Key Features of MDB – Main Distribution Boards. Fully type tested assemblies as per IEC 61439-1; Rated up to 4000A; Rated operating voltage up to 690V; Manufactured in Form 2, Form 3 & Form 4 nstruction; Type tested for 65kA/3 sec, 50kA/3 sec; Top and bottom cable entry; Panels for front or rear access to suit any application; Protection