Understanding the requirements of industrial Ethernet cables

06 November 2017

Ethernet plays a major role in today’s automation systems to pass data through the system at high rates. The digital performance of a network is subject to the active devices within it, such as the switches and controllers. However, it is the passive devices such as cable and connectors which can ultimately determine the performance of a network.

The frequency performance of a component will dictate its performance. Cat 5 cables, for example, will be specified for operating frequencies up to 100MHz. However, the cable is suitable for 100MBIT applications using two pairs and gigabit applications (1,000MBIT) using a four-pair cable in association with the correct connector.

It is easy to confuse MHz performance with the MEGABIT.

Class                 Frequency range           Data rate      Capability
Cat 5/5e               100MHz                        2 Pair                4 Pair
Cat6                      350MHz                       Yes                  Yes
Cat 6A                   500MHz                       Yes                  Yes

Clearly, cables of a higher category (Class) meet all the transmission characteristics of the lower classes, and therefore provide additional performance margin in a given application.

Conductor size
Stranded Ethernet cable does not have the same performance characteristics as solid conductor and this is important when distance is a factor. Ethernet conductors are typically 26AWG and 24AWG, and it is always best to use the largest size when requiring high data rates over long distance. 

The Profinet standard saw 22AWG cables being introduced, with this wire size there is significant transmission performance advantage over 26AWG and 24AWG, full 100m segment length can be achieved with stranded or solid cable types. Three types have been defined within the Profinet standard – stationary, flexible and special application. Care should be taken to select the correct RJ45 connector when using 22AWG cables as many are not compatible with the increased wire size and the larger outside diameter of Profinet cables.

Power Over Ethernet
Spare conductors in an Ethernet cable can be used to provide power to Ethernet devices. Clearly, wire size is important in the calculation of voltage drop in Power Over Ethernet (POE) applications, and is important to bear in mind in cable selection. 

Cable selection
The industrial environment requires suitable cables which have far more robust constructions than those used in office environments. Materials such as PUR are often used to provide mineral oil resistance, high abrasion resistance and are halogen free. 

Ethernet cables are available with solid conductors which are only suitable for static applications. Stranded conductors, typically seven strand, are suited to flexible use applications, while highly stranded – typically 19 strands – are sited to use in continuous movement applications.

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