The ground and neutral being at the same potential is theoretical, but 
practice due to either errors or damage to the wiring, this might not be 
true. The ground and neutral (In the USA) are required to be bonded in 
the main panel legally, and that point should be grounded with a ground 
rod, or possibly a copper buried pipe. But, with the possible presence  
of plastic piping and accessories, like water meters, the ground paths 
may get destroyed, or even never in existence. Also when there is heavy 
current (for wire size) flowing in the neutral, ther will be a slight 
voltage drop due to the resistance of the wire and terminations. There 
is a requirement to install a heavy (#4 or #8 I believe) copper ground 
wire to the piping at both sides of the water meter, but with plastic 
underground piping that is futile, and a ground rod must be used.  When 
I installed our standby generator, I was careful to bond all grounds and 
neutrals starting at one panel point. The service disconnect is not in 
the main panel which has a main switch for the main breaker panel.
Herbert Graf wrote:
> On Thu, 2009-08-06 at 20:04 +1200, Jinx wrote:
>   
>> So I was chatting with the sparkie, as you do, and asked him why
>> the damage was closer to the Active/Live connection. (you can see
>> in the first picture that the damage pretty much stops where the
>> element turns around, ie halfway between A and N). He couldn't
>> say. He said that he asked once at college why, if the current is
>> AC, that Active and Neutral aren't regarded as interchangeable.
>> But the answer he got was "they just aren't"
>>     
>
> In NA the neutral is bonded to ground at the panel. The water in the
> water tank is at the same potential (the ground in the panel is a wire
> connecting to the cold water pipe). So, at the water heater, the
> potential different between the neutral lead and the water is very small
> (perhaps up to a volt or two due to a voltage drop across the neutral
> run to the panel). Obviously the potential different between the hot and
> ground is basically the full amount.
>
> TTYL
>
>   
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