Electromagnetic interference (EMI) poses a critical challenge for industrial control systems. Of course, it’s a problem that’s only exacerbated by the increasingly complex electrical environments we often work with in modern facilities and electrical installations. Nevertheless, from utility substations to renewable energy installations, maintaining signal integrity in control cables is essential for reliable operation.
Below we discuss proven design techniques for reducing EMI in control wires, including shielding methods, twisting strategies, conductor selection, and ground referencing, along with how Allwire’s custom conduit solutions can provide an additional layer of protection.
Naturally, achieving the most effective EMI protection in control cable systems requires implementing multiple complementary techniques rather than relying on any single solution. The most successful designs integrate cable construction methods (twisting, shielding, conductor selection), proper grounding practices, and external protection measures (conduit, separation, routing) into a coordinated strategy.
Understanding Electromagnetic Interference in Control Systems
Electromagnetic interference can disrupt data transmission, cause equipment malfunction, and lead to costly downtime. It occurs when unwanted electromagnetic energy couples into electrical cables, either through radiation or conduction.
Control cables are particularly vulnerable because they often operate in electrically noisy industrial environments alongside high-power equipment, motors, variable frequency drives, and switching power supplies. These sources generate electromagnetic fields that can induce voltages in nearby conductors, leading to signal degradation, data corruption, or complete system failure.
EMI manifests in two primary forms. First, there’s radiated interference, which travels through the air from sources like motors and wireless devices. Engineers also have to worry about conducted interference, which propagates through conductors such as power lines and cables. In industrial settings, control cables face both types simultaneously. For example, it’s not uncommon for substations to experience interference from switching transients and high-voltage power lines, while manufacturing facilities contend with motor drives, welding equipment, and heavy machinery.
Three Techniques for Reducing EMI in Control Wires:
Technique 1: Cable Twisting for Field Cancellation
One of the most fundamental and cost-effective methods for reducing EMI is the use of twisted pair wiring. Alexander Graham Bell patented this technique in 1881, and it remains widely used today because of its remarkable effectiveness. Twisting provides four major benefits: magnetic field cancellation in emissions, magnetic field immunity to external interference, electric field emission reduction, and electric field susceptibility improvement.
How Twisted Pairs Reduce EMI
When two wires carrying signal and return current are twisted together, the magnetic fields generated by adjacent half-twists are equal in magnitude but opposite in polarity, causing them to cancel each other out. This field cancellation dramatically reduces the cable’s ability to both emit and receive electromagnetic interference. The twisting ensures that both wires remain equidistant on average from any interfering source, so any induced noise affects both conductors equally. This common-mode interference can then be rejected by differential receivers.
For differential signaling applications, twisted pairs offer exceptional performance. Differential signals use two complementary conductors where one carries the positive signal and the other carries the inverted signal. When differential receivers measure the voltage difference between these two conductors, common-mode noise that affects both wires equally is automatically canceled, providing excellent noise immunity even in harsh electromagnetic environments.
Optimizing Twist Rate
The effectiveness of twisted pair cables depends significantly on the twist rate, i.e. the number of twists per unit length. Tighter twists with more frequent loops provide better high-frequency EMI rejection because they minimize the loop area that can act as an antenna. While specific twist rate standards vary by application, the general principle is that higher-frequency interference requires tighter twisting. However, excessive twisting can increase cable stiffness and manufacturing costs, so engineers must balance EMI performance with mechanical and economic considerations.
Technique 2: Shielding Strategies for Maximum Protection
Shielding represents the primary defense against electromagnetic interference in control cables. A properly designed shield creates a conductive barrier that either reflects incoming electromagnetic waves or provides a low-impedance path to ground for induced currents. The effectiveness of cable shielding depends on several critical factors: shield type, coverage percentage, material conductivity, and proper termination.
Foil Shielding
Foil shields consist of thin metallic foil, typically aluminum or copper, wrapped around the cable conductors with overlap to ensure complete coverage. The primary advantage of foil shielding is its 100% coverage, making it highly effective for high-frequency applications above 15 kHz. Aluminum foil with polyester backing represents the most common configuration in commercial and industrial applications due to its lightweight nature and low cost.
However, foil shields have limitations. They offer poor flex life and minimal mechanical strength, making them unsuitable for applications involving frequent cable movement or flexing. Additionally, foil shields require a drain wire for proper grounding because the foil itself is too fragile to terminate directly. Despite these drawbacks, foil shielding excels in stationary installations where high-frequency EMI protection is paramount.
Braided Shielding
Braided shields consist of woven copper or tinned copper wire strands that form a tubular mesh around the cable. This traditional shielding method provides 70-95% coverage depending on the tightness of the weave. While braided shields cannot achieve 100% coverage due to gaps in the weave, they offer significant advantages including superior mechanical strength, excellent flexibility, and easier termination compared to foil.
Braided shielding performs best at low to medium frequencies (below 15 kHz) where its lower DC resistance provides effective grounding for interference currents. The woven copper construction creates multiple parallel paths to ground, reducing the shield’s overall resistance and improving its effectiveness against low-frequency electromagnetic fields. Industrial and military applications frequently specify braided shields because of their durability and reliable performance in demanding environments.
Combination Shielding: The Best of Both Worlds
For applications requiring maximum EMI protection across the entire frequency spectrum, combination shields that incorporate both foil and braid offer the optimal solution. These dual-layer shields typically place foil in direct contact with the cable insulation, providing 100% coverage for high-frequency protection, then add an outer braided layer for mechanical strength and enhanced low-frequency shielding.
Combination shields deliver comprehensive performance: the foil blocks high-frequency interference while the braid provides a low-resistance ground path for low-frequency noise and adds physical protection to the cable assembly. This configuration is particularly valuable for critical signal applications in industrial automation, medical equipment, and military systems where EMI-related failures cannot be tolerated.
Technique 3: Conductor Material Selection
The choice of conductor material significantly impacts a cable’s EMI performance, electrical characteristics, and cost. While copper dominates the control cable market, understanding the properties of different conductor materials enables engineers to optimize designs for specific applications.
Copper: The Gold Standard
Copper remains the most widely used conductor material for control cables because of its exceptional electrical conductivity and versatility. Of all metals used in EMI shielding and cable construction, copper is the most reliable because it effectively attenuates both magnetic and electrical waves. Its high conductivity allows copper to absorb and redirect electromagnetic energy with superior efficiency compared to alternative materials.
Copper’s excellent thermal conductivity also provides an important benefit: it dissipates heat generated when electromagnetic waves interact with the conductor, preventing thermal degradation that could compromise cable performance. Additionally, copper forms a protective oxide layer that enhances its corrosion resistance, making it suitable for harsh industrial environments including outdoor installations and high-moisture areas.
For EMI shielding applications specifically, copper braids and foils provide the lowest DC resistance of common shielding materials, creating highly effective ground paths for interference currents. However, copper costs more than alternatives like aluminum, requiring designers to balance performance requirements against budget constraints.
Aluminum as a Cost-Effective Alternative
Aluminum offers approximately 60% of copper’s electrical conductivity while providing significant cost savings and a better strength-to-weight ratio. For applications where weight is a critical consideration, aluminum’s lightweight nature makes it an attractive choice despite its lower conductivity.
Like copper, aluminum forms a protective oxide layer that resists corrosion, particularly in high-moisture environments. This corrosion resistance makes aluminum suitable for outdoor installations and marine applications. However, aluminum’s lower conductivity means that aluminum shields and conductors must typically be larger in cross-section than copper equivalents to achieve comparable EMI shielding performance.
Specialized Alloys
Several specialized conductor alloys serve specific EMI shielding needs. Copper alloy 770 (also called nickel silver) combines copper, nickel, and zinc to provide excellent corrosion resistance and works effectively across the mid-kHz to GHz frequency range. Beryllium copper alloys offer similar shielding properties to pure copper while delivering improved mechanical characteristics and corrosion resistance.
For applications requiring magnetic shielding at low frequencies, materials with high magnetic permeability such as mu-metal or permalloy may be necessary. These specialized alloys are typically reserved for the most demanding applications where standard copper or aluminum shielding proves insufficient.
Technique 4: Ground Referencing and Grounding Best Practices
Proper grounding represents one of the most critical and frequently misunderstood aspects of EMI control in cable systems. Even the best shielding becomes ineffective without correct ground connections because the shield needs a low-impedance path to drain interference currents safely away from signal conductors.
Shield Grounding Strategies
The optimal shield grounding approach depends primarily on signal frequency and cable length. For low-frequency applications below 100 kHz, including audio and many industrial control signals, grounding the shield at one end only is recommended. This single-point grounding prevents ground loops that can occur when the two ends of a cable are at different ground potentials, which would cause unwanted current to flow through the shield and potentially increase noise rather than reduce it.
When implementing single-point grounding, engineers should typically ground the shield at the source end where the signal originates. This choice aligns with the signal voltage reference and provides optimal protection. Only ground at the load end if the signal source itself is not grounded.
For high-frequency applications above 1 MHz, grounding the shield at both ends becomes necessary for effective EMI control. At these frequencies, the wavelength of the interference becomes comparable to cable length, and both-end grounding creates a more effective electromagnetic shield by maintaining a continuous conductive enclosure. The key to successful both-end grounding is ensuring that ground potentials at both locations are essentially equal, or that the system can tolerate any circulating shield currents.
Proper Shield Termination
How shields are terminated matters as much as where they are grounded. The most effective shield terminations provide 360-degree contact between the shield and the ground reference, typically through specialized connectors or cable glands designed for this purpose. Avoid “pigtail” terminations where a short wire connects the shield to ground. This highly inductive connection becomes increasingly ineffective at higher frequencies.
For cables using foil shields, drain wires provide the practical means to establish ground connections. These bare copper wires run in contact with the foil along the cable length and should be terminated with the shortest possible connection to ground. Braided shields can be terminated directly through compression connections or clamp-style connectors that ensure good electrical contact.
Grounding System Design
Beyond individual cable shield grounding, the overall facility grounding system plays a crucial role in EMI control. Best practices include using single-point or star grounding configurations to prevent ground loops, maintaining short and wide ground connections to minimize inductance, and separating analog and digital grounds with a single joining point to reduce noise coupling.
In industrial control panels, proper bonding between cabinets and equipment enclosures allows shield grounding at both ends without ground loop concerns when the bonding maintains equal ground potential. For maximum high-frequency noise rejection, ground braids prove more effective than stranded wire because of the skin effect: high-frequency currents travel on conductor surfaces, so the increased surface area of braided ground straps provides lower impedance.
Integrated Design Approach to EMI Reduction: From Conduit to Conductors
The most effective EMI control strategies recognize that shielding, twisting, conductor selection, and grounding do not operate in isolation. Properly engineered, these parameters synergistically improve system performance and minimize EMI. For example, a control cable system employing twisted pair construction with foil-plus-braid shielding, 7-strand tinned copper drain wire, and single-point ground referencing will achieve electromagnetic performance significantly exceeding the sum of individual technique benefits.
Take Allwire’s substation control cables: such integration is evident in the detailed engineering of products like our multi-conductor shielded 600V and 1000V cables. These cables employ 7-strand construction optimized for high-frequency performance, comprehensive foil-plus-drain-wire shielding certified to ICEA and IEEE standards, and manufactured with meticulous control to ensure uniform twist pitch that maximizes paired-conductor cancellation of magnetic interference.
When installed in Allwire’s cable-in-conduit systems with proper single-point shielding at the control signal source, the complete system delivers comprehensive EMI control from DC through multiple gigahertz.
Custom Conduit Solutions for Enhanced EMI Protection
While proper cable design with shielding, twisting, and grounding provides the foundation for EMI control, the external environment in which control cables operate presents additional challenges. This is where custom conduit solutions offer a critical supplementary layer of protection, shielding cables from mechanical damage, environmental hazards, and external electromagnetic interference.
Expertise in Conduit Design and Selection for Meeting Your EMI Requirements
At Allwire, we understand that selecting the optimal conduit material for EMI protection requires careful consideration of the specific application requirements, environmental conditions, and electromagnetic environment. Our team works closely with customers to evaluate these factors and recommend conduit solutions that provide the appropriate level of protection while meeting mechanical, chemical, and cost constraints.
