Instrument Earth is also called as Reference earth. Instrument Earth all individual shields(Screen) and over all shields single or multi pair cable shall be isolated from electrical earthing due to avoiding interference and terminated different bus bar.
Image courtesy - Erico
Instrument bus bar will be connected to grounding dispatcher by 25 sqmm green- yellow cable.Grounding dispatcher to earth pit is connected to 70 sqmm green -yellow cable.
Electrical earthing also called as dirty earth or protective earth.It is used to protect the power system,electrical equipment and personnel from electric shock.