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[Part 1 begins by defining electrical noise, examining the reasons for its generation and looking at ways to evaluate its effects.]
8.7 Earth loop as a cause of noise
As we have seen in earlier chapters, two different equipment with a communication cable between them and each of the panels connected to a local ground point form an earth loop, which can give rise to noise currents in the communication cable. A typical building electrical system with multiple earth points is shown in Figure 8.16. Note how each panel/equipment in the distribution system is connected to ground at the nearest convenient point of the building grounding system.
Note how two sensitive equipment units (shown in the upper right of the diagram as EDP devices) are connected to ground points A and B with the grounding system's inherent impedance shown between them. The EDP devices have a communication cable running between them with the ends of the cable screen connected to the EDP panel's enclosure. Any stray current in the ground system between A and B will cause a noise voltage between points A and B, which in turn can drive a current through the cable screen that can couple as a noise through the communication cable conductors.
Figure 8.16 Earth connections in building electrical distribution systems causing ground loops
Figure 8.17 shows how a noise can originate in the electrical power supply system. In this case, the HVAC motor winding acts as a capacitance between the electrical system and the motor's grounded enclosure. Whenever the motor starts, this capacitance sends a pulse of current through the insulation into the motor frame, which is grounded through the metallic conduit carrying the cable, leads, to the motor. The random ground connections between this conduit and other grounded metal parts act like a ground loop and create an inter-cabinet potential difference between two sensitive equipment (EDP units 1 and 2). This can cause noise pulses to flow into the serial data cable connecting the two systems, resulting in data errors.
Figure 8.17 Starting of HVAC motor gives rise to noise due to ground loops
8.8 The ways in which noise can enter a signal cable and its control
Electrical noise occurs or is transmitted into a signal cable system in the following ways:
- Galvanic (direct electrical contact)
- Electrostatic coupling
- Electromagnetic induction
- Radio frequency interference (RFI).
If two signal channels within a single data cable share the same signal reference conductor (common return path), the voltage drop caused by one channel's signal in the reference conductor can appear as a noise in the other channel and will result in interference. This is called galvanic noise.
Electrostatic noise is one, which is transmitted through various capacitances present in the system such as between wires within a cable, between power and signal cables, between wires to ground (as we saw in the HVAC motor example) or between two windings of a transformer. These capacitances present low-impedance paths when noise voltages of high frequency are present. Thus noise can jump across apparently nonconducting paths and create a disturbance in signal/data circuits.
Electromagnetic interference (EMI) is caused when the flux lines of a strong magnetic field produced by a power conductor cut other nearby conductors and cause induced voltages to appear across them. When signal cables are involved in the EMI process, this causes a noise in signal circuits. This is aggravated when harmonic currents are present in the system. Higher order harmonics have much higher frequencies than the normal AC wave and result in interference particularly in communication circuits. Radio frequency interference involves coupling of noise through radio frequency interference. We will now describe these in some detail.