What is differential protection and what does it monitor?

• A. It monitors line frequency stability across the system.
• B. It compares currents into and out of a protected zone (e.g., a transformer) and trips on internal faults.
• C. It measures the voltage across a transformer only.
• D. It protects by isolating the entire system from faults.

Answer: (B)

Differential protection works by watching the current on all sides of a protected equipment, like a transformer. It uses current transformers on every connection and compares the currents entering the protected zone with the currents leaving it. When things are normal, the currents balance and no trip is issued. If a fault happens inside that zone, some current is diverted within the equipment, so the incoming and outgoing currents no longer match. That imbalance triggers the protective relay to open the appropriate breakers and isolate the faulted equipment quickly. This scheme is selective for internal faults and will not trip for faults outside the zone, where the boundary currents still balance. It’s not about line frequency stability, it doesn’t rely on a simple voltage measurement across the transformer, and it doesn’t automatically isolate the entire system.