When can a senator be arrested?

Answer 1

When ever he commits a crime. (A Senator cannot be held accountable for any action taken in connection with the execution of his mandate)http://www.senat.gov.pl/k5eng/historia/noty/nota06a.htm

That does not mean he cannot be arrested. If the rest of the senators in office think he should be punished for his actions then he will be, but if the other senators do not think he should be punished, the crime can be put to a pending crime and he can be charged for it when he is released from office.

(Matters that cannot be pursued while a senator is protected by the parliamentary immunity are not subject to the statute of limitations. Therefore, if a proceeding against a senator cannot be continued during his term of office (i.e. he is not willing to forfeit his right to immunity and the Senate does not agree to revoke it), then it can be resumed after the term of office ends.)http://www.senat.gov.pl/k5eng/historia/noty/nota06a.htm

(A senator cannot be arrested or retained without the Senate's permission unless he is caught in the commission of an offence, and that only if the arrest is indispensable to ensuring a proper course of the criminal proceeding.) http://www.senat.gov.pl/k5eng/historia/noty/nota06a.htm