Shut off sensor for cistern

[WellMan]

I have a cistern tank in the attic which is fed from a big powerful pump down in the well. The pump is controlled from a float switch on top of the cistern. The float switch seems to control the pump via a relay.

I am concerned about tank overflows. I have recently had a cistern tank overflow in my rain water system and it was not pleasant, believe me.

I have fitted a nice fat overflow to both cisterns, but on testing it still does not carry the water away as fast as the pump can pump it up from the well.

I am thinking of fitting a side mount float level switch as a fail-safe, to shut off the pump if the water reaches a certain level. I am thinking of getting one of these http://www.ebay.co.uk/itm/Liquid-PP...al_Components_Supplies_ET&hash=item3f2b3f2568 and connecting it in series with the Stuart float switch.

Only thing is I'm concerned about voltages. The spec for the side sensor reads...

Contract rating(Max):50W
Switching voltage(Max):220VDC
Switching current(Max):1.5A
Breakdown voltage(Max):300VDC
Carry current(Max):3.0A
Contract resistance(Max):100 OHM
Temperature rating: -20 ~ +80 C
Material: PP

The float switch is one of these
http://www.stuart-turner.co.uk/media/70542-Accessories-Float-Switch-Datasheet.pdf

So, is this a sensible plan, or am I likely to cause a electrical/water fire disaster?

 

[Burnerman]

The side float switch can only switch low current and low voltage, by the look of it - but you could use it to switch a relay, which could then control the higher current pump.
Its not suitable for mains use.
John

 

[WellMan]

Right, so my understanding is that the current is not a problem, because the existing Stuart float switch is already controlling the pump via a relay, but that the voltage might be.

I have found a float switch which is rated at 250v, http://uk.rs-online.com/web/p/level-sensors-switches/0845976/, so am I correct in thinking that this would be fine connected in series with the existing float switch?

Max. Voltage 250V a.c./120V d.c.
Max. Current 0.6A
Max. Power 25VA resistance load
Electrical life 5 x 105 operations
Max wall thickness 8mm internal, 4mm external
Fluid 5.6 S.G. min 0.85
Thread Size M16 x 2 (19mm long)

 

[ALCPlumbing]

RS components will have everything you need. I set up a similar system in a rain water well to remove ground water from a basement flat. As stated you cannot use mains voltage due to the close proximity of water and the dangers that come with this but it is quite simple to use say a 12v supply through the float to power the coil in a relay which in turn can apply (or cut in your case) the supply to a mains powered pump.

 

[WellMan]

Thank you BurnerMan and ACLPlumbing for your help.

Looks like I need to put a voltmeter across the existing float switch and see what kind of voltage it is running at. If it's low voltage then I'm all good to connect in series, but if not then I have to learn about relays, which will be good for me anyway because the relays already in place won't live forever.