Which of the following voltage systems is solidly grounded?

• A. I only
• B. II only
• C. I & III only
• D. I & II only

Answer: (D)

Solidly grounded means the neutral is bonded directly to the earth with essentially zero impedance. You identify it by a direct neutral-to-ground connection with no resistor, reactor, or other impedance in the grounding path. This direct bond allows a line-to-neutral fault to produce a large fault current, which protective devices can sense and trip quickly to clear the fault.

In the diagrams, the voltage systems that show a direct neutral-to-earth bond, with no impedance in the grounding path, are solidly grounded. The third system includes some grounding impedance or an indirect connection, so it is not solidly grounded. So, the two systems with the direct bond are the solidly grounded ones.