Quench detection and protection, and related safety systems are crucial for reliable and safe operation of superconducting magnet systems, in this case the very large toroids and solenoid of the ATLAS Detector at the Large Hadron Collider at CERN exhibiting a stored energy of 1.6 GJ. The PLC-based magnet control system is the main point of interaction between operators and magnets, deciding on allowed actions and low-level safety measures. Its evolution is continuous, taking into action new demands and interlocks. The magnet safety system is an in-house design, protecting the magnets from fatal errors on the systems themselves. Decisions are made based on machine inputs and overlapping, multiple type sensors. A magnet diagnostics system continuously reads and stores magnet parameters and machine states. Since the prototyping, installation and early operation, valuable experience has been gathered. The magnet quench detection and protection circuits are designed with a twofold redundancy, and feature two physically separated safety chains. Also within a single quench protection chain a more than six fold redundancy is present in detection also using various detection principles like bridge type, differential voltage and superconducting quench detectors for use on superconducting bus links interconnecting coils. Other safety channels look at the status of magnet services like vacuum, cryogenics and electrical circuit components and guarantee proper and safe ramp down of the magnets. Here the experience gained is reported as well as the system response to various events occurred, in particular to the very few true magnet quenches and requested fast dumps. All have gone through long-term live testing on both the toroid and solenoid magnets, following which upgrade proposals are being formulated or already implemented.
