help with my sump

[DEAD_BY_DAWN]

ive just put in a 50g sump on my 90g tank,the sump holds give or take 35-40g of water,my concern is if the overflow blocks for some reason the pump will keep pumping and the result will be 35-40g of water on my floor once the tank overflows is there any sort of divice i can put in the sump so when the water level lowers to a certain point the pump will be switched off ? ..thanks for any help with this john