What is the minimum allowable door closing time according to NSTM 600?

• A. 5 seconds
• B. 6 seconds
• C. 8 seconds
• D. 4 seconds

Answer: (A)

Door closing time is a safety control that ensures compartments can be quickly sealed to limit damage and protect personnel during emergencies. NSTM 600 specifies a minimum closing time of five seconds, meaning the door must fully close within five seconds after being commanded. This five-second floor provides a practical balance between rapid isolation and reliable operation, whether the door is operated manually or by an automatic system. It helps prevent progressive flooding, maintains containment boundaries, and supports damage control readiness. Four seconds would violate the minimum, being too fast to assure reliable closure; six or eight seconds are slower but still meet the requirement, though they are not the minimum value. Therefore, five seconds is the correct minimum allowable time.