A dog will not be determined to be dangerous under section 502-A of the act (3 P. S. § 459-502-A) if:

1. (1) The premises where the dog is kept is a farm.
2. (2) The dog did not leave the farm property to attack.
3. (3) At the time of, and subsequent to, the attack, the farm was and is conspicuously posted at all points of ingress and egress with a sign alerting visitors to the presence of a watch or guard dog.