For appliances having higher workin

For appliances having higher working voltages than rated voltage, for example on the secondary side of a step-up transformer, or if there is a resonant voltage, the clearances for basic insulation are the largest values determined from– Table 16 based on the rated impulse voltage;– Table F.7a in IEC 60664-1 based on the steady-state voltage or recurring peak voltage expected to occur across it, if the frequency of the steady-state voltage or recurring peak voltage does not exceed 30 kHz;– Clause 4 of IEC 60664-4 based on the steady-state voltage or recurring peak voltage expected to occur across it, if the frequency of the steady-state voltage or recurring peak voltage exceeds 30 kHz.NOTE 1 Clearances for intermediate values of Table 16 may be determined by interpolation.If the clearances applied for basic insulation are selected from Table F.7a of IEC 60664-1 or Clause 4 of IEC 60664-4, then the clearances of supplementary insulation shall be not less than those specified for basic insulation.If the clearances applied for basic insulation are selected from Table F.7a of IEC 60664-1, then the clearances of reinforced insulation shall be dimensioned as specified in Table F.7a to withstand 160 % of the withstand voltage required for basic insulation.If the clearances applied for basic insulation are selected from Clause 4 of IEC 60664-4, then the clearances of reinforced insulation shall be twice the value required for basic insulation.If the secondary winding of a step-down transformer is earthed, or if there is an earthed screen between the primary and secondary windings, clearances of basic insulation on the secondary side shall be not less than those specified in Table 16, using the next lower step for rated impulse voltage as a reference.NOTE 2 The use of an isolating transformer without an earthed protective screen or earthed secondary does not allow a reduction in the rated impulse voltage.For circuits supplied with a voltage lower than rated voltage, for example on the secondary side of a transformer, clearances of functional insulation are based on the working voltage, which is used as the rated voltage in Table 15.Compliance is checked by measurement.