For transformer protection, the zone is defined between transformer Start Side winding and its earthed Neutral Terminal. By Darklanlan – Own work, CC BY 4. Differential Protection (87) The most sensitive protection for internal transformer faults: Note: Differential. This is exactly why a transformer protection relay is essential. Setting procedures are only discussed in a general nature in the material to follow. But the effect of a rare fault can be hazardous for the. Protection relays are protective devices used to sense various faults and give signals to circuits (master trip circuit) that execute various functions on the event of a fault, like provide an indication, data to control systems like SCADA, alarm activation, trip the appropriate breakers to isolate.