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Hager breakers are available for that type of fuse box, which clip onto the din-rail, in place of the fuse holders. 1) Electrician will need to switch off, 2) remove the front cover by the two quarter-turn screws, 3) then slacken off the clamp screw on the base of the fuseholder, 4) then release the upper screw clamp containing the circuit phase core. 5) Unclip the fuse holder with an electrician's insulated screwdriver inserted into the little catch at the base of the fuse holder. 6) Clip in the replacement breaker onto the din-rail, 7) tighten the lower clamp screw to engage with the busbar. 8.) reconnect the phase core into the top cable clamp on the new breaker. 9) restore supply.