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EMC Cable Glands: Shield Termination for VFD & Servo
A long pigtail is a 10-lane highway draining into a garden hose. Here's how 360° shield bonding, transfer impedance and the right gland turn your VFD and servo cables from noise radiators into noise drains.
Electromagnetic Compatibility in DrivesIf you've ever chased a servo that loses its encoder position "at random," or a VFD that trips on phantom overcurrent every time a second drive starts, you already know the feeling. The fault logger says nothing, the drive says "no fault," and the machine says "no thank you." Somewhere between the motor and the drive, electromagnetic noise is doing what electromagnetic noise does.
Here's the uncomfortable part: most of the time the culprit isn't the drive and it isn't the motor — it's the termination of the shield at the gland. A VFD's PWM output carries huge high-frequency common-mode currents, and the shield's only job is to give those currents a low-impedance path back to the drive. The moment that path is choked by a long pigtail or a normal (non-EMC) gland, the current finds another way — through motor bearings, encoder wires, and the plant's ground.
This guide covers the one thing that fixes most of it: proper 360° shield termination with the right EMC gland, chosen by transfer impedance, installed at both ends.
1. The Problem With Pigtails: A 10-Lane Highway into a Garden Hose
The pigtail is the single most common — and most damaging — EMC mistake in the industry.
A pigtail is what happens when you peel the braid off a shielded cable, twist it into a wire, and bolt that wire under a screw. It looks fine. It is not fine. At high frequency, the pigtail's inductance and its exposed length make it an antenna — it picks up noise from where it should be blocking it, and it broadcasts the very common-mode current it was meant to drain.
And it shows up in practice. In one documented VFD-motor installation, conducted noise voltage at the input rose as the pigtail got longer:
| Pigtail length | EMC noise voltage at the input | Relative to 360° termination |
|---|---|---|
| 0 mm (360° gland) | 0.82 V | Baseline |
| 50 mm | 3 V | ≈ 3.7× |
| 100 mm | 7 V | ≈ 8.5× |
| 250 mm | 15 V | ≈ 18× |
| 500 mm | 30 V | ≈ 37× |
| Measured conducted noise in one documented VFD–motor cable installation — magnitudes depend on cable construction, pigtail length, frequency, drive switching characteristics and installation geometry, so treat them as indicative rather than universal. Sources: screened-cable/VFD emission test references, 2025. | ||
Read that table twice: in that installation, one 500 mm pigtail degraded the shield by roughly a factor of 37. The exact magnitude depends on cable construction, pigtail length, frequency, drive switching characteristics and installation geometry. Two pigtails (one at each end) are worse than the sum — the antenna effect compounds. In any VFD output lead or sensitive measurement circuit, a pigtail longer than ~25 mm should need written justification.
2. How 360° EMC Glands Work
The fix is conceptually trivial: make the shield touch the cabinet around its whole circumference.
An EMC gland doesn't just hold the cable — it electrically bonds the shield to the enclosure with a large-surface, low-impedance, 360° contact. The shield is clamped around its full circumference, so high-frequency common-mode current flows straight into the enclosure ground instead of detouring through inductance and air.
The engineering metric that grades this is transfer impedance (ZT): how much noise from the outside couples through to the inside. Lower is better. Design differences matter enormously:
| Gland contact design | Transfer impedance @ 100 MHz | Best for |
|---|---|---|
| Spring-finger (beryllium copper) | < 5 mΩ | VFD motor cables, servo systems — consistent contact over temperature & vibration |
| Compression ring | 10–20 mΩ | Fixed, low-vibration installations; performance drifts with age |
| Mesh grounding | 15–30 mΩ | Large diameters and retrofits; more variable |
| Typical manufacturer-published ranges — actual values depend on the specific gland model and test method. Instrumentation applications push lower still — ultra-low transfer impedance (<1 mΩ) from sensor to control room. Sources: EMC gland design comparisons (Bepto, LAPP), 2025. | ||
What it buys you in the real world: in one documented drive retrofit, moving from ordinary glands to spring-finger EMC glands cut motor bearing currents from 15 A to under 2 A and lifted encoder signal-to-noise ratio by ~40 dB. These are case-study figures — results vary with drive, cable and installation. That's the difference between a machine that trips twice a shift and one that runs for years.
3. Terminate at Both Ends — and Nowhere Else
For VFD and servo cables, the shield must be bonded at the motor AND the drive. One end is not enough.
Common-mode current from a PWM drive travels out to the motor and has to come back. If the shield is terminated at both ends with solid 360° contact, it comes back through the shield — which is exactly where you want it. If either end is a pigtail, or an intermediate junction box interrupts the shield, the current detours through the motor's bearings, the encoder, and the machine structure.
- At the motor: solid 360° contact between shield and motor terminal box, via an EMC gland.
- In the drive cabinet: solid 360° contact with the EMC shield busbar, via EMC shield clips or glands.
- Contact surfaces: bare metal, large area (a few cm² minimum), or a short, large-cross-section (≥95 mm²) braided bond where a direct gland isn't possible.
One more rule from the drive makers: don't add intermediate shield bonds in junction boxes between the converter and the motor — each extra bond can reduce shield effectiveness. Keep the path continuous and unbroken from one EMC gland to the other.
4. Selecting the Right Gland Size and Material
The right design with the wrong size or material still fails.
- Size to the cable's shield diameter — an EMC gland works by clamping the braid, so match the gland's contact range to the braid OD, not the jacket OD. Too large, and there's no contact; too small, and you crush the core.
- Match the material to the environment — nickel-plated brass for industrial and corrosive atmospheres, stainless for marine or washdown. Plating that corrodes is a shield contact that vanishes.
- Confirm the braid is exposed — strip the jacket cleanly back so the gland clamps metal, not jacket. Jacket between gland and braid is a pigtail in disguise.
- Verify the contact pressure — a gland that grips lightly has high contact resistance; one that grips too hard deforms the shield. Follow the manufacturer's torque.
5. What the Cable Side Brings: Commodity vs. SORIVO Shielded Cable
The gland only works if the cable's shield is worth bonding to in the first place.
| Characteristic | Market commodity | SORIVO VFD/servo grade |
|---|---|---|
| Shield construction | Unspecified "shielded" | Braid (or foil+braid) with stated coverage % |
| Shield material | Aluminium or thin copper | Tinned copper braid for low, stable contact resistance |
| Flex rating | Not rated | Cycle-tested for servo / drag-chain motion |
| Compatibility data | Not supplied | Shield OD range for gland selection provided |
| Traceability | No metre marking | Metre inkjet, batch traceable |
| A 360° gland is the final step of a chain that starts with the right shield and the right termination discipline. | ||
EMC Gland FAQ
Why is a pigtail shield termination so bad at high frequency?
A pigtail concentrates the shield connection into a thin, long wire with inductance. At VFD PWM frequencies (harmonics into the hundreds of MHz), that inductance and the exposed length turn the pigtail into an antenna — it radiates the very common-mode current it should drain. In a documented VFD-motor installation, a 500 mm pigtail degraded shield performance roughly 37-fold versus a 360° gland; the magnitude depends on cable construction, pigtail length, frequency and installation geometry.
What is transfer impedance and why does it matter?
Transfer impedance (Zt) measures how much noise from outside a shield couples to the conductors inside — lower is better. For EMC glands it grades the shield-to-enclosure bond: per manufacturer-published data, spring-finger designs typically give <5 mΩ at 100 MHz, compression rings 10–20 mΩ, mesh 15–30 mΩ — actual values vary by model and test method. VFD and servo applications need the lowest values you can get.
Should I ground the shield at one end or both ends for a VFD cable?
Both ends, with solid 360° contact at each — motor terminal box and drive shield busbar. The shield must carry the PWM common-mode current back to the drive; a one-ended termination forces that current through bearings and structure instead. Do not add intermediate bonds in junction boxes; keep the path continuous.
How do I choose the right EMC gland size?
Match the gland's clamping range to the cable's braid (shield) diameter, not the jacket OD — the gland must grip metal. Strip the jacket cleanly to expose the braid, pick a corrosion-matched plating (nickel for industrial, stainless for marine), and follow the manufacturer's torque for correct contact pressure.
Does SORIVO supply shielded cable that works with EMC glands?
Yes. Sorivo's VFD and servo cables use tinned copper braid with stated coverage and flex-cycle ratings, and we provide the shield OD range so you can select the matching EMC gland. Send us the drive, cable run and motion profile and we'll confirm the right construction.
Chasing phantom faults? Fix the termination chain.
Tell us your drive, cable length and environment. We'll recommend the shielded cable and confirm the shield data you need to pair it with the right EMC gland.
Sources: Pigtail-vs-360° conducted noise test data (screened VFD cable references); EMC gland transfer-impedance design comparisons (Bepto cable glands, LAPP EMC guide, 2025); Siemens SINAMICS engineering practice for shield bonding and ≥95 mm² braided bonds; drive-maker guidance for two-ended termination. Case figures (bearing current 15 A → <2 A, +40 dB SNR) from published EMC retrofit case studies.