IEC 60358-1-2012 pdf download.Coupling capacitors and capacitor dividers – Part 1: General rules.
Applied general rules:
— The rated positive wet switching impulse withstand voltage is the base for the determination of the minimum arcing distance (external insulation) of the equipment.
— The strength of the external insulation is usually tested wet with the rated short duration power frequency withstand voltage (range I) or with the positive wet switching impulse withstand voltage (range II) (see 10.2).
— The value of the rated lightning impulse withstand voltage is one factor with which to determine the strength of the dielectric of the capacitors.
— For a.c. applications in IEC 60071-1. for each Urn only two standard withstand voltages are sufficient to define the standard insulation level for the equipment:
• range 1 Urn <300 kV: rated lightning impulse withstand voltage and rated short- duration power-frequency withstand voltage
• range II: 300 kV < Urn < 800 kV: rated switching and rated lightning impulse withstand voltages
— For d.c. applications, no standards are available: the insulation level has to be defined between manufacturer and user. Usually for each rated voltage only two standard withstand voltages are sufficient to define the standard insulation level for the equipment:
• range I: USIL <750 kV: rated lightning impulse withstand voltage and corresponding rated power frequency withstand voltage in wet conditions (corresponding to the BIL voltage on table 3
• range II: 11S1L  750 kV: rated switching (wet conditions) and rated lightning impulse withstand voltages
— Due to the non-self-restoring internal insulation of equipment, for range II three standards withstand voltages are specified in Table 3. The short duration power frequency withstand voltage test (resp. d.c. voltage test for d.c. applications) has been specified for range II as a routine test with partial discharge measurement. The stress with a.c. voltage (resp. d.c. voltage for d.c. applications) determines the long term behaviour of the non-self-restoring internal insulation of the equipment.
— The rated short-duration power frequency withstand voltage test (Table 3, column 3), with partial discharge (PD) measurement in range II is an indication for the stress on the insulation of the equipment.
— The rated insulation level is based on the highest voltage for equipment Urn, whereas the thermal requirement of the equipment is based on the rated voltage lip.
— The choice of the insulation level shall be made in accordance with 5.2.1 and IEC 60071-1.
6.2 Other Insulation requirements
6.2.1 Low voltage terminal not exposed to weather
Equipment with a low-voltage terminal shall be subjected to a test voltage between the lowvoltage and earth terminals. The test voltage shall be an ac. voltage of 4 kV (r.m.s. value).IEC 60358-1 pdf download.