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The latest generation of fusion devices needs large superconducting magnets that have to withstand operational voltages of tens of kV. Their insulation typically targets a dielectric strength of about 10 times the operational voltages to cope with degradation during operation resulting from the high electromagnetic loads with cyclic components, thermal stresses due to cycles from 293 K to 4 K, and ionizing radiation. Degradation of dielectric strength is further mitigated by applying solid polyimide overlapped layers in the insulation. The level of Lorentz forces, together with the large superconducting coil sizes, makes high-voltage tests impossible to realize under exact operational conditions. The fact that insulation is very difficult or even often practically impossible to repair in case of electrical fault, requires sound insulation design to reach utmost quality. In this paper, we will discuss the build-up of the electrical insulation in large superconducting magnets both from the manufacturing and operational point of view. In particular, the impact of the insulation design and manufacturing choices on the quality will be discussed in depth. We will also discuss the implementation and the application of insulation systems.