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return for power distribution.Ground Loops
Ground loops can occur from two sources of noise:* Ground potentials* Electromagnetic fieldsThe previous section covered how ground potentials occur. Figure C-4 illustrates how a ground potential can cause interference to an electronic control system.
Figure C-4. Common Ground Impedance CouplingGround current IG, creates the ground potential, VG, through the common ground impedance, ZG. This potential drives current through the loop formed by the source and load wiring. This current creates an error voltage across the load impedance.In a load sharing system, load imbalance is a result of dc ground potentials. Load sharing instability or slow system response are a result of ac ground potentials. Provide separate ground returns for electronics devices.Electromagnetic fields also can induce current into a loop. Figure C-5 shows a ground loop formed by signal leads and circuit common. The field flux cuts through the loop and induces current flow. (This is the same principle that applies to generators.) The resulting voltage across the load causes an error in the measurement.
Figure C-5. Common Loop Coupling from an Electromagnetic FieldThe current induced into a loop is proportional to the field strength and area of the loop. Reducing field strength and loop area are both effective reduction methods. Loop area can be reduced by locating the electronics closer together and routing interconnecting wires near signal ground. Field strength can be reduced by routing wires as far from field sources as possible and orienting wiring at right angles to field sources. Shielding can be placed between the field source and wires to reduce the flux reaching the loop. This will be covered in the shielding section.The most effective (and most expensive) solution to interference from ground loops is to open them. Figure C-6 shows the source floating with respect to ground, so no loop is formed as in Figures C-4 and C-5.
Figure C-6. No Ground Loop with Circuit Grounded at One End OnlyGround loops may be broken with isolated power supplies on the source or receiving circuits. For process signals such as 4-20 mA or 1-5 V, loop isolators are available. Take care in such wiring to prevent grounding of isolated circuits, usually via shields, which defeats the ground loop isolation.Though not related to grounding, electromagnetic fields can also induce current into loops formed by signal lines as in Figure C-7.
Figure C-7. Differential Mode Coupling into Loop Formed by Signal Lead PairLoop area and field strength reduction are again effective methods of reducing interference. Loop area is reduced by using twisted pairs and minimizing the area at terminations. Twisted pairs also offer a cancellation effect on low frequency noise.Summary
A properly designed ground system is the most effective EMI control method at low frequencies. The design should consider using:* Signal ground for electronics separate from power and safety ground to prevent ground potentials.* Minimum loop areas to reduce electromagnetic field coupling.* Electronic wiring located as far as possible from sources of fields.* Elimination of ground loops by single point grounds and isolation from ground.Shielding
Shielding is