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First, Find the Fuse

At about 11 p.m., I received a call from the maintenance manager of a 20-story apartment building. The pumping system had failed, and he could not find a mechanical reason for the problem.

We examined the controller and determined that the display was working normally. Power was going into the control panel, and voltage was present at the output of the power supply. Carefully looking over the circuit board, I discovered a fuse-holder cap. I pulled the fuse and was rewarded with a blown element. It was an odd size, so we were not going to be able to replace it in the middle of the night. The next morning, the maintenance manager purchased replacement fuses. He bought an entire box so spares would be available.

It became apparent that the fuse was not there to protect the circuit board. Its purpose was to protect the 4-to-20-mA loop supplying the water-pressure sensor. In my many years of looking at these kinds of systems, I cannot recall seeing a fuse in this particular location because 4-to-20-mA loops inherently are current-limited. If the design engineer thought it wise to provide a fuse, its location and purpose should have been indicated on the equipment.

The manufacturer's instruction manual was of no help. One troubleshooting recommendation was to check all of the fuses. However, no fuse was shown on the schematic drawing. Instructions on replacing a blown fuse are not much help if you do not know where the fuse is located.

Because the pumping system did not need to be checked every day, operators were not familiar with its routine operation. Also, there was frequent turnover on the maintenance staff. The solution appeared to be attaching a single sheet of abbreviated instructions to the control unit's front cover. Basic operating procedures, routine error messages, reset/restart procedures, and various voltages and pressures that easily could be checked to diagnose a problem, as well as the number for a 24/7 hotline that could be called for troubleshooting assistance, were listed.

If you are going to put electronics in a mechanical-systems component, realize that it will take some time for maintenance workers to become as proficient with digital controls as they are with mechanical systems.

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