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’ recommendations should be followed where appropriate. The control system specification should lay down what standard of installation is required.
The earthing of circuits and cable screens can present problems. Correct earthing depends on the type of cable, the signal frequency and whether the signal is single ended or differential. In a differential circuit, neither of the pair of signal wires is at earth potential; induced potentials from interference are induced onto both wires, and the input of a differential amplifier is able to reject common-mode disturbances. Most BMS analogue and digital circuits are low frequency and single ended, with the ground line at earth potential. For such cables, both the ground line and the screen of the cable should be connected together and earthed at the controller end of the cable only. Earthing the screen at both ends of the cable creates an earth loop, which can cause large 50 Hz currents to be induced, which in turn can induce unwanted noise in the signal cable. Some communication circuits employ differential amplifiers.
Neither signal lead is earthed and the screen should be connected to earth at one end only. Cabling for a standard system should follow manufacturer’s advice. High frequency cables above 1 MHz behave rather differently and the outer screen is earthed at both ends and perhaps at several points along its length.