During ground operations, which power source is used to protect CRT displays for a limited time?

• A. AC power
• B. DC power
• C. External power
• D. Battery power

Answer: (B)

The idea being tested is using a temporary, isolated power source to safeguard sensitive displays during power transitions on the ground. A battery provides this because it can deliver a clean, stabilized voltage for a limited time while the aircraft’s main or external power is being connected or stabilized. This isolation helps prevent damage or erratic behavior in CRT displays from transients, surges, or noise that can occur during ground power changes. Once the transition is complete and the primary power source is stable, the battery-backed protection period ends and normal power takes over.

AC power or external power would bring in the same supply path to the system and can carry in transient or noise during connection, while DC power is part of the ongoing power bus and doesn’t offer the same temporary isolation. The battery’s role is precisely to bridge that critical short interval safely.