A long is substituted for the i in kṣi before the Past Participle in kta when it is not in the sense of the future Passive Participle in ṇyat,
The force of ण्यत् is to denote condition (Impersonal action) and object (passive). When the Past Participle has not the force of ण्यत्, the vowel of क्षि is lengthened. Thus आक्षीणः, प्रक्षीणः, परिक्षीणः all used in the active sense. The क्त is added to the Intransitive क्षि to denote the agent 3.4.72. Thus प्रक्षीणमिदं देवदत्तस्य 'this is the spot where Devadatta perished'. Here क्त is used with a Locative force 3.4.76. Why do we say 'not having the force of ण्यत्?' Observe अक्षितमसिमामेक्षेष्ठाः ॥ Here त is added with the force of condition, and akshitam means 'imperishable'. The vowel not being lengthened the त is not changed to म 8.2.46.,