d) Never close a door or drawer after setting a new combination
until the new combination is worked at least three times and is recorded in
accordance with security directives.
e) On a mounted three-wheel lock, there are 1,000,000 possible
combinations. Some combination padlocks have as many as 125,000 possible
settings. However, because of mechanical and psychological factors, not all
of these combinations are valid. Specifically, follow these guidelines:
(1) No two numbers should be closer than exactly five digits
apart, or multiples of ten.
(2) The third number should not range from O to 20 or from 85
(3) Do not use straight ascending or descending combinations.
Examples: 41-56-71, 81-58-30. It is better to use a sequence that is HIGH-
LOW-HIGH or LOW-HIGH-LOW. Examples: 72-23-81, 33-67-38.
(4) All three numbers should not be even.
(5) All three numbers should not be odd.
(6) Never set a 50-25-50 combination (the standard factory
combination sequence) on the lock of a container that holds classified
material. When removing a container from service, set combination to
(7) Do not use significant dates (birthdays, anniversaries,
etc.), street addresses, telephone numbers, etc., as the basis for
(8) When locking a safe, door, or cabinet, turn the dial at
least four full revolutions in one direction.
Mounted Combination Locks. Underwriters Laboratories' Standard UL
768, Standard for Combination Locks, classifies mounted combination locks as
either Group 1, Group 1R, or Group 2. To qualify for a Group 1R rating, a
lock must be manipulation, radiographic, and thermal resistant. Group 2
combination locks must be reasonably resistant to unauthorized opening.
Combination Padlocks. Combination padlocks are classified according
to Federal Specification FF-P-110 (Series), Padlock, Changeable Combination
(Resistant to Opening By Manipulation and Surreptitious Attack).