shorted insulation centralina compressor

I have a centralina made up of theree compressors.One of them found shorted insulation to earth. each compressor has soft start , current meter inbuilt , low and high pressure cut outs,besides the electrical MCB ,& Rcd . Client need to know the cause ! I still have to dismantle and inspect. It's my first time on this plant. Am I right to write possible causes :

Low voltage / voltage dips of mains supply

Liquid slug due to wet evaporators,

compressor wear and tear of 4 years

High head pressure due to irregular condenser fan motors On/Off through auto reset pressure switches


Any feed back is welcome