Re: [TCML] line feed/generator feed neutral/grnd ?

[Jim Lux <jimlux@xxxxxxxxxxxxx>]

-----Original Message-----
>From: Sfxneon@xxxxxxx
>Sent: Aug 28, 2008 2:46 PM
>To: tesla@xxxxxxxxxx
>Subject: Re: [TCML] line feed/generator feed neutral/grnd ?
>
>Hi David,
> 
>Even though the white and green wires terminate at the same place  in the 
>main panel, they have two very different functions. They just both happen  to be 
>earth grounded, so they are connected to the same terminal but still have  
>different functions. The white, or neutral wire is a current carrying conductor  
>meant to complete the power circuit, while the green, or grounding wire's  
>purpose is safety, by providing a lower resistance path to ground than you  are.


Hence the distinction in the electrical code:
"grounding conductor" -> green wire - electrical safety ground
"grounded conductor" -> white wire - neutral/return

And to amplify on the above connection, the two are connected at ONLY ONE PLACE.. the electrical distribution panel. Extra connections inevitably lead to problems.  The notorious problem is with generators for emergency power.. portable ones have the neutral and ground bonded in the generator, fixed ones don't.  If you use a portable generator for emergency power with a transfer switch that only switches the two hot wires, then you wind up with two connections between neutral and ground (one at the panel, and one in the generator).  If there is then a fault, bad things can happen.



> 
>Normally, the green wire carries NO current...unless there is a  leakage path 
>or a ground fault, such as the "hot" wire coming in contact with  the case or 
>chassis of a piece of equipment. Then the fault current will be  conducted to 
>earth ground through the green wire, instead of your body if you  happen to 
>touch it. The neutral wire can actually be quite a few volts above  ground if 
>the wire is long and the load high, especially during a fault  condition. The 
>green wire is also a reference to ground for GFCI's and provides  a dedicated 
>wire straight back to earth ground for maximum protection from  electrocution. 

Interestingly, GFCIs don't have a green wire connection.  (consider the two wire GFCI on a hair dryer)

They work by comparing the current in the two wires, and if it's more than 5mA different, it trips.


> 
>The only place the white and green should be connected together is at the  
>main disconnect panel, and the white should never be used for both a neutral  
>wire and a grounding wire if the grounding wire is not present. 

The one exception contemplated in the code is for older electric dryers.


Jim
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