I am using 4 core shielded cable. However, there is a strand of what I assume is nylon running parallel to the conductors. A bit like this
What is it for?
That string can be used to strip the jacket off of the cable.
You nick the cable at the end, put the string in the cut, then you can pull the string back and it will cut through the insulation on the way back.
For tough insulation (or soft fingers!) you might want to to tie a loop in the end first.
If the thread is just inside the outer jacket (also called a sheath), it is likely a ripcord. It's meant to be used to strip the outer jacket without harming what is directly under that jacket (shield, bundle of conductors, etc.).
Marker threads (see Andy aka’s excellent answer) tend to be weak.
If it’s a strong nylon thread that’s outside the shield it could be a “ripping cord”. They can be pulled back to slice through the outer insulation. They’re more common in larger cables than the image, but you also see them in UTP:
This thread is actually referred to as "Marker Thread." The tracer filament is nylon fiber that is a non-strength adding member of the cable. During production, it is stranded into the finished product for identification purposes only. Manufacturers have their own distinct color codes, and this tracer helps identify who produced the cable.
Some of the other answers are from the migrated question: Why is there a textile thread in some cables? Each answer seems to have proposed a single use or limited set of use cases.
I am using 4 core shielded cable. However, there is a strand of what I assume is nylon running parallel to the conductors. ...
What is it for?
The company Top Cable has this helpful illustration:
The ripcord allows you to gently tear the outer-sheath allowing you to gently peel it away without damaging the screen. In sheathed wire without shielding screens it's merely a convenience, for screened wire it's important to not damage the screen.
In the blog Insights, by the company Service Thread, their article titled: "What Kind of Rip Cords Are Used in Wire and Cable Manufacturing?" explains:
"Rip cords are industrial yarns located longitudinally just under the jackets that cover conductor wires in cables. The purpose of a rip cord is to cut back the jacket to expose the wires of the cable so you don't run the risk of cutting any of the wires inside (you pull the cord, it cuts the jacket). This helps make splicing cables easier during installations.
Common Types of Rip Cords
Rip cords are made from a wide variety specialty yarns. Some of the most common yarns used are:
- Aramid (Para-aramid)
The cords can be made of a combination of any of these yarns to achieve different strengths and cutting abilities. Along with different base materials, rip cords are typically made in three different constructions: Flat, Monocord and Cabled.
The type of rip cord required is dependent on the strength and density of the cables jacket. For instance, aramid yarn is typically required to rip steel or aluminum cables because of it superior strength. The construction of the yarn used determines the final strength and density of the rip cord. A flat or low twist monocord is not going to give you the same kind of cutting strength that you would gain from a high twist or cabled construction.
You also have the choice of wicking, non-wicking, water swellable, and bonded treatments on the yarn depending on your requirements for moisture migration or yarn stiffness. The range of the rip cords diameter and tensile strength is limitless. Depending on the rip cord you're needing, there is a strength level and material combination to suit the job.".
There are other uses for yarn in wire manufacturing.
In the blog Insights, by the company Service Thread, their article titled: "Importance of Having Accurate Industrial Yarn Specifications" explains:
"Industrial yarn is designed and engineered to be incorporated into products where oftentimes, a functional requirement supersedes aesthetic considerations. Used by non-apparel industry product manufacturers, industrial yarns are typically a component part of a product to enhance the strength, durability, performance and other high-value properties of the product.
Product manufacturers in industries such as wire and cable, web slings, industrial bags, geotextiles, hose reinforcement and many more, are using tons of yarn on a yearly basis - however, it is not uncommon to find that the end user’s yarn specifications on paper do not match the physical product being used.".
In Service Thread's product detail titled: "Yarns for Wire and Cable Applications" they explain:
Binders, Ripcords, Strength Member Yarns, and Indentification and Marker Yarns, engineered just for Wire and Cable Manufacturing
Service Thread designs, develops, manufactures and delivers innovative threads and yarns for wire and cable manufacturing or installation.
Use of color coded threads is for the internal use of the manufacturer, marking of wire for the end user must be done in accordance with local Standards and can be done via printing or laser marking:
So in addition to being a Ripcord similar such fibers can be used to identify which bobbin a wire or set of wires came from when feeding a cable twister and/or to bind sets of wires together for feeding through the bushings. The purpose of binders is to hold the bundle together until the jacketing operation.
Sometimes a plastic fiber is included in a cable to help the individual conductors maintain a particular relationship with each other. I have seen this in "quad core" microphone cable, for example.
Also, I have found twisted paper in many kinds of power cables (e.g., Romex) for pretty much the same purpose. It helps fill in the gaps and make the jacket look smoother and bend more evenly. Sometimes rubber-jacketed power cable has hemp (rope) fibers in it for the same reason.