"Dimitrios Tzortzakakis" wrote in message
...
Yes, that's why in all around the world the neutral of LV distribution
transformers is earthed (so I learned in my studies) so that you get
schocked and know that electricity is dangerous (no joke) AND because in a
neighborhood e.g. someone would be touching a live phase and someone else
another, so that both would be exposed to the line to line voltage which
is
380 V in Europe (220 V in USA).However this happens only in LV, in MV, HV
and EHV there's no need for a reference to ground, because the windings of
the substation transformer are in delta, ungrounded.This doesn't make the
voltage not dangerous, I was told some horror stories back in Kozani.
--
Tzortzakakis Dimitrios
major in electrical engineering, freelance electrician
FH von Iraklion-Kreta, freiberuflicher Elektriker
dimtzort AT otenet DOT gr
---------------
In LV, MV , HV and RHV. the transformer windings can be and often are in Y.-
.. The most common connection is Delta Wye with the HV side in Wye with
direct or impedance grounding. There are good technical and economic reasons
for that. The next most common connection is probably Y-Y -delta for
supplying EHV lines . Note that even at 4160V distribution the supply
transformer is Y on the 4160V side.
Ground faults are the most common faults on transmission lines. This means
that the unfaulted phases will see overvoltages- not good. In addition,
protective relaying problems are increased and arcing faults which are not
detected and often undetectable can occur because of capacitive coupling.
The grounding eliminates these problems as well as problems due to the line
floating up and down due to atmospheric effects (thus stressing the
insulation). The use of a Y system makes life a lot easier.
--
Don Kelly
remove the urine to answer
Ο "Scott Dorsey" έγραψε στο μήνυμα
...
In article , w_tom
wrote:
Ioslation transformer must still have the secondary (the 120
volt side) connected back to the building safety ground. If
not, then the appliance will 'float' - can become electrically
hot. If the transformer is properly designed, then either it
has such a connection OR it has a dedicated grounding wire
terminal to make that necessary connection.
No, that's the point of isolation. Because the appliance floats, it can
become "hot" but the leakage current has no reference to the building
ground so if you touch the hot case, no current will flow through you.
Not that a safety ground isn't _still_ a good idea, mind you.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."