This modular surge arrester is for use in single phase systems.   

The 2 DIN module wide rail mounted type 2 surge device is for TN  connections.

These are easily pluggable surge modules and they are for testing. They meet the requirements of BS7671 amendment 1 section 534.

They can also be wired in to the circuit with the AC isolator.

Keywords: DGMTN275, DEHN, Single Phase Surge Protection