The part that starts and stops your pump
The pressure switch is the thermostat of your well system. A thermostat watches temperature and closes a contact to run the furnace. A pressure switch watches water pressure and closes a contact to run the pump. That's the entire job. It's a small gray metal box, about the size of a deck of cards, bolted to the tee at your pressure tank or to the pipe right next to it, with a thin tube or a short threaded nipple connecting it to the water.
Inside are three things: a diaphragm, a spring, and a pair of electrical contacts. Water pressure pushes up on the diaphragm, the diaphragm pushes against the spring, and at a set pressure the contacts snap open and cut power to the pump. When you run a shower and pressure falls back to the lower setting, the spring wins, the contacts snap closed, and the pump runs again. On a 30/50 switch that means closed at 30 psi and open at 50 psi.
Every part of that is mechanical and every part of it wears. Springs weaken. Diaphragms stiffen and crack. And the contacts arc a little every single time they open and close under a motor load. A house that cycles the pump twenty times a day is putting forty arcs a day across those contacts, and over eight or ten years that's what finishes them. Pressure switches are consumable parts. They fail sooner on systems that short cycle, because short cycling multiplies the number of arcs by ten.
Signs the switch is what's wrong
The pump won't start at all. Pressure in the house falls off, the tank empties, and nothing happens. No click at the box, no faint hum from the well, no movement on the gauge. Contacts that have burnt open, corroded, or a spring that has given up will do exactly this. It looks like a dead pump, and plenty of homeowners have replaced a perfectly good pump because of it.
The pump won't shut off. The gauge climbs past the cut-out setting, keeps going to 60, 70, 80 psi, and the relief valve on the tank finally lets go and dumps water on the basement floor. That's contacts welded shut, which happens when arcing fuses them together instead of pitting them apart. This one is not something to live with for a week. Shut the pump breaker off and leave it off until it's repaired, because a system with no upper limit will find the weakest fitting in the house.
Buzzing, humming, or rapid chattering at the box. A switch whose contacts can't make a clean connection will sit there arcing at the gap instead of closing on it. You'll hear a buzz or a fast machine gun clicking, and sometimes you'll smell hot plastic or see a scorch mark around the cover screw. That's an electrical fire risk sitting on a water pipe, and it gets dealt with the same day.
It works when you tap it. If a light rap on the side of the box starts the pump or stops the buzzing, the switch is done. Tapping jars a stuck mechanism or bridges a pitted contact for a moment. It's a useful diagnostic and a terrible repair. A switch you have to hit is a switch that will leave someone in the shower with no water the week you're away.
The clogged sensing tube that looks exactly like a dead switch
Before anyone buys a switch, the sensing line gets checked. The switch reads pressure through a quarter inch nipple or a small copper tube that ties it into the plumbing. That passage is narrow, it sits in a low spot, and it fills over time with iron, mineral scale, and fine grit from the well.
When it packs solid, the switch goes blind. It's still a perfectly good switch, but it's reading whatever pressure was trapped in the tube the day it clogged. If it clogged at high pressure, the pump never gets the signal to start and you have no water. If it clogged at low pressure, the pump never gets the signal to stop and you're back to the relief valve dumping on the floor. Every symptom in the section above can come from a two dollar brass nipple full of rust.
The fix is to pull the nipple, clear it or replace it, and put the same switch back. It's the cheapest repair on the well system and it's the reason a good tech opens the sensing line before condemning anything. On iron-bearing water it's worth replacing that nipple every time the switch is serviced, since it costs almost nothing while the wrench is already on it.
Plain switch versus one with a low water cutoff
A standard pressure switch only knows one thing, which is what the pressure is right now. It has no idea whether water is actually arriving. If the well runs low and the pump starts pulling air, pressure never climbs to the cut-out setting, so the switch does what it was built to do and holds the pump on. The pump then runs dry, loses the water that cools its motor, and cooks itself. That's how a yield problem turns into a pump replacement in an afternoon.
A switch with a low-pressure cutoff adds a second trip point at the bottom, around 10 psi. If pressure ever falls below that instead of rising, the switch trips and latches off, and it stays off until somebody moves the lever on the side by hand. The pump stops instead of grinding. You lose water until you reset it, which is annoying, and that annoyance is the whole point: it makes you go look at the well instead of destroying a motor.
On a low-yield fractured rock well, the cutoff version is worth having. The price difference between the two switches is a few dollars. The difference in what they protect is a pump, a drop pipe, and a truck with a rig on it. The one caution is that a cutoff switch on a marginal well needs somebody home who knows where the reset lever is, because it will trip on a heavy laundry day.
What 30/50 and 40/60 mean, and why your tank has to agree
Those two numbers are the cut-in and the cut-out. A 30/50 switch starts the pump at 30 psi and stops it at 50. A 40/60 starts at 40 and stops at 60. The 20 psi spread between them is the differential, and it's what gives the tank room to store water so the pump isn't starting every time someone washes their hands.
The pressure tank has to be set to match. An empty tank, meaning one with the water drained out, should be pre-charged with air to 2 psi below the cut-in. That's 28 psi on a 30/50 system and 38 psi on a 40/60. Get that wrong and the tank stops holding a useful volume of water, the pump cycles on and off in short bursts, and those bursts are what burns the switch contacts in the first place. A switch that failed after two years is often a symptom, and the tank is the cause.
It's tempting to chase weak pressure by cranking the big spring nut to 40/60. Sometimes that's the right call, and sometimes it's not. The pump has to be capable of making 60 psi at your depth, the tank pre-charge has to be raised to match, and the rest of the house has to tolerate it. Water heater relief valves are rated at 150 psi but old galvanized pipe, hose bibs and washing machine hoses are the things that give out. Change the setting deliberately, not as a guess.
Switch, tank, or pump, and what a switch job costs
Sorting the three apart is mostly about watching the gauge. A tank problem looks like fast cycling, on for fifteen seconds and off for thirty, over and over, with a tank that feels heavy and solid all the way up and blows water instead of air out of the air valve on top. A pump problem looks like the switch closing correctly, an audible call for the pump, and pressure that never builds. A switch problem is the one where the mechanical side of the system does nothing at the moment it should be doing something, or refuses to stop when it should.
The switch itself is a cheap part, and barely more with the low water cutoff version. What you're buying from a contractor is the diagnosis and the safety. Most switch replacements are an ordinary service call, and it's more if the sensing tee, the gauge and the tank tee are corroded enough to need rebuilding at the same time, which on a twenty year old install is common.
One hard rule: don't open the cover. That box carries 240 volts on two hot legs, both of them live, and it's mounted on a wet metal pipe in a basement where you're likely standing on a damp floor. There's no neutral in there to make it forgiving. Killing the breaker helps only if you killed the right one, and control boxes for three-wire pumps hold a charged capacitor after the power is off. Pulling the cover to look at the contacts is the single most common way homeowners get hurt on a well system.
If you've got a pump that won't start, won't stop, or a box that's buzzing, call 973-853-1550 and describe what the gauge is doing. Half the time that conversation tells us which part to bring before we leave the shop.