My interpretation of the rationale is from my 60335 experience. The idea is the same as Mold Stress testing. When molded, Polymeric materials have stress lines/points in them. The temperature exposure relieves these stresses which can move the material. The temperature is higher for polymeric parts supporting live parts vs an enclosure. We’re looking for reduction of spacings, openings that allow the finger probe, etc. Following is from IEC60335-1, 4th Edition……..
30.1 External parts of non-metallic material, parts of insulating material supporting LIVE PARTS including connections, and parts of thermoplastic material providing SUPPLEMENTARY INSULATION or REINFORCED INSULATION, shall be sufficiently resistant to heat if their deterioration could cause the appliance to fail to comply with this standard.
This requirement does not apply to the insulation or sheath of flexible cords or internal wiring.
Compliance is checked by subjecting the relevant part to the ball-pressure test of IEC 60695-10-2.
The test is carried out at a temperature of 40°C ± 2°C plus the maximum temperature rise determined
during the test of clause 11, but it shall be at least:
– 75 °C ± 2°C, for external parts
– 125 °C ± 2°C, for parts supporting LIVE PARTS.
However, for parts of thermoplastic material providing SUPPLEMENTARY INSULATION or REINFORCED INSULATION, the test is carried out at a temperature of 25°C ± 2°C plus the maximum temperature rise determined during the tests of clause 19, if this is higher. The temperature rises of 19.4 are not taken into account provided that the test is terminated by the operation of a NON-SELF-RESETTING PROTECTIVE DEVICE and it is necessary to remove a cover or use a TOOL to reset it.