The European Spallation Source (ESS) is a high brightness source of slow neutrons which is currently under construction in Sweden. ESS uses a linear accelerator to accelerate protons to 2000MeV. The superconducting cavities of the accelerator are supplied with power by a radio-frequency (r.f.) distribution system. The design, construction and operation of this r.f. system will provide valuable design information for an ADS driven by a linear accelerator. Here we provide an overview of the r.f. distribution system for ESS including protection against electrical breakdown in the waveguides.