that killed a person touching the fence. There have also been reports of kids daring one in the group to urinate on the fence, Oh-My.
 
Thinking of capacitors, I was a components engineer (and design engineer) at a company that used a LOT of capacitors. Some revelations:
a) a reverse voltage on an electrolytic capacitor can cause it to explode. Thus electrolytic capacitors have a venting system to reduce the level of explosion (generally just gassing if the reverse voltage is not severe), however, at one time the companies Siemans and Roederstein made an electrolytic capacitor entirely encapsulated in plastic. Inadvertently, one was mounted in a PC board reversed. The explosion caused a technician to loose site in one eye.
b) high voltage capacitors are shipped with a shorting wire across the terminals. I have seen an oil filled high voltage capacitor after a production fault within it caused it to explode. There was a 1" hole in the side, effectively a grenade. After that instance a plexiglass shield was placed between the test unit and the technician making adjustments.
c) It was considered a prank to leave a capacitor on the bench charged enough that a technician picking it up would get a small shock. Fortunately, those days passed.
d) Be Safe
 
As soon as you touch an electric fence, the voltage drops dramatically because of the series resistance of the source. What you feel then, is a series of pulses of a few mA, with a voltage of maybe 50V on the surface of your skin. Add skin resistance, and the voltage below your skin of maybe 10V
 
Joseph M. Zias said:
It was considered a prank to leave a capacitor on the bench charged enough that a technician picking it up would get a small shock. Fortunately, those days passed.
[Past tense (consider revising)] :woot:
 
So much talk about touching live wires makes me nervous. Kids can find this thread via Google search and conclude that there is no danger.

The OP's question is adequately answered, so this thread is closed.