Digital Multimeters (Part 2)
Objectives
- Describe best practices for cable testing
- Perform a diode test
- Demonstrate advanced circuit parameter measurements
Testing Cables
Cables can fail because of damage, improper assembly or installation. Locate the faults by performing visual inspections and DMM ohmmeter tests.
- Inspect exposed sections of cable for physical and/or thermal damage.
- Inspect shield grounding, cable installation, and termination.
- Test all conductors individually for resistance. It will never be absolutely zero (all conductors have specific electrical resistivity), but it should be very low.
- Test each conductor for isolation from all other conductors in the cable. An ohmmeter placed across isolated conductors will read "OL" meaning infinite resistance between the conductors.
- Try to wiggle the cable during insulation and resistance tests. It may help to detect intermittent problems.
If you have at least three separate conductors, including the conductor under test, and you can isolate them from the system and each other, then you can use this technique.
- Measure for continuity between all three conductors. They should be insulated from each other.
- Put a jumper cable between conductor #1 and #3 on one side of cable.
- On the other side of cable measure for continuity between conductors #1 and #3. If there is continuity, both conductors are good and we can use them for testing other conductors.
- Repeat steps 2 and 3 (using conductor #1 to test against) for the remaining conductors in the cable.
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