When a tank is filled up by an automatically operated way, like a float switch or float ball, an overflow solution is desirable to avoid flood. If the the tank is an underground one, this is highly recommended.
There are more way for sorting this problem, but the most common is the gravity way into a natural depression (pit, river, brook, seeage) via an overflow pipe.
When the tank is located deeper than all of these points, the overflow must be sorted on a boosted way, like a dedicated overflow pump. In this case the power supply of the pump is connected to a dedicated float switch.
Operation, test and maintenance
Overflow pump can be tested by lifting up its float switch a few times. If the pump starts every time, the overflow works. You have to make sure that the float switch is positioned on the same level than before and can float and hung freely when you release in the tank at the end of the test.
There is no way to check gravity overflows sometimes because the outlet of them are not even visible in an underground tank. There are a few thing we can do to insure their working.
If the other end of overflow is known and accessible, check i at the highest water level in the tank. If there is no water flows out, the highest level is set accurately. Then fill over the tank by lifting up the filler float switch or activating the float ball and see if the water starts flowing via the overflow. It tells that the overflow pipe is not blocked.
if the overflow is completely hidden, check the tank area regularly. When the tank is overfilled, the overflow is blocked likely.
Filterize every possible tank inlet and make sure the filters are in good working order. Clean them regularly, especially the rain harvesting filter if it’s applicable.