If a bus relay panel indicates the operation of relay 87, what type of relay has likely operated?

The operation of relay 87 indicates that a Differential Relay has likely operated. Differential relays are designed to detect significant differences in current or voltage between two or more points in an electrical system, typically within transformers, generators, or busbars. When the relay senses that these differences exceed a predetermined threshold—indicating a potential fault or abnormal condition—it will trigger and indicate operation.

The designation "87" specifically corresponds to the ANSI numbering system, which is a standardized way to identify protective relays. In this system, relay 87 is explicitly designated for differential protection. This underscores that when a bus relay panel shows the operation of relay 87, it is indeed signaling that a differential relay has sensed a fault condition, prompting its action.

In contrast, other types of relays mentioned serve different functions: Distance Relays are used primarily for detecting faults based on impedance measurements, Ground Relays are focused on detecting ground faults, and Overcurrent Relays operate based on current levels exceeding a set limit. While all these are crucial functional elements in a protection scheme, they do not correspond to the specific identification of relay 87.