Role playing as chairman of dpc and appointing authority

I am a senior level executive in a famous Central PSU. The DPC Chairman and CMD of the Central PSU (Appointing Authority Capacity) are same person. I doubt the freedom of other DPC members assessing the suitability of candidates for promotion. Please tell me whether the Appointing Authority and DPC Chairman can be the same person?. Which rule is prohibiting ? Is there any Precedent which ban  the dual role. Thank you.