Basic Circuit Troubleshooting

Fully annotated circuit schematic:

“X” represents an open contact or de-energized load

“{\rightarrow}” represents a closed contact or energized load}

Recall that a process switch will be in its resting (“normal”) condition when the stimulus value is less than the trip threshold, and will be in its actuated condition when the stimulus exceeds the threshold.

Therefore,

{\bullet} CR1 coil = de-energized
{\bullet} Blue lamp = energized
{\bullet} Red lamp = de-energized
{\bullet} Amber lamp = energized

A mercury “tilt switch” uses a sample of liquid mercury metal to bridge wire contacts inside of a sealed glass tube when the tube is tipped in a certain direction.

Thought experiment:

{\bullet} Sump starts empty
{\bullet} Level begins to rise
{\bullet} LSL switch tilts; motor doesn’t start up yet because LSH is still open
{\bullet} LSH switch tilts; motor starts up
{\bullet} Level begins to drop as pump moves wastewater out of sump
{\bullet} LSH returns to normal; motor continues to run because it’s control circuit is “sealed in”
{\bullet} LSL retuns to normal; motor stops and remains off until LSH tips again

Predicting the effect of a given fault:

Present each of the following faults to the students, one at a time, having them comment on all the effects each fault would produce.

Identifying possible/impossible faults:

Present symptoms to the students and then have them determine whether or not a series of suggested faults could account for all the symptoms, explaining why or why not for each proposed fault:

{\bullet} Symptom:
{\bullet} — Yes/No
{\bullet} — Yes/No
{\bullet} — Yes/No

Determining the utility of given diagnostic tests:

Present symptoms to the students and then propose the following diagnostic tests one by one. Students rate the value of each test, determining whether or not it would give useful information (i.e. tell us something we don’t already know). Students determine what different results for each test would indicate about the fault, if anything:

{\bullet} Symptom:
{\bullet} — Yes/No
{\bullet} — Yes/No

Diagnosing a fault based on given symptoms:

Imagine the LSH fails shorted in this system (don’t reveal the fault to students!). Present the operator’s observation(s) to the students, have them consider possible faults and diagnostic strategies, and then tell them the results of tests they propose based on the following symptoms, until they have properly identified the nature and location of the fault:

{\bullet} The pump keeps cycling on and off at the low-level switch’s height.