Need helps on multi range AC voltmeter
It's easy to protect this regardless of the isolator insulation.

The input resistors limit current, but NOT voltage; especially because the
gizmo will sometimes be turned OFF (thus, no negative feedback) you ought to
consider clamping pin 2 of the op amp to ground (i.e. pin 3) with a diode
pair.  That pin (under bias) won't be far enough from ground
potential to send any current into the diodes.

Input resistors (R4, R3, R2) might burn up if the diodes DO
conduct, a fuse in series with the input
would be a reasonable precaution.   Fuse resistance can be small and not affect
the calibration.

It won't stop lightning, but random dirt and sparks can bridge insulation,
and a little preparation is cheaper than a repair.