Which component is grounded last in the unit substation grounding sequence?

• A. Ground line side of fuse
• B. Ground load side of fuse
• C. Open ACBs
• D. Ground secondary bus

Answer: (D)

Grounding in a unit substation is done in a deliberate sequence to keep workers safe and to prevent unwanted currents from circulating through control circuits while you’re working on high-energy parts. The high-energy paths—the line and load conductors and the switchgear (open ACBs and the fuses’ sides that can connect to the source or to the load)—are tied to earth first so any residual voltage is forced to flow through the intended protective paths and not through the control wiring.

The secondary bus, which carries the low-voltage control and metering circuits, is grounded last. Once the main energy paths are solidly earthed and verified safe, tying the secondary bus to earth completes the grounding with minimal risk of ground loops or voltage differences affecting the control circuitry. This sequencing protects the control system and ensures the safety of personnel working on the station.