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Electromagnetic Compatibility problems (EMC) – where and why do they occur?

16 October 2017

What is Electromagnetic Compatibility?
An Electromagnetic Compatibility (EMC) problem can occur when one cable is adversely affected by the operation of another. The problem is commonly caused by a power cable installed in close proximity to a data or control cable, corrupting the flow of data.

EMC problems are considered to be more likely to occur when un-armoured and or un-screened cables are used in close proximity. The best situation is when both the influencing power cable and the influenced control cable are separated by the maximum possible distance and/or both cables are either armoured or screened.

In our experience, EMC issues with armoured or screened power cables are few. This is largely because the electromagnetic field is contained within the earthed envelope of the steel wire armour or screen, which limits interference with adjacent cables.

In order to minimise EMC problems with power cables, We recommend the following general principles should be adopted:

  • Achieve balanced circuits by ensuring the phases carry equal current (referred to as balanced loads)
  • Achieve the largest separation possible with sensitive circuits, such as data or telecoms cables
  • Earth all screensand armour of cables

Bonded Cables
A cable with solid bonding such as Afumex BS6742 or FP200 Gold will have a very low surrounding magnetic field. The trefoil arrangement of three core cables will tend to cancel out the magnetic field, particularly if the phase currents are balanced. Also, the eddy currents and circulation currents generated in the armour will produce a field tending to oppose the primary field and eliminate it. Since the magnetic field is dependant on the current in the conductor and not the voltage, the magnetic field induced by an 11kV cable will be no more than that around an LV cable with the same current.

More Than Just Cables
Electromagnetic Compatibility is an issue relating to more than just cables. Cables are components when incorporated into an apparatus or system. It is the apparatus as a whole that is required to comply with EMC regulations, so the original equipment manufacturer or system designer should specify cables and installations methods that ensure compliance with the regulations and directives.

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