... you don't have the mess of a common "neutral"
digsys: I am not sure what the problem is here.
Refer the 2 options -
In option A, the OUTPUT has a potentially DIRECT connection to the Isolated 240V. Our "student" has dialled
up 12VAC and is happy that it's safe !! BUT one side of his output is at A2 or N2.
The "neutral" (in rabbit ears) was to convey the fact that this connection is still at mains reference, just being lazy.
Option 2 is totally isolated at the 12VAC.
Some may think this is pedantic, but I live with SAFETY in education all the time. Anyone is quite FREE to voice
whatever opinion they desire, and it's saved here on the forums for ever !!
OK, I can see that in "A" you can get a shock if you :-
(1) Are across A2 & N2,or the equivalent points on the Variac winding so you receive the whole 240 volts from the isolation transformer secondary.
(2) Are across the entire Variac winding,so you may receive a shock of greater than 240 volts.
(3) Are between the wiper & the A2 connection for a 228 volt shock
(4) As(3) but between the wiper & the extended (unused) part of the Variac winding for a shock of greater than 228 volts.
(4)Are across the tap & N2 when the last (12v) section of the Variac winding becomes open circuited,elevating the wiper point to 240 volts.
All these possibilities except (4)are extremely unlikely because the Variac is heavily insulated.
In no way is there a lack of insulation between the output of this setup & the "Street" mains.In the case of "B"
If you stay at the output of the Isolation transformer,you can get a shock if:
(1) The Variac is wound up enough & you get across the two sides of the transformer secondary while there is a dangerous voltage present.
(2) As in (4) above,the last section of the Variac winding becomes O/C,increasing the voltage to the Isolation transformer's primary,although having most of the Variac winding in series with its primary will stop it seeing the full 240volts.
Neither of these setups are a safe method of obtaining low voltages for permanent use!I agree that the connection of "B" is the better one,but more from considerations of practicality than anything else.
The most logical setup is to have the Isolation transformer & Variac as separate units,so they may be used where only one or the other is required.