What Is a Float Switch? Types and Selection Criteria
A float switch is a cabled level switch that tilts with the liquid level to open and close a contact; it is used for tank filling, drainage and pump dry-run protection.
A float switch is a level switch whose body, hanging on its cable, tilts as the liquid level rises and falls, opening and closing the contact inside it. It is used in water tanks, wells and drainage pits to run a pump in the filling or emptying direction, to prevent the pump from running dry and to detect water collecting in an elevator pit. A float switch provides an electrical contact; it is different from a float valve, which mechanically shuts off water at the tank inlet.
Types of float switches
| Type | Where it is used | Series in the catalogue |
|---|---|---|
| Cabled float switch | Water tanks, wells, drainage pits; pump control | Tense FLT: 1.6, 3, 5 and 10 m cable |
| Elevator level float switch | Detecting water collecting at the bottom of an elevator pit | Tense ASFLT: 1.6, 3 and 5 m cable |
The alternative to a float switch is electrode-based level control: electrodes work in a conductive liquid with a liquid level relay and have no moving parts.
Selection criteria
- Cable length: Add the distance to the connection point to the working depth. If the cable is too short, a joint is needed inside the tank; choose the right length from the start.
- Contact rating: The product data of the Tense FLT models states 8 A / 250 V AC, 50/60 Hz. Because a motor's starting current is several times its rated current, run the pump through a contactor and let the float switch control only the contactor coil.
- Function: For filling a tank, use the contact that closes when the water drops; for emptying a pit, use the contact that closes when the water rises.
- Cable and protection: The product data of the FLT models states IP68. For the ASFLT models the cable is given as 3 × 1.5 mm² rubber (H05); the protection rating and contact rating are not stated, so check the manufacturer's datasheet.
- Liquid: The models in the catalogue are specified for water. For sewage, fuel or chemicals, ask the manufacturer for a declaration of suitability.
Common installation mistakes
- Wiring the contacts the wrong way round: If the filling and emptying leads are mixed up, the pump runs with reversed logic. Before commissioning, lift and lower the float by hand and check the leads with a meter.
- Not setting the free cable length: The difference between the levels at which the pump starts and stops depends on the free cable length between the fixing point and the float. If it is too short, the pump starts too often.
- Restricting the float's movement: If the float catches on the pump, a pipe, a ladder or the pump cable, it stays in the wrong position.
Standards
IP68 is defined in IEC 60529 (TS EN 60529). The depth and duration of the continuous immersion test are agreed between the manufacturer and the user and must be more severe than the IPX7 test (1 m, 30 minutes); check the depth at which the float switch can be used in the manufacturer's datasheet.
Float switches in the Welibaba catalogue
All 7 active products in the Float Switches category are Tense:
- 4 cabled float switches: FLT-1.6, FLT-3, FLT-5, FLT-10
- 3 elevator level float switches: ASFLT-1.6, ASFLT-3, ASFLT-5
Prices and stock status are shown on the product pages.
Frequently asked questions
How is a float switch wired for filling and emptying?
On a three-wire float switch, one pair formed with the common lead closes when the water rises and the other closes when the water drops. Use the first for emptying and the second for filling. Confirm which lead closes in which position on the product label or datasheet.
How does a float switch protect a pump from running dry?
The float switch in the well or lower tank is wired on the contact that stays closed while there is water and stops the pump when the water runs low. In a two-tank system this contact is wired in series with the filling contact of the upper tank's float switch, so the pump runs only when there is water in the lower tank and the upper tank is empty.
Float switch or liquid level relay?
A float switch is a mechanical switch that works regardless of the liquid's conductivity. A liquid level relay detects the level with electrodes immersed in a conductive liquid; Tense specifies its own relays for conductive liquids. Choose electrodes for narrow wells or where moving parts are unwanted, and a float switch where simple pump control without a separate relay is wanted.
What does an elevator level float switch do?
It detects water collecting at the bottom of the elevator pit and sends a signal to the elevator control panel. Which input it connects to is given in the control panel's manual.
Can a float switch cable be extended?
If needed, the joint is made above the water level in a sealed junction box. The safer option is to choose a length that needs no joint; the Tense FLT series goes up to 10 m.