Neutral Link

Change the name of it, then you can use it in Phase side in AC circuits.

On sigle-phase circuits, isolation of the phase is enough to stop the current. So single-pole devices (eg. MCBs in the IEC world) are used on the phase, and the neutral is not interrupted. A neutral link gives you the option of removing it to isolate the circuit completely, generally for maintenance.

UH DO YOU MEAN WHY ARE THERE TWO WIRES ?

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This is only used in some part of the world - say - Old countries-poor countries-and countries have knowledge but no money.

Old countries?..........poor countries?

Now I understood your point old civilization, rich in culture