BS 10621 Thief resistant locks lockable from outside only
A niche variant: the lock can be secured with a key from outside but not locked from inside. Used where internal locking must be prevented.
What it actually decides
- Outside key locking only
- Same attack resistance family as BS 3621 / BS 8621
Applies to these lock types
5-lever mortice deadlock (BS 3621)
Why a retrofit designer cares
Rare, but it explains why some doors cannot accept a motor that also locks: the specification deliberately forbids inside locking.