IET wiring regulations – Seventeenth Edition; Amendment 3:2015
559.5.6 Protection against heat and UV radiation effects within luminaries. External cables and cores of cables connected within a luminaire or passing through shall be so selected and erected that they will not suffer damage or deterioration due to heat and UV radiation generated by the luminaire or its lamps (e.g. shielding of the cable from heat and/or UV by means of heat/UV-resistant sleeving).