Backyard Aquaponics
A place to discuss aquaponics

I'm about to set my AP system up and am thinking of using 3 electronic timers that have 8 on/off programs, and use them sequentially timer1- timer2- timer3-1-2-3 etc to get my 24 * 15 min on periods over a day. If one fails I will only miss out on every 3rd pumping cycle, rather than missing out on pumping for 8 hours straight if each timer was used to cover an 8 hour period. The ones I use in the house will switch 10A (@230V) but I also have a more compact one rated for 30A... either one is more than enough for a ~100W pump drawing under 1A, so the contacts should last a long time.

Will they be plugged into each other? - i cant think of another way to have them sequential.
if they are, then it only will take one to fail for the whole system to go down. - that depends on how they fail, you would be lucky for them to fail, stuck in the on position.

No they are not sequential- they will be in parallel. Each one will close the circuit to turn on the pump in turn. I'll build them into a waterproof electrical housing with a 3 pin plug on a lead to plug into my power point, and there will be a 3pin socket to plug the pump into.
The outputs will be in parallel, so that any one turning on will activate the pump. That will enliven the other 2 leads in parallel back to the other 2 timers, but since they are not connected to anything (the timer is off), there is no problem with that. Even if one failed and happened to be stuck on, it is at the same potential, so no problem- the pump would then be running continuously.

If 1 timer fails and is off, I still get 15 mins pumping for 2 out of every 3 hours. In the unlikely event of 2 failing, I would still have 15 mins (or whatever I have it set to) pumping every 3 hours. I'll be using an 8000l/hr pump, and will require up to about 2000l/hr max, but will need to test it to see the actual pumping rate through the poly pipes.
I know, I should put my plans into an intro thread, and will do soon