Cable assemblies are utilized in a variety of industrial applications, which can exhibit electrical noise. Electrical noise can lead to Electromagnetic Interference (EMI) in cable assemblies. When a cable assembly receives or generates electromagnetic interference, it can cause poor signal quality and loss of data. It can also disrupt the function of electronic equipment and other components around it. Hence, it is necessary to use special shielding to protect cable assemblies from EMI.
Shielding for Protecting Cable Assemblies from EMI
Cable shielding is used to resolve the issue of EMI in cable assemblies. A cable shield encapsulates the conductor within the cable to provide effective protection. The shielding impacts EMI in two ways.
- The first way is by reflecting the electromagnetic energy.
- The second way is by picking up noise, and grounding it.
Although some energy will pass through the shielding, but it will be so highly attenuated that it won’t have any impact on the assembly.
Types of Shields Used In Cable Assemblies
Various types of shields are used in cable assemblies based on the application they will be used in. Commonly used shields in cable assemblies are:
- Foil Sheaths: Foil shields comprise a thin layer of aluminum or copper attached to a carrier such as a polyester or Polypropylene film. A foil sheath provides 100% coverage to the cable or components. The foil shield consists of a drain wire for terminating and grounding the EMI.
- Metal Braids: A braid shield is constructed as a woven mesh around the cable core. It is constructed from bare or tinned copper wires. For special applications, stainless steel or silver plated copper braid construction is also used. Metallic braids provide improved termination and lower DC resistance than foil sheaths, which makes them ideal for low frequency interference.
A cable assembly needs to have appropriate shielding according to the application it will be used in. However, it is also important that the equipment connected using the cable assembly should be properly grounded.
If you are designing products for military purposes, then you will need to use MIL-Spec cable assemblies. However, it is not just simply about using these types of cables. You have to use wires and cables with exact specifications according to the product and the application’s requirement. Choose wrong, and the product may not function properly. This is why when creating a military electronic product, you need to pay attention to the military standards.
What are Military Standards?
Military standards, also known as MIL-Spec, are a group of specifications created by US Government organizations. They are the regulations that should be kept in mind when using cable assemblies in harsh military environments. They are particularly important for electronic devices that are exposed to chemicals, abrasive materials, or extreme temperatures. Mil-Spec wire can be used in various applications, including aircrafts, weapon systems, and military ground vehicles.
What do the Standards Comprise?
These standards are used primarily during the manufacturing of military cable assemblies. Cable manufacturers refer to the standards when deciding on various aspects of the wire or cable, such as:
- Type of Application
- Application Environment
- Manufacturing Materials
- Cable Construction
The standards are also referred to when choosing Mil-Spec electronic components. These include:
- Ground wires
- Airframe wires
- Point to point wiring
- Shielding and protective covering
Primary MIL-Spec Standards Organizations
There are a variety of military standards used by cable assembly manufacturers. However, the two most important organizations are the National Electrical Manufacturers Association (NEMA), and the Qualified Products List (QPL).
- NEMA: This organization focuses on developing public policies for cable manufacturers, as well as technical standards.
- QPL: As the name of this organization suggests, the QPL focuses on identifying different types of materials, which can be used in military projects. It also identifies cable product suppliers who can be approved for government projects.
There are a variety of regulations that come into play when creating Mil-Spec cable assemblies. By following the military standards, you will ensure that your products will comply with all the requirements of Department of Defense organizations.
An overmolded cable assembly is a type of cable assembly, which is seamlessly constructed to combine the cable and the connector into a single cable assembly. The process of overmolding a cable involves injecting molding. It utilizes a molten material that is injected into a mold cavity. This molten material conforms to the contour of the mold cavity, and solidifies into a desired shape when cooled.
Custom Solutions for Overmolded Cable Assemblies
The mold cavity can be designed with markings to produce custom molded cables. Custom cable molding options can include a company name, logo, extended area for strain relief, and flanges to be used as attachment points.
Custom cable molding also allows to encapsulate cable connectors in almost any shape. Also, the cable assembly can be produced in various materials, colors, and finishes. Primarily thermoplastic materials are utilized to produce custom cable assembly solutions. These include:
- Polyvinyl chloride (PVC)
Rubber or ABS (Acrylonitrile-Butadiene-Styrene) material is also used for certain unique applications.
How are Overmolded Cable Assemblies Produced?
The production process involves premolding, potting, and overmolding. This is followed by a careful review in order to determine the perfect combination of characteristics and performance to meet the customer’s expectations.
Generally, a two-cavity system is used to produce overmolded cable assemblies. In most cases, each cavity is identical that creates the same shape. Instead, unique forms of cavity can also be used to create different aspects of the assembly during a single cycle. A particular application of this includes an initial pre-shot of non-colored natural materials, which is followed by a colored, finishing material for the outer mold. Moreover, overmolded cables can be designed with straight, right-angle, or any type of connectors for custom applications.
Overmolded Cable Assemblies offer Enhanced Features
Overmolded cable assemblies can enhance your application by offering the following features:
- Flexible support
- Abrasion resistance
- Increased pull strength
- 360 degree strain relief
- Water, moisture, and heat resistance
- Increased EMI (Electro Magnetic Interference)
Speak to a manufacturer to understand how custom overmolded cables can improve your applications.
There are several factors to consider when choosing an electrical wire or cable. However, the most important factors to take into account are the electrical requirements of your application.
Selecting an Appropriate Wire or Cable
Mentioned below are the electrical requirements to consider when selecting an appropriate wire or cable.
- Current Carrying Capacity: This term refers to the calculation of the amount of current a cable can carry. The current carrying capacity of the wire or cable is directly proportional to its size. Current is expressed in amperes (Amp). The larger the wire size, the more current it can carry.
- Inductance: Inductance is the property of a wire or circuit that induces an electromotive force when the value of current changes. It causes the voltage to lag behind. This can occur in the same wire, or in a neighboring wire. It is expressed in microhenries per foot (μH/Ft). Higher inductance can hinder the flow of current in the wire.
- Impedance: Electrical impedance is the total opposition that a circuit or wire presents to the alternating current (AC) when voltage is applied. Impedance is expressed in Ohms (Ω). The length of the cable has no effect of its impedance, but the impedance at the transmitting and receiving end should be same.
- Capacitance: Capacitance is the measure of the amount of charge a cable can store within itself. Capacitance is expressed in Pico farad per foot (pF/ft). High capacitance can hamper the performance of a cable at high frequencies.
- Resistance: The electrical resistance of a cable is a measure of the difficulty it faces in passing an alternating current (AC) through it when voltage is applied. It is expressed in Ohms (Ω). There should be little to no resistance in a cable or wire.
- Velocity of Propagation (Vp): This term refers to the speed at which a signal travels through the length of a cable compared to its speed in free air. In practical applications, a cable slows the speed of electrical signals. This slowing effect is directly dependent on the dielectric material of the cable. Vp is expressed as a percentage of the speed of signal in free space. The lower the Vp, the longer the delay. Hence, choose a wire with high Vp percentage.
- Voltage (Volt): It is the measurement of electrical pressure along the conductor wire. The voltage rating is directly proportional to the insulation thickness of a cable. The thicker the insulation, the higher the voltage rating. Following are the generally used insulation thicknesses and maximum permitted voltage:
- 0.016″ insulation = 300 volts (UL Standards)
- 0.010″ insulation = 600 volts (Military Specifications)
- 0.016″ insulation = 1000 volts (Military Specifications)
- 0.032″ insulation = 600 volts (UL Standards)
Following these guidelines will help you choose a high-performance, and durable wire or cable. If you need further assistance, contact an expert manufacturer, he will be happy to help you choose the appropriate cable for your application.
The growing complexity of modern electronic and electrical systems, along with the ever increasing distance signals are required to travel, have increased the electrical interference in cables. In order to diminish the effects of signal interference in cables appropriate shielding is vital.
Types of Signal Interferences
The primary function or the shielding is to limit the signal from escaping the wire or cable. Besides, it also prevents external interference or signals from flowing into the wire. Depending on the kind of application, cables can be affected by different types of signal interferences. These include:
- Electrostatic interference
- Electromagnetic interference
- Radio frequency interference
Types of Cable Shielding Popularly Used in the Industry
Mentioned below are the most popular types of cable shielding that are used in the wire and cable industry today:
- Foil Shields: They are generally constructed of a Mylar and aluminum composite tape. They are lightweight, economical, and easy to use. They are normally grounded with a drain wire, and provide 100% shield coverage. Although foil shielding provides good flexibility, it can be damaged in high-flex applications.
- Braid Shields: This is the oldest form of shielding, and is still used in the industry. It is a woven pattern consisting of small gauge wires. Braid shields are mostly made up of bare, tinned, or silver plated copper, and aluminum or steel. They have a typical coverage of 55% to 95%.
- Spiral Shields: This shielding is made up of single strands of wires, which are spirally wrapped around the cable core. This shield is more flexible, and easier to terminate than the woven braid. They have a typical coverage of 95% to 98%. As they are most effective in the audio frequency range, they are typically used in audio and microphone cables.
- Tape Shields: This shielding is made from various types of materials. They normally comprise a laminate of aluminum and polyester layers, which is combined together with an adhesive. These shields are fragile, and offer considerable mechanical strength. Some of the popularly used tape shielding are:
- Bonded Aluminum Polyester
- Aluminum/Polypropylene (AP)
- Aluminum/Polyester/Aluminum (APA)
The cable shielding types mentioned above serve specific applications. Hence, you must consult a manufacturer or supplier to understand which shielding can suit your needs.