If ground is a reference point and it is not physically connected how can it protect someone touching the wire from electric shock (I probably misunderstand what that symbol really means)
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In the context of your [idealized] diagram, the ground symbol doesn't imply protection from electric shock. There is a special type of diagram called isolation diagram, which is used in the industry during safety analysis.
In the context of your diagram, the notion of ground is just an engineering shorthand. A convenient node have been selected in the circuit, to which many other parts are referenced, which makes it easier to discuss the circuit. But ground is not fundamentally absolute (unlike 0°K is the absolute zero). You can move the ground symbol to another node in the circuit, the circuit will behave in the same way. It will only be more difficult to understand, less convenient.