After Large Hadron Collider will be turned OFF, a new, more energetic machine will be needed in order to explore unknown regions of the high-energy physics. For this reason, the project Future Circular Collider (FCC) has started, with the goal of developing a 100-km-circumference collider of 50 TeV proton beams. The Eurocircol collaboration is part of the FCCstudy under the European Community leadership, and it aims to develop a conceptual design of FCC till 2019. One of the main targets is to design a bending dipole able to reach 16 T operation magnetic field, in order to accomplish the size and energy constraints. Such a magnetic field can be reached using Nb3Sn conductors. One option under exploration is the Cos. dipole, by INFN of Milano and Genova. Because of the high stored energy and the large current densities due to the conductor performances, quench protection is one of the most challenging aspects of the design. In this paper, the quench protection of the cos. design is presented. A standard quench protection study is accompanied by a less conservative study which includes ac effects on the power dissipation inside the coils and on the magnet inductance, in order to not exclude preventively more convenient designs, and to develop a more performing magnet as possible.