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Reply To: Wiring Riddle No.6 – EMI in buildings


    The cabling that connects components of a sensetive system is susceptible to interference from adjacent power cables and from radio frequency interference. Techniques for avoiding interference include the use of screened cables, care with routing cables, segregation of power and signal cables and correct earthing practices. Many types of screened cable are available. A comprehensive treatment of data and other types of cabling is given in Electric Cables Handbook. Table below gives an indication of the performance of different types of cable in attenuating magnetic and electric fields. It is important to remember that screening with a conductive material, such as copper, is effective at attenuating high frequency electromagnetic fields, but has little effect on power frequency magnetic fields. To attenuate 50 Hz magnetic fields it is necessary to shield with a magnetic material such as steel, or to use twisted-pair cables. In practice, a low cost, foil-screened twisted pair cable is suitable for most BMS applications for frequencies below 1 MHz. For IT and data communication circuits operating at frequencies above 1 MHz, such as Ethernet, the cable type will normally be specified by the equipment supplier. It may be coaxial cable or suitable twisted pair. The use of coaxial cable is not recommended for low frequency use as noise induced in the screen will be added to the signal. BMS signal cables should be kept as far away as practicable from sources of interference. In particular, untwisted cables should not be exposed to magnetic fields from high current equipment such as transformers. The IEE Wiring Regulations permit signal and power cables to share the same conduit, providing the signal cable has adequate insulation. The regulations are written from the point of view of electrical safety, and do not concern interference. Sharing a conduit or trunking makes for economical installation and some field bus systems, e.g. EIBus, are designed so that this will not introduce any EMC problems. However, in general it is recommended that signal cables without screening should be separated from power cables by a minimum distance of 150 mm. Ideally signal and power cables should be routed in separate trays or trunking, and cross at right angles where they meet. Table below presents general recommendations for different types of screened signal and power cables; manufacturers