DC power versus AC power

David Lesher wb8foz at nrk.com
Mon Dec 30 02:06:54 UTC 2002

Unnamed Administration sources reported that Michael Painter said:
> I've laid across the buss-bars before...definitely an uneasy feeling, but never
> felt it unless I was sweaty. <g>
>  Capability of thousands of Amps, but it's the old "power transfer"
> deal...internal resistance of the source vs. internal resistance of the load (your
> body).

True. It's all Ohms law. But for those of you who (unlike me)
speak C and mean a language, not 3E8m/s..... let's look at
13.8v car battery and a 120v line cases.

Ohms are what resist the flow of [I'll say so even though I'll
wince] electrons.

I = E/R. I = current, E voltage, R resistance.

People are not good conductors; let's just say you have oh 100Kohm
from one DRY thumb to another. Most of that is skin. Get inside
the ?epidermous [sp]? and it goes down fast. Get even slightly
sweaty, and same deal.

So I = 13.8/100k is not much current. Not enough to hurt.

I = 120v/100K is 1.2ma. That's not enough to cook you, but
may be enough to screw up your heart. That's why the "one hand
rule" exists. Loop one hand through belt at your ass, work with
other. Iffen you get bit finger to finger on the same hand,
it won't pass though your trunk. (Assuming rubber shoes...)

Now, consider that dropped screwdriver. Say the total resistance
of it across the contacts is 0.05 ohms. (It's dirty...)

I = 13.8/0.05 = 276 A. Sparks fly. Tool melts, you hope.

I = 120v/0.05 = 2400 A. Here, the 20A breaker should clear as
fast as it can, and it's likely only going to be oh 1000A until
it does. Still shit flies and if your eyes are in the way...

Now, milliseconds later [if the fuse/breaker has not cleared]
that initial current gets enough things hot, melts/cleans parts,
LOWERING THE RESISTANCE as it ..improves..  the connection... 

Worst case, you get a plasma arc that LOVES to conduct.

In that case, the resistance goes to near zero. The current is
only limited by other resistances in the battery string, and you
have good tight connections and large cables, right? 

If you are lucky, the screwdriver is consumed and the short stops.
If not, well soon you get badly burnt. Wrenches are a lot more
massive; and will do more damage.

Some major burn cases live.  I'm not sure they are the lucky ones.

Short summary: 120vac line may "electrocute" you easily; a high
current lower voltage system can cook you just as dead. Neither
is "safe" to futz with casually.

A host is a host from coast to coast.................wb8foz at nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

More information about the NANOG mailing list