Why do cables have shields?
To prevent signals in cables from radiating and causing interference.
To prevent outside signals from affecting signals inside the cable.
To maintain controlled impedance so that fast-changing signals can travel long distances without electrical “echos.”
Shielded cable assemblies have an increased chance of defects in the manufacturing process. Listed in the table below are some of the key challenges and recommended best practice testing solutions for dealing with shielded cable assemblies.
SHIELDED CABLE CHARACTERISTICS
CHALLENGES AND RISKS
- 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.
Latent and intermittent shorts are much more likely than in non-shielded cables.
Perform hipot test on assemblies with shields to ensure appropriate “Insulation Resistance.”
Shield provides shell-to-shell connection on cables with metal shells.
Shell-to-shell connection does not get tested.
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.”
Shield connected at only one end of the cable.
Since only one connection to the shield exists, the cable passes a continuity test even if this connection is missing.
Verify presence of shield with capacitance measurement between wires and shield.
Shields that looks like center conductors during assembly.
Swap of center wire and shield at both ends of cable. Assembly passes continuity test.
Set resistance thresholds to detect the difference in resistance between conductors and shields (4-wire Kelvin test may be required).
Multiple shields in one assembly have potiential to be mixed.
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.
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).
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.
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.
Use ‘LINK’ commands to eliminate false hipot failures. Easy-wire testers have low voltage LINK command to eliminate false continuity failures as well.