Surge protection devices of category 2 or SPD (Surge Protection Devices) are installed in power distribution networks to protect people, systems, and devices from unacceptable, short, impulse-like surges. They can be used in network systems between L-N and L-PEN. A defect is indicated by a red color in the viewing window. The status is provided as a potential-free switching contact via a plug in the accessory package.