Why silver coating is given to copper strands in a cable? Is it improve the conductivity or for solder ability of conductors or for manufacturing ease to put insulation over conductors. Does tin coating is better over silver in terms of cost?

Industrial customers require tinned Cu wire. Some chemical industry and pulp industry cause that there is something corrosive in the air.
Insulation and sleeve help little bit against corrosion but there is always some visible Cu if wire is not tinned. There it change to green and when you pull wire it can broke easily. I believe tinned Cu-wire is enoug good for most use (and cheaper than silver). Silver is better for high corrosive environment and for high frequency.
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In case of cables the visible metallic white plating is Tin only not Silver. Silver can never be used to protect Copper, unless the coating or plating of Silver is thick enough so that not even minutest portion of copper is exposed to corrosive atmospheric. For example a copper coin can be fully protected from corrosion by thick enough silver coating, however, silver itself is not 100% corrosion proof. Only Gold and Platinum can be considered as almost corrosion proof but there again these cannot be used for protecting copper by being used as protective coating unless the coating is thick enough so as to cover copper 100% literally. This is not feasible in case of copper cables.

After discussing with my seniors I came to know some info.
When the wire size is selected based upon Ampere capacity, we are not increasing the conductivity by coating it with silver. Definitely it is for anti corrosion purpose and for solder-ability. Also it was told that to put Teflon/kapton insulation over the conductor, silver coating helps.
The coating of silver is a practice in aerospace/space companies for wiring conductors.
@Jari vetrtanen can you tell us sir, why do you think at higher frequency silver coating helps?
@ANAND KUMAR GUPTA , Silver forms a stable oxide, which is also a good conductor and by going to SPC no adverse effects. The coating of silver is up to around 10-15Um thickness is normally given.

Since "aerospace/space" is mentioned, I would like all to search Google for "red plague in silver plated copper wires". In space applications the practice of Silver coating of copper wires was practiced until the destructive effects of this practice were discovered and the problem that arises is "RED PLAGUE". Rather than me describing this phenomenon, it would be better if some brain storming takes place.

It must be borne in mind that in science and technology there are no set rules and as more and more understanding is reached about various physical, chemical, electrical, electro-chemical phenomenon applications are decided on a case-by-case basis, rather than being generalized.

Dear Gowrishankara, I find this from internet. Perhaps high frequency means communication/radio technology.

ELEKTRISOLA silver plated copper wire is composed of a copper core covered by a concentric silver plating. This material combines the advantages of copper, such as the conductivity, with the bright and shiny surface of silver. In addition, the silver coating provides high corrosion resistance. All these advantages make it the favorite choice for high frequency wire, as well as, colored textile wire.