By TEBAOFLEX Jyen. Originally published on April 17, 2026.
A good DC connection can be a poor high-frequency connection
VFD switching produces common-mode currents that depend on the drive, motor and cable. Long narrow shield connections add inductance, so DC continuity alone does not demonstrate an effective high-frequency bond. ABB’s drive guidance recommends circumferential shield connections and short, low-inductance bonding paths.
Cable transfer impedance is a frequency-dependent characteristic normally expressed per unit length. A gland or termination has its own impedance, and an installed drive system has emissions and immunity requirements. These are related but not interchangeable measurements.
Illustrative pigtail reactance
Assume a lead inductance of 150 nH solely for illustration. XL= 2πfL gives 9.425 Ω at 10 MHz and 28.274 Ω at 30 MHz. Actual inductance depends on geometry and the return path; length alone does not establish exactly 1 nH/mm.
At 30 MHz the free-space quarter wavelength is about 2.5 m, not 150 mm. A short lead can still couple unwanted energy without being a quarter-wave resonant antenna. The arithmetic does not prove a 90% shielding loss or a specific emission reduction.
Make the shield connection part of the enclosure boundary
Follow the drive and motor instructions for 360-degree glands or shield clamps, conductive mounting surfaces and bonding at the relevant cable ends. Keep required protective-earth conductors intact. EMC bonding is not permission to remove PE or rely on a braid that has not been assessed for fault duty.
Select a gland by its actual cable and shield diameter range, thread, sealing, strain-relief and environmental rating. Thread size alone does not define an interchangeable clamping range. For hazardous areas, the complete approved gland and enclosure arrangement is a separate requirement.
Commission the complete installation
Check shield continuity through intermediate isolators, connector backshells and junction boxes. Inspect contact surfaces and tightening instructions; do not invent a universal jacket-strip length or contact-resistance threshold.
If potential differences drive unwanted shield current, investigate the equipotential bonding network with qualified personnel. Do not cure a noise symptom by disconnecting safety earth. Verify signal separation, routing, filter arrangements and equipment operation under representative load.
Request test records stating frequency, method, sample length and termination arrangement. A low cable transfer impedance or a passing continuity test does not certify the installed system to CISPR emissions limits.
Primary references and specification evidence
The references explain the cited technology or manufacturer example. They are not TEBAOFLEX product approvals or evidence that a supplied batch has passed the tests. Request the exact product drawing, specification edition, acceptance criteria and traceable records.
Discuss your application
Discuss the duty, installation and required documentary evidence with TEBAOFLEX. WhatsApp: +86 185 9591 3361. Product suitability and test obligations must be confirmed for the specific order.