The Merz-Price principle is applicable to a multi-terminal zone such as a busbar. The principle is a direct application of Kirchhoff's first law. Usually, the circulating current arrangement is used, in which the current transformers and interconnections form an analogue of the busbar and circuit connections. A relay connected across the CT bus wires represents a fault path in the primary system in the analogue and hence is not energised until a fault occurs on the busbar; it then receives an input that, in principle at least, represents the fault current. The scheme may consist of a single relay connected to the bus wires connecting all the current transformers in parallel, one set per circuit, associated with a particular zone, as shown in Figure (a). This will give earth fault protection for the busbar. This arrangement has often been thought to be adequate.
If the current transformers are connected as a balanced group for each phase together with a three-element relay, as shown in Figure (b), additional protection for phase faults can be obtained. The phase and earth fault settings are identical, and this scheme is recommended for its ease of application and good performance.
Protective Relays Application Guide, ALSTOM