Troubleshooting Residential Submersible Pump Systems

Dec 1, 2008 12:00 PM, By David Herres, Master Electrician

Practical tips for applying job-specific troubleshooting techniques to your next submersible pump project

Why is it that residential deep well submersible pump system malfunctions are notoriously hard to diagnose? For one, a pump/motor assembly suspended 10 feet from the bottom of a 300-foot well brings new meaning to the word “inaccessible.” The symptoms also have a troublesome way of overlapping so that precise diagnosis can be elusive at first. Invariably, however, persistence and logic prevail for skilled electricians.

Photo 1. This type of submersible pump control box contains a capacitor, relay, and associated electronics.

In this article, we'll take a look at the 3-wire 240V single-phase submersible pump system for drinking water applications, typically set between 50 feet and 300 feet below grade. The pump is fed down a steel well casing that extends through the earth until bedrock is encountered — at which point the rock itself becomes the casing.

The wiring system

A 3-wire system (actually there is a fourth equipment ground conductor that is not counted in the number of wires) implies there is a control box inside the house, containing a large electrolytic capacitor, microprocessor, and other electronics (Photo 1). In contrast, a 2-wire system omits the in-house controller so that the capacitor is inside the hermetically sealed underwater motor. Although this arrangement makes for a cheaper initial installation, when it comes to repair, options are limited.

To efficiently diagnose one of these systems, it's necessary to clearly understand the wiring arrangement you're dealing with — at least external to the control box and motor. A 240V branch circuit from a double-pole breaker within the entrance panel is run to an off-the-shelf double-pole pressure switch, which is actuated by low cut-in and high cut-out water pressure inside the pressure tank. This switched power goes to the input terminals of the control box.

The control box output exits the building, is buried in the same trench with the water pipe, and emerges from the ground in metal or PVC raceway where it enters the well cap through a passage designed for the purpose. Inside the well cap, it's usually spliced with twist-on wire connectors, and then goes down along the water pipe to the pump motor, which comes furnished with a pigtailed set of three wires (plus equipment ground). These are connected to the pump cable by means of crimp-on connectors and heat-shrink tubing, which is the same as an underground splice kit. The spliced wires should be routed inside the torque arrester — an expandable rubber cylinder that keeps the pump/motor assembly centered and minimizes counter rotational motion and wire chafing.

The pump cable, which comes from the control box output terminals, consists of red, yellow, and black (plus green) twisted conductors. Because they are not jacketed, you must run the indoor segment in raceway. As for the outdoor portion, you can direct bury it, as far as the NEC is concerned. However, state and local well installation regulations may call for UL-listed gray RNMC or similar protection to the wellhead.

Photo 2. Control box with pressure switch and disconnect regulates supply of power to the pump motor at the bottom of the well.

Many people assume that the well cable is made up of two hot legs with a neutral; however, that is not the case. The red is start, the black is run, and the yellow is common. One of the functions of the control box is to energize the red for a short period of time so that the pump motor can get up to speed, after which black is switched online.

One large manufacturer offers the motor and control box, which are used by many pump manufacturers. The pumps are bolted onto the motors (with matching splined shaft). However, one major manufacturer makes its own motor and control box, which are not interchangeable. As for the others, motors and control boxes may be interchanged as long as horsepower, phase, and voltage are a match.

The great advantage in the universal control box is that the cover contains the electronic components so you can replace them without doing any rewiring.

Stepping through a diagnosis

In troubleshooting a nonfunctioning submersible pump system, you must strive to diagnose and repair the problem without unnecessarily pulling the pump out of the well or digging up the buried line.

Let's say a homeowner has a drilled well with a submersible pump located 500 feet from the house set at a depth of 300 feet. The complaint is that when a faucet is turned on, there is no water. The homeowner says the system has been in place for several years, and there has never been a problem. Because it has rained a lot all summer, lack of water in the well can be ruled out. There is a good probability the pump has failed due to sand preventing the impeller from turning or because the motor has seized up. But because pulling a pump that is set at 300 feet is a fairly difficult task, other possibilities are pursued first.

It is a 3-wire system, and all electronics are mounted inside the control box cover. After verifying with a voltmeter that there is power to the input terminals — indicating house wiring and pressure switch are functional — a new cover of the correct horsepower rating is acquired. The cover contains a capacitor, relay, microchips, and other electronic components (Photo 2 on page C26). When it's snapped in place, it supplies power to the pump motor via 3-wire (plus equipment ground) underground pump cable. Unfortunately, this procedure did not cause water to flow to the pressure tank, so other avenues must be pursued.

At this point, it's time for the “divide-and-conquer” troubleshooting technique. The idea is that when a system consists of a number of components connected serially — and the endpoint is not receiving power — the most efficient strategy is to perform a test at the midpoint to narrow the focus, not necessarily spatially but in terms of probable causes. The bad half can be tested in turn at its midpoint, allowing the technician to methodically zero in on the problem location. This technique is especially useful when working on large complex machinery or wiring systems.