[Prev][Next][Index]

Re[2]: Dupla Cables

>From: nguyenh at nosc_mil (Hoa G. Nguyen)
>Date: Thu, 7 Mar 96 10:16:22 PST
>Subject: Re: Dupla Cables
>
>>From: KB Koh <KB_Koh at ccm_ipn.intel.com>
>>>From: chapman at SEDSystems_ca
>>>
>>>In the first place, electricity does not just take the path of least
>>>resistance -- it takes every available path.  You can see that this is true
>>>from your own experiences -- you can plug several different items into a
>>>power bar, and electricity flows into all of them, even though each has a
>>>different resistance.
>>
>>Electric current actually take the path of least resistance. If one of your
>>several items has a short circuit to ground, all the current will flow
there and
>>none to the rest of your items. The sum of current to all items is equal to
the
>>input current. Something got to do with Kirchoff(sp?) Law.
>>
>>>The purpose of the ground wire is to make a low resistance path to ground
>>>available so that if a fault occurs, a large current will flow and result in
>>>a blown fuse or circuit breaker.  If the cable has an internal fuse, as the
>>>salesperson implied, that is its purpose.  No fuse can tell if current is
>>>flowing through your body or not.
>>
>>Partly true because the path to ground has the least resistance. All currents
>>flow to the ground and none through your body. The ELCB would then trip. The
>>fuse would blow only if the ELCB malfunction or the ground wire is open, as
>>mentioned by the sales person.
>>
>
>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.
>

OK, my mistake in simplifying it too much. The wording should be "almost none to
the rest of your items." For a few MegaOhms of body resistance comparing with 
hundreds of Ohms for Ground, very little current would flow to cause any severe 
damage to human in such a short period of time before ELCB trip. It would take 
something like 1 Ampere to kill someone. Kirchoff Law still hold true.

>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 period of 
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. 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. Remember, the original poster was 
talking about substrate heater using main 110V ;-) Again, I'm simplifying a lot 
here, removing lots of details. 

Lets get back to talking plants instead.
rgds..kbkoh