SBD Differential Voltage Relay. Description. The Type SBD11B is a single-phase, high-speed differential relay. It is specifically designed to provide bus differential protection, but may also be used for differential protection of shunt reactors. A high impedance, voltage measuring circuit with overcurrent supervision is used for fault detection. The utilization of static circuitry results in very short operating times. Output isolation is obtained with a telephone-relay, and a target seal-in unit is provided. A voltage selecting link allows the relay to operate on 48, 125, or 250 VDC. The relay is packaged in an S-2 type drawout case and requires no auxiliary CT's.