One End? Both Ends? No Ends?

Introduction

When developing electrical or electronic products that use shielded cables, a common question arises: Where should cable shields be grounded? At one end? Both ends? Or not at all?

The following discussion aims to offer a starting point for formulating your own response based on your specific situation.

Cable shields play a critical role in maintaining electromagnetic compatibility (EMC) and minimizing interference, so let us explore some cable shield grounding options.

Important Note: All shield grounding options discussed below assume the shield is not the signal return conductor.

Grounding Option 1: Shield Grounded at One End Only

“Grounding Option 1: Shield Grounded at One End Only” is commonly used in scenarios involving low frequencies, specifically audio frequencies and those below 100 kHz. In such situations, if the shield were grounded at multiple locations or both ends, unwanted noise current could flow through the shield due to differences in ground potential between these points.

Reference 1 illustrates how induced noise voltage in 50/60 Hz systems leads to the flow of loop current. Specifically, during a 10,000 A unbalanced fault in a system, there exists the potential for 100 A of unwanted current to flow through the communications cable ground path.

Where should you position the single ground when opting for Option 1 (Shield Grounded at One End Only)? According to Reference 2, it is advisable to ground the shield at the source end. This choice aligns with the signal voltage reference. Only ground the cable shield at the load side if the signal source is not grounded.

Proper Grounding of Cable Shields: A Crucial Consideration

When it comes to cable shields, how you ground them is just as critical as where you ground them. Let us explore this essential aspect:

Common Pitfall: Tying the Shield to Circuit Ground

• Often, the cable shield is inadvertently connected to the circuit ground.
• Unfortunately, this incorrect practice undermines the shielding effectiveness of both the cable and the enclosure.
• It creates a near-unobstructed pathway for high-frequency energy from the surrounding environment to enter the equipment.
• This mistake can potentially lead to upset or damage due to electromagnetic interference (EMI).

Shield as an Extension of the Enclosure

• Best Practice: Treat the cable’s shield as an extension of the shielded enclosure.
• Terminate the shield to the enclosure using the lowest impedance connection possible.
• Think of it as ensuring a seamless continuation of the enclosure’s shielding to ensure its effectiveness.

360-Degree Connection: Ideal, but Not Always Achieved

• While achieving a complete 360-degree connection (bonding) is ideal, it is not always feasible.
• Especially with non-military style connectors, a perfect 360-degree bond may be challenging.
• However, strive for the most comprehensive connection possible.

Avoid “Pig-Tail” Terminations

• To maintain robustness against EMI threats (both low- and high-frequency), avoid “pig-tail” terminations.
• These highly inductive connections can introduce unwanted impedance and compromise shielding effectiveness.

Grounding Option 2: Shield Grounded at Both Ends

When dealing with frequencies above approximately 100 kHz, the practice of grounding the shield at both ends (and potentially at multiple locations) becomes relevant. Let us explore the considerations:

Stray Capacitance and Ground Loop Completion

• Even if the shield is not physically grounded at both ends, stray capacitance effectively completes the ground loop.
• This completion can make maintaining ground isolation at the unterminated end challenging.

Filtering Out Low-Frequency Noise

• At low frequencies, any small noise voltage arising from a difference in ground potential is easily filtered out.
• The shield’s connection to the enclosure at both ends ensures effective shielding against EMI.

Skin Effect

At frequencies above approximately 1 MHz, the skin effect becomes significant. In this scenario, common impedance coupling is reduced. Noise currents predominantly flow on the outside surface of the shield, while signal currents remain confined to the inside surface of the shield, never intersecting. This effect can be leveraged.

Grounding Option 3: Shield Grounded Combination

In scenarios involving both low- and high-frequency signals, the “Shield Grounded Combination” technique comes into play. Let us explore its features:

Objective

• The goal is to break the ground loop at low-frequency while maintaining effective cable-shield-to-connector-shell termination at high frequency.

Hybrid Grounding Technique

• Instead of relying on stray capacitance (as in Option 2), we introduce an actual physical capacitor with a value of approximately 47 nF (a relatively small capacitance).
• The impedance of this capacitor behaves differently at different frequencies.

Low-Frequency Behavior

• At low frequencies, the capacitor’s impedance is large.
• This effectively achieves Grounding “Option 1: Shield Grounded at One End Only.”

High-Frequency Behavior

• At high frequencies, the capacitor’s impedance becomes low.
• This aligns with “Grounding Option 2: Shield Grounded at Both Ends.”

Enhancing Shielding Effectiveness with Option 3: Shield Grounded Combination

Balancing low and high frequencies is crucial for robust EMC performance. Pay particular attention to the following:

Inductance and Its Impact

• Inductance introduced by components in series with the capacitor(s) can hinder the benefits of Option 3.
• To mitigate this, focus on minimizing lead lengths—keeping them as short as possible.

Parallel Capacitors for Reduced Inductance

• Parallel capacitors are crucial for mitigating the inductance limitation of a single capacitor connection.
• By placing many identical capacitors in parallel, we reduce overall inductance.

Integrated Connectors with Capacitors

• Often, achieving optimal results means building the cable connector with the capacitors installed.
• This ensures a direct, low-impedance path for shielding.

Shielding Effectiveness Up to 1 GHz

• With Option 3, and by incorporating parallel capacitors in the connector shell, shielding effectiveness can be maintained up to 1 GHz.

Grounding Option 4: Shield Grounded at No Ends

To the best of the author’s knowledge, there are no situations where a cable shield is present, and “Grounding Option 4: Shield Grounded at No Ends” is a viable cable shield grounding termination method.

Summary

This article succinctly addresses the question: Where to Ground Cable Shields? One End? Both Ends? No Ends? Here are the key points:

Low Frequencies

• Ground the shield at one end (typically the source side).
• This approach provides effective cable shielding.

High Frequencies

• Ground the shield at both ends.
• Ensures robust shielding against interference.

Combination Approach

• For scenarios involving both high and low frequencies, use a small-value physical capacitor(s).
• At low frequencies, the capacitor’s large impedance achieves a single-point ground.
• At high frequencies, the capacitor acts like a short, creating a ground at both ends of the cable.

References and Further Reading

1. Roberts, J., Weber, M., Avoiding Magnetic Induction Issues in Communications Cabling, SEL Application Guide Volume II, AG2001-06, Date Code 20090708. Retrieved from https://selinc.com/api/download/4976/?lang=en
2. Ott, Electromagnetic Compatibility Engineering, Wiley, 2009.
3. Paul, C.R., Scully, R.C., Steffka, M.A, Introduction to Electromagnetic Compatibility, 3rd Edition, John Wiley & Sons, 2023.
4. Williams, T., EMC for Product Designers, 5^th Edition, Newnes.