Isolated AC voltage measurement
If the line switch is closed, but the neutral is open, what will happen depends upon a) whether the neutral wire happens to be grounded, and b) whether any part of your circuit happens to be grounded. Neutral wires are often grounded at the utility transformer. Whether your circuit is grounded anywhere depends upon how you design your circuit.
If both the neutral wire, and your circuit have a ground in them, then you may expect to see your AC voltage even when the neutral switch is open, if the line switch is closed.
Note that standard practice is that if a power circuit is not fully insulated from the outside world, it should be enclosed in a metal container, which container should be grounded. If your circuit is intended for use by someone else, this may be a requirement, but I am not a lawyer, nor an electrician.
Also note that &quot;solid-state switches&quot; like triacs/SCRs are not perfect switches. Even when they are &quot;off&quot; or &quot;open&quot;, they will conduct a little. if your ADC is sensitive in that it needs little current to make a measurement, it may &quot;see&quot; some of the AC supply voltage (reduced of course by your voltage divider) in spite of the fact that the triacs are &quot;off&quot;.