Manufacturing, Testing & Quality
Shield termination at one end or both: first check frequency, equipotential bonding and the termination path
Summary: A cabinet drawing marked “shield grounded” can still omit two details that affect the result: how far the shield current travels before reaching the metal enclosure, and whether the reference potential at the other end is the same. Comparing only “one end” with “both ends” does not distinguish the path for high-frequency interference from low-frequency equalizing currents.
This note is for controls and application engineers reviewing cabinet modifications, instrument connections or communication interference. It helps you place the termination requirements from equipment manuals on a single path diagram and decide whether to check termination geometry, equipotential bonding or the sensor circuit’s specific requirements first. It is a guide to design documents and drawing review. It does not report MTTJ hardware testing or provide instructions for changing wiring on energized equipment.
First separate three paths on the drawing
The signal reference, shield connection and protective earth may be connected inside equipment, but they serve different purposes. Mark the signal circuit, shield-to-enclosure connection, and equipment protective earth and equipotential bonding paths separately. A single “ground” symbol with no endpoints identified hides the actual connections inside the interface.
The shield must not replace a protective earth conductor. Schneider Electric’s SH3 installation instructions explicitly retain this boundary. Weidmüller’s UC20-M manual also requires separate equipotential bonding and prohibits using the cable shield for this purpose. Connecting the shield to an enclosure or busbar associated with PE does not allow you to omit the PE conductor required by the equipment. SH3 installation instructions, UC20-M manual, page 47
Begin the review by obtaining the exact equipment models at both ends, manual revisions, interface grounding diagrams and site equipotential bonding drawing. If these documents conflict, or the shield has become the only metallic connection between two devices, refer the issue to the person responsible for electrical design first. Do not disconnect PE to investigate noise.
Why continuity on a multimeter may still be inadequate for high-frequency shielding
Phoenix Contact discusses the resistive and inductive components of a shield connection separately. Low DC contact resistance describes only part of the connection; the inductance of the connection path becomes increasingly significant as frequency rises. Twisting the shield into a long, thin pigtail and routing it to a distant terminal adds to this path. A short connection over a large contact area at the cabinet entry helps provide a return path before interference reaches the internal wiring. Shield connection guidance: Low-impedance shield connection / Pigtail
An ideal-component illustration makes the difference clear. Assume a path inductance of 100 nH. Using the inductive reactance formula Xₗ = 2πfL gives approximately 0.63 Ω at 1 MHz and 6.28 Ω at 10 MHz. The 100 nH value is an arbitrary teaching assumption, unrelated to the length or measured value of any particular pigtail. These numbers cannot predict the attenuation of the complete shielding arrangement and are not acceptance limits. They only explain why the same DC reading is insufficient to represent high-frequency performance.
During drawing review, therefore, trace the shield connection all the way to the enclosure. Check where the clamp is fitted, how the busbar connects to the mounting plate, and how the mounting plate connects to the enclosure. Look for long wires, insulating mounts or metal parts connected only at their far end. A part called a “shield clamp” does not establish that the entire path is short enough. Contact stability also depends on the correct clamping range, surface condition and installation method; check these against the documentation for the selected component.
Compare three terminations under the same conditions
The comparison below helps select the design path to check. It does not replace equipment-specific installation instructions. Motor output cables, fieldbuses and precision analog sensors, in particular, cannot be treated as the same type of circuit simply because they all use shielded cables.
Scroll tables horizontally to read every column.
| Termination path | Conditions in which to consider it | Main purpose | What still needs to be checked |
|---|---|---|---|
| Direct connection at one end, with no direct shield connection at the other | Permitted by the equipment; for example, certain circuits with low-frequency electric-field coupling, low-level signals or passive sensors | Avoids a closed DC/low-frequency shield path between the two reference points while providing the corresponding electric-field shielding | Whether the interference also contains high-frequency components, the cable’s electrical length at that frequency, and whether another shield connection exists inside the interface |
| Direct, short and wide connections at both ends | Required by the equipment/bus installation instructions; reference potentials and equipotential bonding at both ends have been addressed | Provides high-frequency shield connections at both ends and maintains the cable–connector–enclosure path | Whether a potential difference causes equalizing current to flow through the shield, and whether connectors or intermediate enclosures interrupt the shield |
| Direct connection at one end and a designed capacitive connection at the other | A hybrid arrangement permitted by the equipment, where both the high-frequency path and low-frequency circulating currents need to be addressed | The capacitor presents different impedances at low and high frequencies, directing these two types of current along different paths | Selection of the capacitor and protection network, installation parasitics and system requirements; this connection does not perform the function of a PE or equipotential bonding conductor |
Single-end termination has a specific circuit context. Section 9 of ADI’s Hardware Design Techniques uses low-frequency electric-field pickup and passive sensor examples to explain single-point connections, then discusses electrically long cables, high frequencies and hybrid termination. Its “Sensors and Cable Shielding” section emphasizes the sensor configuration and the nature of the interference. An individual example must not be turned into a rule that every analog cable should be connected at one end only. ADI, pages 76–77 and 80–88
Industrial control documentation may start from a different set of assumptions. The cable shielding section for Siemens ET 200eco PN V20 uses connections at both ends and requires a separate equipotential bonding connection where a potential difference exists. Where equipotential bonding cannot be implemented, it discusses capacitive grounding at one end, explicitly noting that this arrangement addresses only high-frequency interference and that the values in the diagram must vary with the system. This retains its industrial control context; it is not a universal rule for every instrument circuit. Siemens V20: Cable shielding
A capacitive arrangement is not a matter of picking a value from either manual and simply adding it. At high frequencies, a real capacitor also has lead and installation parasitics. The equipment and system design must determine the permitted termination conditions, ratings and protection requirements. This table screens the alternatives; the next step is still to apply them to the specific interface.
Two drawing-review examples with different next steps
Example 1: equipment in the same cabinet, with manuals already requiring shield connections at both ends. Assume the equipotential bonding design is complete, but the shield extends into a long pigtail after entering the cabinet before connecting to a distant terminal. Checking the entry clamp, connector shell and busbar-to-cabinet path first directly addresses the structural difference already identified. Cutting off one end just because interference has appeared also changes the required shield arrangement, without answering why the path is too long. First complete a termination drawing that follows the equipment instructions, then arrange verification; this makes the results easier to explain.
Example 2: a remote analog sensor and its receiver are in different power-supply areas. Assume the circuit documentation permits single-end shielding and low-frequency fluctuations occur at the site. First check the signal reference, whether the sensor is floating, the receiver input type and the reference potentials at both ends. Do not copy the direct connections at both ends from the previous example. If a high-frequency problem is also found, evaluate hybrid termination, isolation or routing changes within the equipment’s permitted arrangements. Without evidence of the interference frequency band and circuit paths, fluctuations that coincide with a variable-frequency drive starting do not establish the shield as the sole cause.
Both examples are hypothetical drawing-review scenarios, unrelated to actual faults at a customer installation. They demonstrate different next steps: the first corrects a known termination path; the second first establishes the circuit and potential conditions.
Deliver a termination drawing another engineer can check
Add a simple diagram beside the existing wiring drawing; there is no need to create another record system. At a minimum, another engineer should be able to trace the following on it:
- the interface shell and shield connection point at end A;
- the cabinet entry;
- every intermediate connector or junction box;
- the shield connection point at end B;
- the separately identified equipotential bonding and PE paths.
Show any direct or capacitive connections already present inside the equipment. Otherwise, what looks like “single-end” termination on the drawing may already be connected at both ends in the actual system.
For the location you intend to modify, attach the relevant equipment manual section, the interference symptoms and observation conditions, photographs or dimensions of the current connection, and the proposed structural change. The review is complete when the requirements at both ends can be met simultaneously; the roles of the shield, signal reference and protective earth are clear; the shield does not replace equipotential bonding; and the next verification can distinguish the effects before and after the change.
The verification plan should hold the equipment state, load, cable route and observation window constant. State in advance whether you will observe communication errors, changes in instrument readings or the results of a specified EMC test. Change one interpretable condition at a time and retain the actual results. Reduced symptoms can support further investigation of this configuration, but a single normal run is insufficient to declare that the complete equipment has passed EMC compliance verification. Appropriately qualified personnel must carry out site work, isolation and measurements in accordance with the equipment instructions.
Recheck this drawing if the equipment, cable length, connectors, cabinet location or power-supply area changes. If the problem ultimately points to bus reflections or receiver states, continue with RS-485 bench troubleshooting: termination, stubs and idle bias to avoid continuing to use shield connections to solve a different kind of circuit problem.
English summary
This note helps controls and application engineers review cable-shield termination before a cabinet change. It compares single-end, direct both-end and hybrid capacitive connections under their actual circuit conditions, explains why DC continuity does not establish high-frequency impedance, and separates shielding from protective earth and equipotential bonding. Two hypothetical cases show how those conditions change the next design check. The numerical inductance example is illustrative; no MTTJ hardware testing or EMC compliance result is claimed.
Sources checked: October 19, 2026. This article was researched and compiled with AI assistance. The filename in the cited Weidmüller link retains “2024”; the manual actually returned during this check was version 2922760000/03/02.2026. ADI’s document is a historical design handbook used to explain the principles.