Float switch

Two types cover almost every installation: the tethered float and the vertical float. They switch on the same principle and behave differently in the chamber.

The tethered float, sometimes called a ball float, hangs on its own length of cable from a fixed point and swings out with the surface. It tips at one level on the way up and a lower one on the way down, so the free length of cable sets how wide that band is. Give it less free cable and the two levels move closer together.

The vertical float, or rod type, rides up and down a stem and closes a magnetic reed contact at a fixed point. It has a narrow, fixed switching band and it needs almost no room sideways, which is why it goes into narrow chambers and packaged tanks where a tethered float has nowhere to swing. Calpeda lists a vertical magnetic float switch as an option on the GQG grinder for that reason.

Each costs you something. A tethered float gives you a wide band and a long, slow pump cycle, but it wants clear space and it will foul on rag. A vertical float fits anywhere and fouls less, but its narrow band means the pump cycles more often for the same inflow.

A wet well has three levels to set, not one: where the pump stops, where it starts, and where the alarm goes off. A stop float sets the bottom of the working range, a start float sets the top, and a high-level float sits above both. On a duty and standby station a further float brings the second pump in when the first has not held the level.

The simplest set-up collapses the first two into a single float doing both jobs. It makes on the way up to start the pump and breaks on the way down to stop it. That is what you get on a small drainage pump with a float on the end of the cable, and it is fine for a cellar sump. It is not enough for a foul chamber a building depends on.

The high-level float is not a control device and should not be wired as one. It is an alarm. When it makes, the pump that should have dealt with the inflow has not, and something needs looking at: a blocked impeller, a shut valve, a failed start float. Wiring the alarm float to start a pump hides the fault you wanted it to report.

On a submersible set the stop level is a mechanical protection, not a convenience. Set it too low and the pump runs with its motor jacket out of the liquid, and on a wastewater duty that is one of the three things that kills pumps in service, alongside rag and grit.

The mechanism matters. On a centrifugal pump, and most submersibles are centrifugal, the usual result of dry running is a failed mechanical seal, because a mechanical seal needs liquid to lubricate it. Take the liquid away and the film between the faces goes with it, and the faces follow. A rotary lobe pump fails the same way for the same reason. There is no design fault anywhere in that sequence. It starts with a stop float hung lower than it should have been.

So set the stop level against the manufacturer's minimum immersion figure for that frame, not against the floor of the chamber. Calpeda gives 300 mm as the minimum immersion on the GQG, and the small GQS frames want less than that, so the figure is per pump rather than a rule you can carry between them. There is more on the failure itself in what happens when a submersible pump runs dry.

The volume held between the start level and the stop level is the drawdown volume, and it is what decides how often the pump starts. Inflow divided by drawdown volume gives you starts per hour. That is a wet well sizing calculation, and it is the one people skip.

Manufacturers cap it. Calpeda allows 30 starts an hour at regular intervals on the GQN and the GQG. Exceed that and you cook the motor long before anything hydraulic goes wrong, and you wear out the contactor doing it. A chamber sized on flow alone, with no thought to the drawdown volume between the float levels, will exceed it.

You cannot simply open the gap up as far as it will go, either. A big drawdown means sewage stands longer between runs, which lets solids settle and lets the contents go septic. The float levels are a compromise between cycling the pump to death and letting the chamber stand. Where the two pull against each other, the starts-per-hour limit wins.

On the Calpeda GQ range the single-phase builds arrive with the float already fitted. The GQNM carries a float switch, a thermal protector and a built-in capacitor on 10 m of H07RN-F cable, and the single-phase GQGM comes with a control box and a float switch on the end of the cable. Plug it in and it controls its own level.

Three-phase is a different proposition. The float is not fitted as standard, and Calpeda lists a three-phase pump with an integral float switch as available on request. Where you buy the pump without it, Calpeda is explicit that you add a float switch wired to the control panel to set the start and stop levels wherever the water level is not under direct visible control. That makes the float a panel decision on a three-phase station rather than something that arrives in the box.

Check which you have before the pump goes in the chamber. Finding out on commissioning day that the level control was never in the order is a common and avoidable delay.

Floats fail stuck far more often than they fail electrically. Rag wraps the cable, the float wedges against the chamber wall or a bracket, or grease builds up until it no longer rides the surface. The pump then either never starts, or never stops.

Treat a float that has stopped swinging as urgent rather than as an annoyance, because the version where it never stops is the version that takes the seal out. Put the floats on the routine service visit alongside the impeller and the oil chamber, check the cable is free to move through its full travel, and confirm the pump actually stops at the level you set rather than the level you assume.