Where should the lighting arrestor be placed in distribution lines?

It is used to protect the equipment from the strike ( excessive over voltage).
we don't put lightening arresot on line we usually we put surge arrestors.
The lightening arrestors are usually installed on the top of lighting pole or on the building roof to protect the installed equipments under from the strike .
The distributions, height, and numbers of them are based on something called shielding study

Near distribution transformers and out going feeders of 11kv andincoming feeder of 33kv and near power transformers in sub-station

at entry of substation in case of incoming high voltage transmission lines