Disconnectors Symbol in Single Line Diagrams (SLD)
The moving part is almost always on the load (de-energised) side, for the following reasons:


If it is three-position disconnector (closed-open-earthed), the load must be connected to the moving part so that it can be earthed.
If you have a fuse-switch-disconnector, the fuses are typically on the moving part. You want the moving part to be de-energised so you can safely access/inspect the fuses.


There may be more reasons but those are the main ones that come to mind right now.

Be aware that the orientation shown on the drawing often does not reflect reality, and at higher voltages many disconnectors are either rotary (double-break) or centre-break types, even though they might be shown on the SLD using the standard disconnector symbol.