Main - Tie - Main Arrangement With Tie Close

I suppose both the mains are from two different sources.

with tie closed.............

adv: your system is 100% reliable, even if one of the source fails i.e one of the main feeder fails, power to the load is always maintained through one of the sources since tie is closed.

Dis adv: fault level will increase. Difficult to set the relays.

For the protection you will need directional over current/earth fault protection in addition to the normal 50/51/50N/51N protection.