Which device prevents simultaneous connection to primary and alternate sources on an SDC?

Which device prevents simultaneous connection to primary and alternate sources on an SDC?

• A. Interlock switch
• B. Fused disconnect
• C. Circuit breaker
• D. Positive locking bar

Answer: (D)

The main idea is preventing backfeed by making sure only one power source connects at a time. A positive locking bar provides a physical interlock that blocks the transfer switch from closing both sources simultaneously. It ties the two positions together so you can’t have primary and alternate connected at once, even if someone tries to move both handles. This hard, mechanical lock is what guarantees exclusive connection and protects the system from backfeed or parallel operation. Other devices don’t offer that same level of guaranteed separation. A fused disconnect or circuit breaker protects and isolates a source, but doesn’t prevent the mechanism from being positioned to connect both sources at once. An interlock switch may sense position or gate control, but it isn’t the definitive mechanical barrier that ensures only one source is connected.

The main idea is preventing backfeed by making sure only one power source connects at a time. A positive locking bar provides a physical interlock that blocks the transfer switch from closing both sources simultaneously. It ties the two positions together so you can’t have primary and alternate connected at once, even if someone tries to move both handles. This hard, mechanical lock is what guarantees exclusive connection and protects the system from backfeed or parallel operation.

Other devices don’t offer that same level of guaranteed separation. A fused disconnect or circuit breaker protects and isolates a source, but doesn’t prevent the mechanism from being positioned to connect both sources at once. An interlock switch may sense position or gate control, but it isn’t the definitive mechanical barrier that ensures only one source is connected.