Indentify Faults in Simple Lamp Circuit including Failed Wires

Suppose the lamp refuses to light up. A voltmeter registers 24 volts between test points C and D:

Simple Lamp Circuit

First, list all the possible (single) faults that could account for all measurements and symptoms in this circuit, including failed wires as well as failed components:

Now, determine the diagnostic value of each of the following tests, based on the faults you listed above. If a proposed test could provide new information to help you identify the location and/or nature of the one fault, mark “yes.” Otherwise, if a proposed test would not reveal anything relevant to identifying the fault (already discernible from the measurements and symptoms given so far), mark “no.”

• Measure V_CF
• Measure V_ED
• Measure V_AB
• Measure V_AD
• Measure V_CB
• Measure V_EF
• Measure current through wire connecting A and C
• Jumper A and C together
• Jumper B and D together
• Jumper A and B together

Finally, develop a rule you may use when assessing the value of each proposed test, based on a comprehensive list of possible faults.

Answer:

Here is a comprehensive list of faults, each one individually capable of accounting for the symptom (no light) and the measurement of 24 volts between C and D:

• Lamp burned out (failed open)
• Wire failed open between A and C
• Wire failed open between B and D

Based on this short list of possible faults – assuming only one of them is actually true – the value of each proposed test is as follows:

• Measure V_CF –> NO
• Measure V_ED –> NO
• Measure V_AB –> YES
• Measure V_AD –> YES
• Measure V_CB –> YES
• Measure V_EF –> NO
• Measure current through wire connecting A and C  –> NO
• Jumper A and C together  –> YES
• Jumper B and D together  –> YES
• Jumper A and B together  –> NO

A good rule to apply when evaluating proposed tests is to ask the question: “Will this test give me the exact same result no matter which one of the possible faults is true?” If so, the test is useless. If not (i.e. the results would differ depending on which of the possible faults was true), then the test has value because it will help narrow the field of possibilities.

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