Abstract: A power supply system for supplying electrical energy at an I.S. level to an electrical machine e.g. of a mineral mining machine, comprises two low speed generators A and B mechanically coupled together at coupling 3 and rotatable by means of a drive shaft 2, the mechanical input at the drive shaft 2 producing a separate, isolated electrical output at each of the generators A and B, the output from a first generator A being fed to an I.S. barrier 10, whereby an I.S. power supply is made available to power a first circuit 4, the output from the second generator B being fed to a non-I.S. supply circuit 8, and an I.S. coupling device 7 between the I.S. circuit 4 and the non-I.S. circuit 8, to achieve electrical and physical segregation of the circuits 4 and 8.