What is the difference between an intermediate class to station class surge arrester?

Station Class: Station class arrestors are typically used in electrical power stations or substations and other high voltage structures and areas. These arrestors protect against both lightning and over-voltages, when the electrical device has more current in the system than it is designed to handle. These arrestors are designed to protect equipment above the 20 mVA range.

Distribution Class: Distribution class arrestors are most commonly found on Transformers, both dry-type and liquid-filled. These arrestors are found on equipment rated at 1000 kVA or less. These arrestors are sometimes found on exposed lines that have direct connections to rotating machines.