[sdiy] Neutral ground

[The Old Crow]

  First a quick explanation of where "neutral" comes from...

  On a 3-phase AC generator, with its windings wired in what is called 
the "delta" configuration, a voltage is induced across each coil 120 
degrees apart.  These voltages, call them Vphase(A~C), are typically 
208VAC in the states.  This is straight out of the generator.

  For short-distance transmission of the generator power, as from a 
utlity pole to a few houses, another configuration is used.  The 
generator's "delta" (called that because the three connected windings 
look like a triangle) goes through a large 3-phase transformer where the 
power is transfered to a "wye" configuration.  This is visualized by 
drawing a "Y" shape using the vertices of the delta triangle and having 
the three lines meet in the middle.  It is this middle point which 
interests us.

  The result of calculating the voltage at this middle point from the 
three new voltages (vertice A to middle + B to middle + C to middle) 
happens to result in the middle being zero volts, provided the phases 
are balanced.  Each vertice-to-middle voltage ends up being Vphase/1.732 
(square root of 3), so a phase-to-neutral (calling "neutral" as it 
happens to be zero volts) turns out to be 208V/1.732, or...120VAC.  
(Google "delta to wye" if you want to be harangued by the math)

  Since the neutral is zero, there is no need to bring a wire from the 
pole to the house.  Both the pole-neutral (transformer) and house 
neutral (house breaker panel) are earth-grounded, usually by a 8' stake 
driven into the earth and a heavy copper buss wire clamped to it.

  The safety ground of a socket is ultimately connected to earth as 
well.  The point of the safety ground is to provide a redundant path 
aside from the neutral.  This way a chassis can be grounded, but not by 
the (old way) of neutral-to-chassis.  Neutral to chassis was just one 
reversed plug (hence the polarizing) from asking for disaster.

Crow, who in earlier life was a heavy power systems sort of fellow...

/**/


Woody Wall wrote:

> So does this mean that neutral and earth ground are the same? Or am I 
> missing something? It's been a long time since I studied this in school.
>
> Woody