from wiki "In electronics testing and servicing an isolation transformer is a 1:1 (under load) power transformer used for safety. Without it, exposed live metal in a device under test is at a hazardous voltage relative to grounded objects such as a heating radiator or oscilloscope ground lead (a particular hazard with some old vacuum-tube equipment with live chassis). With the transformer, as there is no conductive connection between transformer secondary and earth, there is no danger in touching a live part of the circuit while another part of the body is earthed."