Why do some electrical devices have a ground and others don't?
The IEC suggests 5 protection classes that are followed by the electrical appliance manufacturing industry.

According to IEC 61140, equipments does not require a safety connection to earth because the exposed parts are single insulated Class “0”, and double insulated class “II” (NOT class 2). 

Class 0 appliances can be sold to dry areas only, but in some countries sales or imports prohibited at all. Class II mark is like this (two squares)



EDIT

The simplest earthing design and install solution against insulation faults for mobile, semi-fixed and portable loads is this:



You can see the companies transformer (Y), common and earth.
Does not require coninues monitoring. Just a periodic check on the RCD, which is the device that protects the persons. Each insulation fault results an interruption in the supply of power.

The bellow connection it is not adviced for souch loads.