Low Pressure Cut-Off Pressure Switch Adjustment

I am maintain more than a dozen homes with submersible and shallow well pumps. On two that can frequently develop low/no water I have installed low pressure cut-off swithes so that the pumps won't run continually when there is no water. My problem is that every so often when there is plenty of water available and everything is OK the low pressure cut-off kicks in for a reason I can't fully fiqure out. They will go for weeks with no problem and then cut-out and then be fine again after re-setting.

I reasoned that perhaps when the pressure is close to cut-in pressure and there is a larger than normal demand (ie. two toilets flushed at once) the pressure drop is too dramatic and bingo it cuts-out. This I can manage to duplicate but it is with the drain valve on the tank opened fully. Is there a way to adjust the cut-off pressure lower than factory setting to overcome this problem? I am using FSG2040 (Sq D) switches.

Thanks to anyone who can help. Sq. D has no help that I can find.

How long does the pressure have to be below the "low pressure cutoff" to actuate?

It could be caused by tanks that are waterlogged, or where the air pressure in the tank is higher than the start pressure. There may be cases where a waterlogged tank or improper air pressure could result in a drop in pressure before the pump can get up to speed to prevent the pressure switch from cutting off the pump.

Thanks for your reply. The tanks are captive air and fairly new with the precharge set appropriately. The cut-out seems to happen at the moment the cut-in should be taking place. As I said it behaves perfectly for weeks on end, then it will "happen". In one case the head pressure from a reservoir is about 15 lbs. static and the pump is used just to develop pressure. In the other situation the pump is "pulling" water from a lake with just about 3 feet of lift.

I've only been able to make it happen twice and both times I had many drain valves opened at once.

If you have enough valves open to exceed the pressure tanks and replenishment from the pump, the pressure will drop. Sounds like you might need a bigger pump to provide enough flow and keep it above the minimum cutout. Once you've used all of the pressurized water in the storage tanks and the pump can't keep up, the pressure continues to drop until it reaches equilibrium.

Valveman

Let some more air out of your tank. If you have the air charge set correctly, then if you have a 30/50 pressure switch there is 28 PSI in the tank. Because of this, as soon as the system pressure drops to 28 PSI, the bladder hits the bottom and the low pressure switch cuts off even if the low pressure setting is 10 PSI. If you drop the air charge in your tank to 20 PSI, the bladder won't hit the bottom till much later. This gives time for the pump to catch up with what you are doing. If this does not work, you need a bigger pump because you are just using more water than the pump can supply.

Just for your info, the FSG2-M4 switch is not 100% protection. So keep that in mind for future reference. I never liked the M4 because of the nuisance tripping you are experiencing. Just like you said, a little more use of water than normal, pressure drops a little more and boom your out of water until someone goes to the switch and lifts the lever for a while.

bob...

Thanks to all for your ideas. The pumps are certainly adequate in size because they can handle full open valves, it is just that the pump does not come on in time to keep the water flowing. I think Speedbump has "hit the nail on the head" with his last comment - it seems that nuisance trips can be characteristic of the low pressure cut-off switches. I will also try reducing the static pressure in one of the tanks next time I am called. This will effectively give me more water after the cut-in pressure. Thanks for the idea Valveman.
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Valveman

There are other ways to protect the pump from a dry well. A Cycle Sensor looks at the amp draw of the pump and shuts the pump off when the well runs dry. Then it can be set to restart the pump in one minute increments. So nobody has to go restart the pump, it just restarts itself after say 3 minutes or 1 minute, depending on the setting. A low pressure shut off switch is not fool proof. If for instance you turn off the faucet just at the same time the well runs dry, the pressure does not drop and the pump continues to run. It is not pumping water so the pressure switch can't shut it off at high pressure. You are not using water so the pressure stays higher than the low pressure setting, and the low presssure safty can't shut the pump off, and the pump melts down. The Cycle Sensor will shut the pump off if the well runs dry regardless of the pressure. Then it will restart on it's own in a couple of minutes. http://www.cyclestopvalves.com/products.html#cyclesensor