What system that makes sure no branch becomes too powerful?

In theory the separation of powers, which is interwoven into the constitutional fabric. there are three principle branches- the Legislative, which makes the laws, the executive, which carries them out- including the presidency, and the Judicial. There are overlaps and checks and balances familiar to all students of PAD ( problems of Americ an Democracy) The interlocks work like- well certain powers are denied to the states ( coining money, for example) states are not supposed to have border taxes for interstate commerce ( but, in practice they have tolls on tunnels and bridges which have the same effect) automobiles are still ( at this writing) licensed by individual states, but aircraft, ships and radiOS ( including amateur and CB sets) are federal. so it goes. there are a number of overlaps- I have mentioned the fact that border taxes are not only illegal but unconstitutional, but this is rationalized as maintenance fees on Bridges and tunnels- which often cross state lines! so it goes.