Insulation of wire we say like wire has 5KV insulation,similarly in transformer we say 5KV isolation. both this sentence referring the same meaning that transformer wire having a 5 KV insulation

Insulation : the potential difference between the core & polish of wire

The varnish, (coating or "polish") of winding wires is there to insulate one wire from an adjacent wire for the purposes of functional insulation. Now, it may well be that there exists a wire with a varnish capable of standing off 5kV but this does not mean it's suitable as safety insulation.
The processing of wire in factories means that the varnish can undergo cracking or wearing as it is wound, therefore it is usual practice to provide a barrier of insulating material between layers of a magnetic component that operate at separated voltages. Soaking the windings in an impregnating varnish after assembly is usual practice. The barrier usually is comprised of a "distance though insulation" and a "creepage distance". For windings that can be contacted by a person, the insulation is most often "double" or "reinforced" meaning that there are several independent layers of insulation, or one system giving the same level of protection.
The values of creepage distance and isolation voltage are mandated by the safety standard governing the equipment and the "working voltage" across the isolation barrier.
So, a 5kV isolation voltage doesn't really mean much by itself even if it is a big number.
There is one type of construction however where you would take this into account: Pre insulated wire. For safety, the norm is to have two layers of insulation, any one of which can withstand the Hi-Pot test, or 3 layers of insulation, any two of which can withstand the hi-pot test. So, to use something like Furukawa Tex-E, you decide what is the isolation voltage requirement, then you select a wire from the chosen supplier to suit.

Safety agencies are concerned only for insulation between Chasis ground, motor stator or transformer yoke to AC line . Insulation between wire to wire is reliability part of the design.
In SMPS, the secondary is considered at chasis ground potential for safety evaluation. So we use triple insulated wire as one of the winding (primary or secondary) for all the transformers crossing the primary & secondary boundry. For transformers whose all the windings stays on one side (primary or secondary) , it does not need triple insulated wire. But if the potential between two windings are high (high side mosfet drive XFRM), use tape insulation between winding or for tight coupling use single insulated ( not vanished) wires.