Mains voltage comes with two wires, the PHASE and the NEUTRAL. 230VAC is between PHASE and NEUTRAL. If you want to connect a motor or a lamp to mains, then you do it to these two lines.
While the NEUTRAL is normally connected to all metallic stuff like water pipes, etc., at the point where the mains cable enters your house and does not present a serious danger, if at all, when occasionally touching it, PHASE does!!. The reason for this is, that you are nearly at the same potential as NEUTRAL, when touching the floor with your feets.
Both wires, means PHASE and NEUTRAL must NEVER be connected to your microcontroller circuit, unless you do it to your mains transformer, if used!! They are only for lamps, motors, transformers and other mains voltage stuff.

Additionally to PHASE and NEUTRAL there's a third line often used in mains voltage installations, SAFETY EARTH. It's connected to NEUTRAL, at the point, where the mains cable enters your house. But then SAFETY EARTH is routed totally independently of NEUTRAL in your house! This SAFETY EARTH, which is routed by yellow-green wires can be used by the designer of a circuit to fullfill certain saftey requirements:
If your circuit is powered by mains voltage and must fullfill "Safety Class I" standard, then any metallic enclosure must be connected to SAFETY EARTH. If you also use a "Safety Class I" transformer, then the core of transformer (if touchable) and signal ground at the secondary side of transformer (0V of your circuit) must also be connected to SAFETY EARTH. If you use a "Safety Class II" transformer, on the other hand, then signal ground at the secondary side of transformer (0V of your circuit) need not to be connected to SAFETY EARTH. You can do it, then SAFETY EARTH is called "functional earth", but you don't need it.
If you use a "Saftey Class II" transformer in combination with metallic enclosure, then you only need to connect the metallic enclosure to SAFETY EARTH, if you cannot provide double or enhanced safety isolation between mains wiring and metallic enclosure.
So, in most cases "Safety Class II" transformers are used with plactic enclosures, where the SAFETY EARTH is not needed anywhere in this application. In this case only PHASE and NEUTRAL are needed, SAFETY EARTH can be omitted.
In most cases where a "Safety Class I" transformer is used, on the other hand, this part is put into a metallic enclosure and everything, means enclosure, core and signal ground at secondary side of transformer (0V of your circuit) are connected to SAFETY EARTH.


OV of your circuit, or the "component's GND" as you called it, should only get a connection to the ground pin of voltage regulator! Don't connect 0V of your circuit neither to the center tap of transformer nor to elswhere, otherwise you will suffer from ground noise, due to the huge rectifier pulse currents.
Of course, the rectifier section must be connected to the transformer's secondary windings and sometimes to an eventual center tap, but this is not the right place to reference 0V of your circuit to!

Kai