To Shield or Not to Shield – Cables

Shielded vs. Unshielded Cable

Imagine yourself in a bar with a friend during happy hour.  The atmosphere is vibrant, but you strain to be heard over the noise.  When your friend responds you can barely hear them.  Now imagine it is a weekday afternoon and the same place is nearly empty.  It is easy to have a conversation.

Serial Communication PLC

In a nutshell, this is the same effect electromagnetic interference (EMI) has in the electrical industry.  The difference is that noise does not refer to what can be heard by the human ear, rather it is the electrical noise experienced by PLCs, servo motors and other devices communicating between each other in manufacturing.

Manufacturing is filled with high-voltage equipment.  Any object that uses electricity has the potential to generate electrical noise.  This noise permeates throughout the plant and has the potential to interrupt the signals of other equipment.  Some equipment remains unaffected by this such as simple motors and photoelectric sensors; however, other devices are communicating larges amounts of data at high rates of speed.  In these instances, the electric pulses of those transmissions can be easily interrupted by a stray bit of noise much like a conversation at a noisy bar.

Data transmissions take place over copper cables and if we think about a cable in its simplest form, it is merely a long strand of thin metal.  Another everyday object of a similar design is a radio antenna.  Cables can act as antennas by capturing and exacerbate EMI.  This can appear as incomplete data on the receiving end, like the static interference you can hear on a radio.

RJ45-RJ45 Double Layer Shielded LAN Cable

To get around this issue, cable shielding is necessary.  Cable makers have worked around this issue for quite some time.  In fact, some of the first electrical shielding developed was by Alexander Graham Bell.  To prevent noise in his telephone lines running near power cables, he twisted pairs of wire together.  This improved the signal and allowed the lines to be run across longer distances.

Cable manufacturers still use this method nearly 140 years after Bell invented it.  Cut open any Ethernet cable and you will see it still used in modern technology.  This is what is referred to as unshielded twisted pair (UTP) and shielded twisted pair (STP) for Ethernet cables and it prevents noise from the other wires in the cable.

This does not always prevent external noise.  It is necessary to conduct the EMI away from the wires themselves.  To do this, manufacturers protect the twisted pairs in a thin layer of foil, which transfers stray noise from the cable to a ground.

Depending on the environment this may not be enough.  Manufacturers also wrap the entire cable in a protective layer of braided copper mesh and aluminum foil to combat any extraneous noise in the environment.  This allows the unimpeded transfer of data and even allows a device to transmit at high rates of speed such as Cat 6.

MISUMI offers both shielded and unshielded cabling for the roughest industrial environments.  Some of these are available cut-to-length and can be used in applications ranging from power and robotics all the way to standard communication and high-speed communication such as Industrial Ethernet.  Visit our website to find these products and more.

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