If the output disconnect is closed when the 28 VDC is lost, the disconnect shall open within how many seconds?

• A. 1 second
• B. 2 seconds
• C. 3 seconds
• D. 5 seconds

Answer: (B)

When a DC output disconnect is responsible for isolating a powered bus, the key requirement is rapid and reliable disconnection after the source is lost. If the 28 VDC supply disappears but the disconnect remains closed, energy could continue to flow from any downstream sources or stored energy, create arcing on the contacts, or keep equipment energized when it shouldn’t be. The specified behavior is that the disconnect must open within two seconds, ensuring quick isolation of the load and preventing back-feed or extended energization. This two-second window strikes a balance between fast protection and the practical actuation time of the switch, avoiding unnecessary nuisance trips while meeting safety and fault-isolation needs. Choosing shorter times would be more demanding on the mechanism, and longer times would leave the circuit energized longer than allowed, increasing risk.