If the shielding layer is not grounded, the consequences are not just "nearly" but rather, it directly turns "shielding" into "attracting lightning." In essence: at the very least, there will be signal packet loss; at worst, equipment breakdown, and there's also the possibility of electrocution. Specifically, four things may occur:

The interference has been amplified

The shielding layer becomes a receiving antenna, and the EMI from external motors, frequency converters, and cell phone base stations directly couples into the signal line, with no attenuation of low-frequency magnetic fields. Gigabit Ethernet, HDMI, and encoder pulses begin to experience packet loss, bit errors, and screen distortion

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The radiation is also high

Internal high-frequency signals (USB3.0, DP, servo drive PWM) cannot find a low-impedance loop and can only radiate outward in a common-mode manner, immediately exceeding EMC testing standards and causing adjacent devices to crash

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Electrostatic accumulation → High-voltage discharge

Long-distance shielding layer friction induction or capacitive supercritical coupling, tens to hundreds of volts of static electricity released instantly, can directly break down PHY, drive chip; there have been cases in industrial fields where ungrounded shielding layers have broken down the IGBT gate of servo drives

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personal safety risk

If the shielding layer of a medium-voltage variable-frequency power cable is suspended in the air, it can carry hundreds of volts of touch voltage when insulation breakdown occurs. The specification clearly states that this "may lead to fatal danger" and requires at least one end to be grounded

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Therefore, the shielding layer should either not be constructed, or if constructed, it must be grounded in accordance with the specifications

Low frequency, short distance: single-ended grounding to avoid ground loops;

High frequency, long distance: double-ended grounding, plus ground isolator;

High-voltage power cable: Both ends must be reliably grounded, and the cross-sectional area must also meet the short-circuit current requirements.