CSFi - Din Rail Compact Surge Filter the CSFi is designed for mounting in distribution boards to provide secondary protection against power surges caused by external sources such as lightning strikes and substation switching as well as providing a measure of protection from surge events generated on the secondary side of the filter. In addition, the unit also provides filtering of the line harmonics, noise and RF transmitters with a cut-off frequency of 10kHz and a nominal attenuation of 45dB.