Which principle does the #87 differential relay operate on?

The correct answer is based on the principle that the #87 differential relay functions through the concept of current balance in an electrical system, which is fundamentally related to Kirchhoff's Law. This law states that the sum of currents entering a junction must equal the sum of currents leaving that junction.

In the context of a differential relay, it monitors the current entering and leaving a protected zone. The relay operates under the assumption that, under normal conditions, the current entering a transformer or other equipment should equal the current leaving that equipment. If there is a difference, typically attributed to a fault within the equipment, the differential relay will detect this imbalance and activate to protect the system.

While Ohm's Law relates to voltage, current, and resistance, and Faraday's Law pertains to electromagnetic induction, neither of these directly governs the operational principles of the #87 differential relay. Newton's Law primarily deals with motion and forces, which are not relevant in the analysis of systems protected by differential relaying. Thus, Kirchhoff's Law is the foundational principle driving the operational integrity of the #87 differential relay, making it the correct choice.