secondary ties ?

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peter_egypt

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cairo,Egypt
hello,
i have question it will help me to understand some pints:

what is the different between secondary ties and parallel operation of transformers ?

when transformers can be considered as separately derived systems ?
is outside the building means uncovered as not inside a room? so which be subjected to lightning ?
 
It is my understanding that a "tie" is where the primaries are switched separately; i.e., separate circuits, and the secondaries are tied together.
 
so is is the same thing like parallel ,so why code differs between them ?

i want someone clarify this also for me , in grounded systems regardless separately derived or not ,grounded means neutral connected to ground,
at services EGC+NEUTRAL connected to ground

but if i do connect transformer frame to neutral there would be an objectionable current ,through neutral ,and even didn't connect neutral there will be through the electrodes !! so that 250.6 will contrast with 250.21


in separately derived systems it is cleared connection to neutral to frame in one place and also to electrode in one place that's why there is not any objectionable current
 
There will not be objectionable current unless neutral is connected to ground in more than one place. In your transformer, SDS or not, there is only one ground to neutral bond.
In as SDS there is only one connection from neutral to ground, no connection directly to primary neutral and its ground-neutral bond.
In a non-SDS the secondary neutral is connected to primary neutral but not locally bonded to ground or EGC.

It is a tie if the two primaries come from different sources (different POCO service or even substations, or in some cases just a very different feeder.)
It is parallel if the primaries come from the same immediate source, whether there is independent disconnect or OCPD or not.
 
GoldDigger said:
There will not be objectionable current unless neutral is connected to ground in more than one place. In your transformer, SDS or not, there is only one ground to neutral bond.
In as SDS there is only one connection from neutral to ground, no connection directly to primary neutral and its ground-neutral bond.
In a non-SDS the secondary neutral is connected to primary neutral but not locally bonded to ground or EGC.

It is a tie if the two primaries come from different sources (different POCO service or even substations, or in some cases just a very different feeder.)
It is parallel if the primaries come from the same immediate source, whether there is independent disconnect or OCPD or not.
Click to expand...

so connection in this picture is a violation ?
a.jpg
 
If, as it appears to me, the whole transformer belongs to the utility, you have an earth-neutral bond at the secondary, which is correct, at the meter, and at the main panel. Although the last two bonds can only coexist under limited circumstances, that has no bearing on the existence of the parallel return path to POCO secondary through the earth.
The current is something you have to live with in our earthing system, but it will not be more than a fraction of the total neutral current and there will not be a major potential difference around the earth electrodes unless the wire neutral path from panel to transformer is missing or compromised.
The example of 5V offset implies a wiring failure, not a code violation, IMHO.
 
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