, 100 ms for some 400 kV metal-clad substations up to 600 ms for lower voltage levels. The protection must remain stable during through-faults (outside the bus-zone), especially in the case of CT saturation and switching operations. The relay uses a setpoint to. This may vary from, i. Due to the high ratio of through-faults to. Busbar differential protection is the primary method for detecting and isolating faults within the busbar zone of electrical substations using Kirchhoff's current law principle. A. IEC 61439 is a standard developed by the International Electrotechnical Commission (IEC) that covers design verification for low-voltage electrical products and assemblies.