Re: Aquatic Plants Digest V1 #310
>From: KB Koh <KB_Koh at ccm_ipn.intel.com>
>Subject: Re: Dupla Cables
>>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...
>I have to partly disagree here. This is true to a small power supply but would
>apply little to the utility supply. I don't know what is the rating in the
>United States, in Malaysia we are using 240V with 60A main fuse. For home
>supply, the utility voltage would not have the time to drop when grounded.
>Without ELCB, the current shoot up to 60A very fast and cause the main fuse to
>blow. The voltage don't event have the time to drop during that short
>time. It would take a very hugh current and no fuse or circuit breaker all the
>way to power generator to cause the huge voltage drop and eventually cause a
>shutdown to the entire electrical grid.
Right, but I had that case covered. Note that I had "either" and clause (b)
was that the fuse blows. So I said the protection is derived from either
clause (a) and then clause (b), OR clause (b) directly. ;-)
> The reason the current is so low when
>flowing through your body is because almost all the current flow to the ground
>and not because the output voltage went low.
Now here you get confusing again! The ONLY reason the current is ever low
when flowing through your body MUST be due to the drop in voltage! Remember
Current = Voltage / Resistance
and your body resistance isn't changing! To have a low body current
requires you to go back to case (a).
So if we're talking about a direct hookup to the 220V powergrid (and the
voltage isn't dropping), then the only protection you have is via the fuse.
There is no instance of a low current through your body due to the existence
of a ground path! And you'd better hope it's a very quick-acting fuse! Or
have a Ground-Fault-Interupt in the circuit (I do).
Okay, NOW let's get back to plants.