IDEC FL1E SmartRelay - Manual (Page 41)

IDEC SmartRelay installation and wiring
IDEC SmartRelay Manual 27
6. Using a screwdriver, push the interlock to the left. In its
end position the slide interlock engages in IDEC
SmartRelay base module.
Repeat steps 3 through 6 to mount further expansion
modules.
Note
The expansion interface on the last expansion module must be
covered.