As I understand, the limited liability that police enjoy, requires
that people bringing civil cases against police must prove that the
police person should have had a reasonable knowledge of the civil
rights that you accuse him of breaking, for the civil case to be
This isn't quite right.
The test you are referencing is the one for qualified immunity from civil liability under 42 U.S.C. § 1983, which imposes liability on government officials only for violating a "well-established" constitutional right.
A rule of law is "well-established" when there is controlling case law in that jurisdiction when a factually similar binding precedent exists in that jurisdiction to show that the alleged conduct is unconstitutional.
This test is employing the legal fiction that police officers are familiar with all of the binding precedents in the jurisdiction regarding what constitutes a violation of a constitutional right, which is held against officers. Of course, in reality, almost no police officers have that exhaustive a level of understanding of the law.
What the test does, however, is to prevent police officers from being held civilly liable for money damages when they take action which, in fact, violates a constitutional right, but which no case law in a factually similar case that was binding precedent established before the incident took place. Thus, police officers are relieved of liability for incidents that they would have to predict that a future court would find violated a constitutional right. This is sometimes phrased as being justified because a reasonable police officer could not have foreseen a new rule of constitutional law or a novel application of an existing rule of constitutional law to a new situation.
One of the reasons that the qualified immunity rule is controversial, however, is that courts have the discretion to decide a case on qualified immunity grounds without determining if the underlying action indeed did violate a constitutional right, and this prevents constitutional law from evolving normally over time to applications in new factually novel situations.
The key point, however, is that this requirement that a constitutional right be well-established to be enforceable in a civil action is an "objective" test in that it is decided without any reference to what the particular individual being sued actually knew about the law in the particular circumstances presented.
A police officer who acts without actually knowing the law does so at his or her peril.