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Re: Dupla cables

On Fri, 1 Jan 1999, Mortimer Snerd wrote:

> My understanding was that both voltage and amps could kill you, if they were
> high enough, and that the higher voltage of the Dupla system was a reasonable
> compromise, as it allows a lower amperage.

No, it's really the current that kills.  Voltage is just the enabler of
the current, and you have to examine how the voltage affects the current
for the particular situation.

I posted an answer here on the APD a few years back (I don't think it was
complete, but some of the points were valid), a quickie-discussion about
the effect of voltage.  Basically, the water acts as a resistor too, just
like the heater, and the amount of current that can pass through the water
(and thus, your hand) I = V / R... it's *proportional to the voltage*.  
Higher voltage => Higher Current through your hand => Higher danger.

Now combine this with the fact that the cable resistance for a given power
is proportional to V^2 (the 120 volt heater is 50 times more resistive
than the 24 volt one)... depending on where the insulation wore off on the
heater wire, you could get a current through the aquarium somewhere
between 5 and 50 times stronger with the 120 volt cables.

> "Adam R. Novitt" wrote:
> > I'm not sure if the analogy is totally accurate but... If electricity were
> > water, amps would be pressure (PSI), and volts would be volume (GPH).

Actually, it's more like voltage is pressure (say you have a water tower
-- this would be how high off the ground you need to build it), current is
flow rate (GPH), resistance is pipe length and diameter, how many twists,
etc. Power would be pressure * flow rate, which is kind of goofy to
visualize. :)

The dupla system is a smaller diameter pipe than a car battery but much
larger than a 120-volt system, so you need to apply the most *pressure* to
the 120-volt system to drive it through the teeny pipe (say 50 psi); you
need to apply the least pressure to the 12-volt system (say 5 psi), but
it's going to have ten times as much water flowing through it.  If you
poke a hole in the side of the pipe, you'll get a much slower leak on the
12-volt battery system, you might be able to plug it with your finger. The
most dangerout leak is on the 120-volt system.

You could stick your finger over the leak in the 12-volt system and you
probably won't feel anything.  But you might get hurt with the 120-volt 

  - Erik

Erik Olson
erik at thekrib dot com