Which device surrounds a wire to detect the electromagnetic field due to current?

• A. Incremental output
• B. Current sensing relay
• C. Building automation system output
• D. Flow switch

Answer: (B)

Detecting current by surrounding the wire with a sensing element relies on magnetic coupling: when current flows, it creates a magnetic field that a sensor around the conductor picks up and converts into a usable signal. A current sensing relay is built for this purpose, often using a clamp-on or through-pass arrangement that surrounds the wire to sense that field and produce a proportional electrical signal or trigger a control action. The other options don’t fit: an incremental output describes a signaling format from a position encoder, not a field-sensing device; a building automation system output is a control signal, not the sensing hardware; and a flow switch detects fluid flow, not electrical current.