SHIELDED CABLE
CHARACTERISTICS |
CHALLENGES AND RISKS |
SOLUTIONS |
- Removal of cable outer jacket can nick wire insulation.
- Loose strands from shield.
- Wire strand debris from trimming braided shielding.
- Over-molding that moves wires and shielding tape added before over-molding can create near shorts.
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- Latent and intermittent shorts are much more likely than in non-shielded cables.
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- Perform hipot test on assemblies with shields to assure appropriate "Insulation Resistance".
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- Shield provides shell-to-shell connection on cables with metal shells.
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- Shell-to-shell connection does not get tested.
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- Make sure that shell of test connector is a live test point. (It must show up as a connection in the test program) Use Cirris adapters with "testable grounded shells."
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- Shield connected at only one end of the cable
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- Since only one connection to shield exists the cable passes a continuity test even if this connection is missing.
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- Verify presence of shield with capacitance measurement between wires and shield.
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- Shields that looks like center conductors during assembly.
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- Swap of center wire and shield at both ends of cable. Assembly passes continuity test.
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- Set resistance thresholds to detect the difference in resistance between conductors and shields (4-wire Kelvin test may be required).
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- Multiple shields in one assembly have potiential to be mixed.
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- Multiple shields in one assembly and shields that can be intermixed with connections The assembly passes a continuity test but fails in application because of errors in wiring the shields. Wires can be swapped between two different shields or wires end up outside of their shields.
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- Use capacitance measurements, including relative capacitance, to differentiate between shields. Capacitance usually varies enough to differentiate. (< 20 pf per ft for wires within shields and > 30 pf per ft for shields)
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- Multiple shields in one assembly are connected to a common point. It is ok for the shields to be shorted together but they don't have to be.
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- Continuity test often fails due to inconsistencies of shields touching each other. Assemblies fail test but are not bad, yet the ground still needs to be tested
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- Use 'LINK' commands to eliminate false hipot failures. easy-wire testers have low voltage LINK command to eliminate false continuity failures as well.
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