Practical Industrial Data Networks:Design, Installation and ...

20Practical Industrial Data Networks: Design, Installation and Troubleshooting2.5 Grounding/shielding and noise2.5.1 Sources of electrical noiseTypical sources of noise are devices, which produce quick changes (or spikes) in voltageor current, such as:• Large electrical motors being switched on• Fluorescent lighting tubes• Lightning strikes• High voltage surging due to electrical faults• Welding equipmentFrom a general point of view, there must be three contributing factors for the existenceof an electrical noise problem. They are:• A source of electrical noise• A mechanism coupling the source to the affected circuit• A circuit conveying the sensitive communication signals2.5.2 Electrical coupling of noiseThere are four forms of coupling of electrical noise into the sensitive datacommunications circuits. They are:• Impedance coupling (sometimes referred to as conductance coupling)• Electrostatic coupling• Magnetic or inductive coupling• Radio frequency radiation (a combination of electrostatic and magnetic)Each of these noise forms will be discussed in some detail in the following sections.Although the order of discussion is indicative of the frequency of problems, this willobviously depend on the specific application.Impedance coupling (or common impedance coupling)For situations where two or more electrical circuits share common conductors, there canbe some coupling between the different circuits with deleterious effects on the connectedcircuits. Essentially, this means that the signal current from the one circuit proceeds backalong the common conductor resulting in an error voltage along the return bus whichaffects all the other signals. The error voltage is due to the capacitance, inductance andresistance in the return wire. This situation is shown in the Figure 2.1.