IEC 60664-1 is the basic safety publication that establishes how insulation is dimensioned in equipment connected to low-voltage supply systems up to 1000 V a.c. or 1500 V d.c. It provides the framework used by a very large number of product standards, which refer to it rather than restating the rules. The central idea is that insulation must withstand the transient overvoltages that the installation can deliver, so the standard defines overvoltage categories that describe the position of the equipment in the supply network, from the service entrance to the protected secondary circuits. From the rated impulse voltage chosen for each circuit, and taking into account the pollution degree of the environment and the comparative tracking index of the insulating material, the standard derives the minimum clearances and creepage distances, the required thickness of solid insulation and the values used for dielectric and impulse testing. It also treats the special cases of insulation between circuits at different voltages, coated and potted assemblies and printed board design. Both the computational rules and the verifying tests are given.