> As an electrical engineer, I must agree with the first poster, that
> "electricity takes all available paths." Given a voltage source with
> parallel paths to ground, electricity will flow through all paths, with
> current varying inversely to the resistance in each path.
> The reason the ground wire works is that no voltage source is an ideal
> voltage source. If we have a ground path with near zero resistance, the
> current drawn by that path will be so great that either:
> (a) the voltage drop across the internal resistance of the voltage source
> (every non-ideal voltage source has a non-zero internal resistance)
> approaches the total voltage, so the output voltage is reduced to near zero.
> It won't be zero, because there it will reach an equilibrium determined by
> the ratio of the resistances (load/ground vs. internal). Since the output
> voltage is so low in this stage, the current through your body will be
> unnoticeable (although non-zero), but it will most probably be in this state
> a very short period of time, because...
> (b) the large total current causes a fuse to blow.
> Here's an ASCII representation of the whole circuit:
> --------/\/\/\-----o ------------------------------------
> | Internal | |
> | Resistance | | |
> | \ \
> Voltage (V) Output Ground / Your /
> source | voltage wire \ body \
> | (very low / (high /
> | | resistance)| resistance)|
> |________ _________o_____________________________________|
> ~ |
> Fuse ___ Ground
I agree with the above (except that I doubt you'd find a fuse in the ground
circuit - it should be shown in series with the voltage source internal
resistance) in the case where the cable failure results in a low-resistance
connection between the voltage source and ground. The ground wire greatly
improves the safety of the Dupla cables.
Sometimes, though, I think failures may occur that do not result in blowing
of the fuse -- a partial breakdown in the insulation, perhaps or some other
scenario that doesn't result in large currents flowing to the ground wire.
This is still dangerous to anyone coming in contact with the tank...it
doesn't take much current to put your lights out.
To guard against that, you must install ground fault circuit interruptors on
all outlets supplying equipment placed in the tank -- but then, we've all got
those already for our power heads, heaters, and everything else electrical in
the water, right?
Paul Chapman in Saskatoon, SK, CA, where its still about -30 C.