3rd protection stage is an essential part of 3-stage overvoltage protection cascade. The typical representatives of this protection type are for example transient overvoltage protections- range PI-k* and PI-3k*. The products reduce overvoltage (thanks to their inside connection-high protection, filter, low protection) to the level UP<0,8 up to 1kV, which is safe for the final appliances. They are usually constructively fitted on DIN rail 35mm.
It is important to place these products as near to the protected appliance (for example flat switchboards) as possible. The distance between switchboard and appliance must not overreach 15m. When there is a longer distance it is necessary to use other class III overvoltage protection devices (for example protection sockets or overvoltage protection on DIN rail) cca 10m far away from each other along the protected socket line.
On the other hand protected sockets are in no way equivalent substitution for 3rd stage protection with high-frequency filter.