Oct 2, 2017
Shielded PVC Multiconductor Cable Layer Attributes and Characteristics
Table of Contents
- Shielded PVC Multiconductor Cables Overview
- Copper and Tin Layer of Multiconductor Cables
- Insulation Layer of Multiconductor Cables
- Shielding Layer of Multiconductor Cables
- Jacket Layer of Multiconductor Cables
- Attributes of Shielded PVC Multiconductor Cables
- Characteristics of Shielded PVC Multiconductor Cables
When cables have multiple layers they tend to get confusing so this article is meant to ease the confusion of shielded PVC multiconductor cables. Imagine running 6 wires from A-to-B and letting them all hang loosely for the life of the application. First of all, it’s not a fashionable way to install the wires, and second, they could use some added protection against impact, heat, voltage and Electromagnetic Interference. Wires conduct electricity in order to make electronics and machines function, which in turn, create heat, voltage and movement, all of which can damage wires and complicate the copper conductor.
The copper conducts electricity, so the cable is nothing without the copper. As the strands of copper get smaller they become more flexible, so there will sometimes be multiple small strands used for added flexibility in the application. The layer of tin over the copper strands limits corrosion, decreases the chances of heat problems and makes it easier to solder the wires to an application. Adding hot solder to the wire will slightly melt the tin coating and adhere the two together well.
A layer of insulation is placed over each conductor of copper, or each group of flexible strands. Multiple small strands equal that of a solid larger strand as long as both diameters are equivalent. We don’t want any damage to occur to the copper, so the insulation is the first layer of protection. This particular insulation will be PVC (Polyvinyl Chloride) which is a specific mixture of elements used to protect the copper against a certain level of heat, voltage, some liquids and environmental impact. For example, the style UL1007 will resist 105C and 300 volts while UL1015 resists 105C and 600 volts. Both are PVC insulation but one is thicker than the other. However, as you can see, the melting point is the same in this case. There are numerous types of insulation that resist different levels of heat, voltage, liquids and impact, and as you might guess, they each get more expensive and less available as the list of requirements gets longer.
Aluminum foil can deflect sunlight into a different direction, and if you think about that concept it will help you understand how a PVC cable shield functions. When two cables run side-by-side without shield protection their waves can overlap each other causing electromagnetic interference. You may have experienced this in the past as you’re listening to the radio and it changes back and forth between two different stations. The interference is due to the cables being too close to each other because the magnetic field is larger in circumference than the cable itself. So, a single aluminum polyester shield is wrapped around all of the conductors in order to deflect the magnetic field inward and to avoid colliding with the cable next to it.
As you begin to understand shielding and shielded cables you’ll see that there are different kinds of shields that work in slightly different ways. The most basic of shields is mentioned in the last paragraph which mostly looks like aluminum foil. The next level of protection uses the same material but individually wraps pairs of wires and wraps the entire set of conductors. There is also a braided shield that uses tinned copper strands weaved like a Chinese finger trap toy and wrapped around all of the conductors. And third, there is a foil and braid shield that uses both shields on the same cable for increased protection.
Imagine the jacket of a multiconductor cable as the outermost layer that simply mimics that of the insulation over the copper. It brings everything together as a single unit but when the jacket is removed there is a shield and multiple conductors to work with. The resistance levels of the outer jacket mimic that of the insulation because they will be placed in the same spot for the application. For example, when would the jacket need to resist 200C while the conductors need to resist 300C? I’ve been told to never say never, so there are a few circumstances when the jacket needs a different set of approvals compared to the insulation, but it’s rare. For instance, when the cable runs 100 feet from the inside of a house to a barn, the portion of the cable that runs outdoors will need additional protection from either sunlight or everything underground.
Notice the braided stainless steel layer over the standard PVC jacket in the picture below:
Pick your own attributes. Let your supplier know that you need to resist a certain temperature, voltage, liquid or environmental condition and they will find you the cheapest option that fits your needs. The cable might have to be 600 volts instead of exactly 460 volts, but it will be covered. Wire harnesses are used in ovens, near car engines, in radios, inside the walls of your home, in your computer, cash registers, telephone poles, etc. So there might be a long list of things to resist and every wire and cable specification sheet lists exactly what it was designed to resist. If you have a requirement that didn’t make it to the specification sheet you can raise the question for the design engineers at the manufacturer’s facility. It should be answered promptly.
Believe it or not, colors matter. Some equipment manufactures look beyond the functionality of the cable and need the outer jacket to be white in order to match the entire desktop computer. Some need the cable to be orange for safety reasons. Some harnesses are specific about the color combinations of each wire, different from the standard colors used in the industry. Some applications need a smaller than normal outer diameter in order to fit inside of their application. These characteristics can usually be met with much higher costs of design, new materials and the expense of a minimum run. In order to change the jacket color to orange they must buy that material, take the current job off the machine, test and approve the orange cable to the same specifications, run this job for only you, and probably run it only once. There will surely be an added cost and decent lead time for this project, but it can certainly be done!