In misfire cases, what is the rule for expiry time to return to the blasting area?

• A. The longer of 30 minutes or twice the number of minutes equal to the meters in the longest fuse used
• B. Exactly 30 minutes
• C. Twice the burn time of the fuse in seconds
• D. One hour

Answer: (A)

The principle being tested is safety timing after a misfire: you must wait a written minimum period that accounts for the longest fuse used, so you don’t re-enter the area too soon. The rule uses the longest fuse as the basis because it represents the worst-case burn time for the shot. You take the burn time of that fuse, double it, and then compare it to a fixed minimum of 30 minutes; you wait for the longer of the two times. This approach protects against delays in detonation or delayed energy release and adapts to how long the fuse actually takes to burn.

If the longest fuse would burn for longer than 15 minutes, doubling that burn time will exceed 30 minutes, making the wait longer than the fixed 30-minute minimum. Conversely, if the longest fuse burns quickly and its doubled time is less than 30 minutes, you still need to wait at least 30 minutes. That combination ensures a safe window tailored to the specific fuse used.

Why the other options aren’t suitable: a fixed 30-minute wait doesn’t account for longer fuse burns and could be unsafe if a longer fuse is involved; using twice the burn time in seconds mixes units and ignores the standard minimum of 30 minutes and the fuse length; a flat one-hour delay ignores the actual burn characteristics of the longest fuse and is unnecessarily conservative in many cases.