What is the objective?

Determine the average quality of the system's insulation? or
Measure the actual capacitive leakage current for sizing neutral groudnign resistors?

An insulation resistance measurement could be done on a de-energized system by jumpering  three phases together, removing any connection to earth and apply 1,000 VDC  with an insulation resistance test instrument.  (Commonly called a Meggar in North America).  The result will be the combined insulation resistance to ground of the system that was connected at the time of the test.  It would not provide much useful information about the condition of the individual equipment items: wires, transformers, panels, etc.

I don't recommend trying these test unless you really understand the risks.  Besides the obvious risk of working on electrical equipment, some devices connected to your system might get ruined by a DC overpotential test.

Capacitive charging current measurements can be done several ways on energized systems. Let us know what you are trying to do.
 

Hi there,
Sorry for the late replay, It came under my attention that the electricians do not know how to do the earth leakage test.
I need to draw up a one page prosedure on how to test and proof the operation off the earth leakage.
Yes I agree with you that to do a Megger test on the cable and motor as is de-energised and safe.
Will let you know on what we desided in South Africa.
Thanks
Pieter