Plan b works during ovulation? if it's the case , a sperm can fertilize an ovule in hours , it can still be effective?

The Plan B: Causes a delay in ovulation, so that the egg should not get released and therefore not be available to the sperm to fertilize it. If a woman had already ovulated, then the Plan B wouldn't work.

Plan B can be: taken at any time during the menstrual cycle. “Plan B temporary stops the release of an egg from an ovary. Prevents Fertilization. Prevents a fertilized egg from attaching to the uterus.” REF: http://planb.ca/how-it-works.html