Earthing

i am assuming electrical earthing refers to using the metal structure as an earth return thus saving wires. as used only on low voltage systems ie cars

normal earthing possibly refers to the connection of the metal structure to the ground thus preventing the structure becoming live in the event of a fault

IEEE 1100 would be a good reference

Dear Sir

Electronic earthing is a dedicated earthing for electronic equipments seperate from the normal electrical earthing of metal parts.

It is normally through copper unarmoured cables/bus and are connected to dedicated earth pits.

But there are some firms which do not recommend seperate ie dedicated earthing for DCS/UPS etc.

In our organisation we follow seperate earthing irrespective of manufacturer's recommendation.

Further as guided by others u may refer to IEEE for more clarity on the subject.

regards

IEEE

http://www.ieee.org/portal/site