Earthing of low voltage

Iffat Nahar said:

Is it possible to convert the TNS earthing system to IT earthing system and vice versa?

Yes, it's possible. Quite how will depend on the circumstances, it might be as simple as removing the N-PE link (or replacing it with a suitable impedance) on an exiting system, or you might need an entire transformer to keep the secondary side separated from Earth.

Is there any IEC standard about what safety precautions and switchgear protection system we should use on this ?

Again it depends on the circumstances - and what particular standards cover the situation. If for example it was part of building wiring, it would likely fall under national wiring standards. For the UK that would be BS 7671 and section 411.6 gives some of the general requirements for IT systems (often you'd need an insulation monitoring device of some kind, but there is a lot more to it than just that). That might well map back to IEC/HD 60364, but the details may differ, as may other countries interpretations in their national regs. IT systems are often used in places like Hospitals and Railway signalling systems - which often have their own standards (either instead of or in addition to national wiring regs), and if it's part of a particular item of equipment there may well be equipment standards that apply,

   - Andy.