B-757 minimum fuel ground
 

B-757 AMM:
The three identical heat exchangers are in the wing fuel tanks and use fuel as a coolant. The left and light system heat exchangers are at the inboard end of the left and right main fuel tanks, respectively. The center system heat exchanger is adjacent to the left system exchanger. A minimum of 600 gallons (4020 pounds/1827 kilograms) of fuel is required in each main tank to provide hydraulic fluid cIooling.
Is this restriction applied for flight OR both in flight and on the ground?
Thanks!

The minimum amount of fuel is always necessary when your hydraulic systems are on, to provide them with cooling. So yes, both ground and air; however I don't think many flights are having less than 2 tons in the mains when still in the air.

Not 2 tonns.
There should be 1827 kg in EACH main tank. So, 3654kg that apprx 3700 kg.
In accordance with FPPM in might be more than 1.5 hour reserve.
In this case regulation's fuel reserve requirements might be less than manufacture's requirements. That is an item to think about that.

There is no minimum fuel limitation in our 757 FCOM. The "Low Fuel" EICAS comes on with about 1000 kgs remaining in either main tank.

I advised AMM information.
FCOM indicate Low Fuel situation. It's different.
Fuel less than indicated in AMM will cause increasing hydraulic liquid temperature and will cause increasing pump temperature in some ambient conditions.
Here is another item to think about...