hi,
i mean that when u r not using an overflow box.
using a partition and a hole drilled in the main tank, dun even hv to worried abt overflow box siphon breaks or nt.
when the pump failed, u will only worried abt certain amt of water being auto siphon back to the sump.
see pic:
lets say when my pump fails....
water from the main will be siphon back to the sump till the YELLOW line as i had drill 2 holes on the outlet pipe to let air suction.
in sump,
GREEN line is the actual water level when the pump still works.
Water siphon from the main will stop at the RED line.