On an SDC, what prevents power from being supplied to both the primary and alternate power source?

On an SDC, what prevents power from being supplied to both the primary and alternate power source?

• A. Manual disconnect switch
• B. Interlock relay
• C. Positive locking bar
• D. Dual-input fuse

Answer: (C)

In a setup with two possible power sources feeding the same bus, you must prevent both sources from being connected at the same time to avoid backfeeding or fighting between sources. A positive locking bar provides a physical, positive lock between the two source sections so that only one can be closed at a time. It mechanically interlocks the handles or contacts, blocking simultaneous closure and ensuring that energizing one source automatically prevents energizing the other unless the lock is released. This direct, physical constraint is why it’s the most reliable way to prevent both sources from supplying power at once. A manual disconnect switch can isolate a source but doesn’t inherently prevent the other source from being closed and backfeeding. An interlock relay can achieve mutual exclusion electronically, but the question emphasizes the mechanical barrier that the positive locking bar provides. A dual-input fuse protects conductors but doesn’t prevent both sources from being connected at the same time.

In a setup with two possible power sources feeding the same bus, you must prevent both sources from being connected at the same time to avoid backfeeding or fighting between sources. A positive locking bar provides a physical, positive lock between the two source sections so that only one can be closed at a time. It mechanically interlocks the handles or contacts, blocking simultaneous closure and ensuring that energizing one source automatically prevents energizing the other unless the lock is released. This direct, physical constraint is why it’s the most reliable way to prevent both sources from supplying power at once.

A manual disconnect switch can isolate a source but doesn’t inherently prevent the other source from being closed and backfeeding. An interlock relay can achieve mutual exclusion electronically, but the question emphasizes the mechanical barrier that the positive locking bar provides. A dual-input fuse protects conductors but doesn’t prevent both sources from being connected at the same time.