EMC in cables

In industrial environments, electromagnetic behaviour can have a disruptive impact on sensitive motors, controllers and welding machines. Problems can arise especially in long electrical power or data transmission wires running between the individual components, and it is important to take suitable protective measures against this.

Because they behave like antennas, such cables can pick up electromagnetic interference, which becomes superimposed over the useful signal (to or from a sensor or rotary encoder, for instance). The result is EM interference in the connected devices – causing anything from inconspicuous corruption of isolated measurements to total failure of a production wire. Conversely, such cables and wires can also behave as transmitters of EM interference.One proven countermeasure has been to install electromagnetic components in an earthed switch cabinet and to connect them using shielded (EMC) cable. One commonly observed problem with this, however, is that the very site where the cables pass into the switch cabinet becomes an EMC weak point in itself. Inadequate contact between the cable shield and metal housing here frequently negates the desired shielding effect entirely. Take good notice of this fact!

EMC in cables and their shielding

Generally, it is not possible to distinguish between wire-bound and field-bound interference. Field-bound interference, which can be directly given off by or conversely act upon a PCB for example, can be effectively mitigated by installing electrical or electronic components into closed metallic housings such as switch cabinets. 

If there are no large openings or breaches in housing, then a Faraday cage is formed, which offers effective protection against EM interference. In industrial practice, this type of shielding most often comes at a very high price, and can hardly be achieved for moving machine parts. An alternative is to use cables featuring an EMC shielding braid. The quality of the cable shielding in this case depends greatly on the structure and density of the braid. Also, suitable mechanical fastening elements must be used to achieve the closest to ideal connection of the cable shield to the housing wall, so as to prevent any interference running along the cable shield from getting in. 

The crucial factor here is the leakage resistance, i.e. the resistance a transverse electromagnetic wave “sees” on the cable shield when it reaches the cable-housing interface. Use of suitable cables can therefore ensure EMC protection.