Our overflow alarm failed recently, after more than five years, it was this one:
Serendipitously, I bought this a few days earlier just because it was a good price:
It’s a simple unit:
The first one would sound an alarm continuously until it was turned off, so you needed an electrician to wire it alongside with the water pump so that it’ll turn off when the pump was turned off.
This one is a little smarter, it has a voice announcement as if you’re in a railway station and it announces just once, in Hindi and English and I assume it waits for the next overflow event before announcing again.
For both of these, you’ll need a length of wire to the overhead tank. Copper or tinned copper is unnecessary for this, aluminum service wire works just fine. I used 2.5mm sq because I couldn’t find anything thinner.
Infact, for outdoor wiring (like for lighting etc), I’m finding aluminum service wire to be preferable because it doesn’t corrode as easily as copper, it is weather resistant and double insulated.