When is (DC) electricity dangerous? confused by what I read so far

[ricardocello]

Don't strip a live telephone where with your teeth, ask me how I know!

(Yes, I'm well aware that you should never strip any wire with your teeth )

Let me guess, telemarketer call....

 

[max2k]

Well they have to be connected at some point don't they? or you mean get them into position disconnected and connect them in series as the last task?

you got it- just imagine the possible current flow and break it where it's convenient along the path. Then the system simply becomes pile of disconnected parts which is safe to work on. After completing the work restore the broken connection and all is well The trick is to separate work activity and turning the system back on. Working on live system is much more adventurous: even some accidental spark / loud sound / etc can make your hands move involuntarily and get under actual voltage.

 

[ULR]

The old-school POTS telephone lines run at -48VDC for safety reasons as well.

Bell did a bunch of tests to figure that out, too. And the 50V nominal cut between the simpler low-voltage rules and the full-bore ones in the electrical code was apparently chosen based on the phone company standard and its research: Just high enough that the phone company came in under the cut.

Just don’t hold the wires in your hands when a ring occurs, that is like 90 VAC and you feel it tingle.

Typically starts out as 130V 20 Hz square wave - on one side of the pair only. But there's a lot of resistance in miles of fine wire and the bells pull substantial current, so it's lower at the phone (if a mechanical bell is connected).

PV DC voltages above 60V I will not touch, that’s potentially dangerous.

It's current that kills. (Also timing with respect to the heartbeat.) About 20 mA up the left arm can start fibrilation: A patch of heart stops beating and the contraction signal circles around it and radiates out in a spiral, causing the heart to vibrate and not pump - until it runs out of its own oxygen and quits. (A large shock can cause the heart to contract hard all over at once and then (sometimes) get out of the failure mode and restart, which is how defibrilators work. But slamming a victim's hand back into the equipment is not normally effective. B-b )

At an auto plant where I consulted, an electrician claimed that the motors mostly ran on 440V, not just to save copper, but because somebody touching it would tend to be thrown away from it by muscle spasm rather than latching an involuntary death-grip onto the wire. But I take that as a (politically convenient?) rumor rather than anything I'd bet my life on.

At EE school we were entertained with a cautionary tale about a couple freshmen engaged in a bet over whether you could be electrocuted with a 1.5v battery. Story goes they tried it out with a couple buckets of salt water, one for each hand. Worked fine, i.e. stopped the "you can't do it" bettor's heart. Unfortunately they didn't get it restarted...
[/QUOTE]

 

[SparkyJJO]

All I know is I stuck the legs of a capacitor with around 300V+ DC charge on my palm (because "it is off I've done this hundreds of times..." super cocky young stupid person I was) and it felt like a searing hot knife.
OK I respect those big fat capacitors more now.
I still say it was my coworker's fault for jinxing me by asking if that power supply held dangerous voltages or not. I never got nailed until he asked that

 

[doing it different]

My experience with caps is depending when you remove them on the sine wave. They can have alot of juice, or little...

 

[HighDesertOffgrid]

 

[max2k]

interesting how calmly the operator took it, must be ordinary event over there

 

[HighDesertOffgrid]

interesting how calmly the operator took it, must be ordinary event over there

According to one comment: the cameraman was his son and thought he was pranking. Called ambulance and the guy survived.

 

[J.H.]

Current kills, not voltage. Higher voltage pushes more current, so the higher the voltage, the more dangerous it can be. But it's the amount of current flowing that does the damage. I'm sure you've seen at least one video of a pretty lady standing on a pedestal with lighting shooting from her fingers. That's around 100K-1M volts, but with almost no current.

AC or DC - it doesn't matter. They are both dangerous. And in fact high voltage DC is a lot more damaging than AC because it's "always on", as opposed to AC that hits zero volts every 1/120th (60Hz) or 1/100 (50Hz) of a second.

The DC that comes from your PV panels is likely to take a big bite out of you. They push sometimes a few hundred volts DC, and can follow that up with sometimes over 20A. Not sure how likely it is to kill you.

The 48V DC batteries aren't likely to hit you hard unless your hands are sweaty, and then it'll probably hurt, maybe really bad. Don't lick it unless you have a death wish.

The DC from your 12V battery is not so bad. Lean across a LFP battery with sweaty arms and touch both terminal, and you'll feel it. It gave me a zing, but not painful. But again, don't lick it.

The AC coming from the grid or your inverters is enough to easily kill you, if that current passes to close to your heart. It's not the current here that is so bad, assuming you aren't conducting a crazy amount of it. But actually the frequency disrupts the signal between your brain and your heart and it just stops beating after a few minutes. Of course, the current will eventually kill you itself if you can't get away from it within a second or two.

I watched a kid in high school go to the hospital after touching his tongue to a 1.5V dry cell (those big cylinders used to start hobby engines like the Cox .049). His tongue swelled up like a tennis ball in his mouth.

I also survived grounding out my fuse panel though my body as a small child. It was before I have memory (so maybe 2-3yrs old). But my mom has told me the story many times about me disappearing, and her finding me stuck to the fuse box, humming involuntarily. She swung her mop at me with all her might to break me free from it, knowing that touching me herself would also put her in danger.

Now that’s a cool mom… yer lucky…

 

[max2k]

According to one comment: the cameraman was his son and thought he was pranking. Called ambulance and the guy survived.

No way, hardy individual, the fall alone would break anyone's spine

 

[hwy17]

I have it in my head that another reason DC is more dangerous is AC has a skin affect. And although the skin effect is not named after human skin, it applies to us as well. So when you accidentally grab 120v on your right hand with system ground on your left, the current that travels through your chest mercifully prefers to go around the surface of your body on your chest and back while much less of it goes through the middle where your heart is.

I don't know how true that really is.

 

[jgh]

I don't know how true that really is.

Just search on "skin effect 60 Hz" to find out what sort of depth is involved.


Meanwhile, DC has the problem that it tends to make muscles contract. So if the contact was on the palm side of your hand, you grip on hard and cannot let go.
In that sense, AC is slightly less dangerous.

 

[hwy17]

Just search on "skin effect 60 Hz" to find out what sort of depth is involved.

What do you want me to read about it?

I read that the skin effect at 60hz is about 8.5mm in copper, and this is why it's insignificant in household wiring sizes.

I don't know from that what the skin effect in a human chest is. If it was 8.5mm that would be great, most of my heart is more than 8.5mm deep.

The muscle contraction thing I'm aware of and I'll set aside my thoughts on as as separate issue to the skin effect discussion.

 

[Andrewr05]

Let me guess, telemarketer call....

I don't know, I didn't hold it in my mouth long enough to see if I could hear the sound of the incoming call lmfao

Talk about bone induction lol

 

[max2k]

What do you want me to read about it?

I read that the skin effect at 60hz is about 8.5mm in copper, and this is why it's insignificant in household wiring sizes.

I don't know from that what the skin effect in a human chest is. If it was 8.5mm that would be great, most of my heart is more than 8.5mm deep.

The muscle contraction thing I'm aware of and I'll set aside my thoughts on as as separate issue to the skin effect discussion.

human body has much less conductivity than copper and 60Hz AC will go through it, not just on surface.

 

[J.H.]

What do you want me to read about it?

I read that the skin effect at 60hz is about 8.5mm in copper, and this is why it's insignificant in household wiring sizes.

I don't know from that what the skin effect in a human chest is. If it was 8.5mm that would be great, most of my heart is more than 8.5mm deep.

The muscle contraction thing I'm aware of and I'll set aside my thoughts on as as separate issue to the skin effect discussion.

Everytime I see your post I strangely think of Sammy Hagar in that black Ferrari….

 

[Bop]

Just search on "skin effect 60 Hz" to find out what sort of depth is involved.


Meanwhile, DC has the problem that it tends to make muscles contract. So if the contact was on the palm side of your hand, you grip on hard and cannot let go.
In that sense, AC is slightly less dangerous.

Back in the 'bad old days' when I was doing my apprenticeship (elec fitter), it wasn't unknown for the older tradies to 'check for voltage' with their hand!!!

(Do NOT do this!!!)

They would touch the BACK of their hand to a contact to see if it was live at 240v!!!
If it was, your arms muscles would contract, and pull your hand away from the contact...
One tradie I trained under warned that even then you could get hurt doing it- he had punched himself in the face LOL

(this was back before meters were commonplace- even when I was doing my training (1980's) the railway workshops only had a single Avo8 multimeter for the entire workshop which was kept in the 'toolroom' and logged out for use...)

 

[canadian0]

(this was back before meters were commonplace- even when I was doing my training (1980's) the railway workshops only had a single Avo8 multimeter for the entire workshop which was kept in the 'toolroom' and logged out for use...)

Hmm really? I get a lot of hand me downs from my grand father as my mother keeps everything and one was a multimeter which looked like it is from the 1950s as a guess by the design. It is a needle one. It was functional but quickly switched to a newer one mainly because of the wires falling to bits. The cheap new one was just as bad in that regard though as those wires both disintegrated after a few uses and now have now got myself a nice Uni-T 210E. Loving that thing mainly for the DC ammeter. So much easier to see what is going on with the system with a simple clamp.

 

[Bop]

Hmm really? I get a lot of hand me downs from my grand father as my mother keeps everything and one was a multimeter which looked like it is from the 1950s as a guess by the design. It is a needle one. It was functional but quickly switched to a newer one mainly because of the wires falling to bits. The cheap new one was just as bad in that regard though as those wires both disintegrated after a few uses and now have now got myself a nice Uni-T 210E. Loving that thing mainly for the DC ammeter. So much easier to see what is going on with the system with a simple clamp.

I didnt get a digital one until the 1990's- my first was a 'cheap' DSE analogue one in the 1980's (and that was close to a hundred bucks!!!)
LOL- found a pic of the exact type mine was!!!


The Flukes did have a digital model by the late 80's- but they were eyewateringly expensive (over $500)- when my first takehome pay as an fully licenced electrician was under $350 for the week!!!)

 

[DPC]

the railway ..... Avo8 multimeter

I had to double check where you were from ( Canadian lol ? )
Wording railway... and AVO meter lol

 

[Berdink]

I may be reposting what's been said (I'm new to all this), but I'm suspecting as all this becomes less expensive, more worser stupidider dangerouser things are going to be done.

 

[davros]

I used to work with an engineer who started on phones and Telex lines. Telex was +60v to -60v @ 40mA. He said it would really spoil your day if you were up a phone pole, in the rain, and came across a Telex pair!

And there were repeater units for really long lines +/- 120v IIRC