How does the European Code of Police Ethics separate the police from the criminal justice system?

The police must be clearly separated from the courts, the public prosecutor and the prison system, and have no power of control over them. Outside emergencies, the police may not act as prison staff.

This is the separation of powers applied to criminal justice. Each stage has its own role:

• the police investigate and collect evidence,
• the public prosecutor (Staatsanwaltschaft) decides whether to charge,
• the court decides guilt and sentence,
• the prison system (Strafvollzug) carries out the sentence.

If the police could control the later stages, the body that suspects someone could also convict and punish them, and nobody independent would check its work. Keeping the police out of prisons, except in an emergency, prevents the same concentration of power at the other end.

Go deeper:

• Wikipedia: Separation of powers — why the bodies that investigate, judge and punish are kept apart.