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In this chapter, we will learn about noise in electrical circuits, the reasons for their generation, types of noise and mitigation. We will cover shielding as a means of noise control and the role played by grounding and how properly designed grounding can reduce noise. We will learn about zero signal reference grids for noise-prone installations. We will briefly deal with the subject of harmonics and how they affect power and electronic equipment and about ways of controlling them.
8.2 Definition of electrical noise and measures for noise reduction
Noise, or interference, can be defined as undesirable electrical signals, which distort or interfere with an original (or desired) signal. Noise could be transient (temporary) or constant. Unpredictable transient noise is caused, for example, by lightning. Constant noise can be due to the predictable 50 or 60 Hz AC 'hum' from power circuits or harmonic multiples of power frequency close to the data communications cable. This unpredictability makes the design of a data communications system quite challenging.
Noise can be generated from within the system itself (internal noise) or from an outside source (external noise). Examples of these types of noise are:
- Thermal noise (due to electron movement within the electrical circuits)
- Imperfections (in the electrical design).
- Natural origins (electrostatic interference and electrical storms)
- Electromagnetic interference (EMI) - from currents in cables
- Radio frequency interference (RFI) - from radio systems radiating signals
- Cross talk (from other cables separated by a small distance).
From a general point of view, there must be three contributing factors before an electrical noise problem can exist. These are:
- A source of electrical noise
- A mechanism coupling the source to the affected circuit
- A circuit conveying the sensitive communication signals.
Typical sources of noise are devices, which produce quick changes (spikes) in voltage or current or harmonics, such as:
- Large electrical motors being switched on
- Fluorescent lighting tubes
- Solid-state converters or drive systems
- Lightning strikes
- High-voltage surges due to electrical faults
- Welding equipment.
Figure 8.1 shows a typical noise waveform and how it looks when superimposed on the power source voltage waveform.
Figure 8.1 Noise signal (top) and noise over AC power (bottom)
Electrical systems are prone to such noise due to various reasons. As discussed in the previous chapter, lightning and switching surges are two of these. These surges produce high but very short duration of distortions of the voltage wave. Another common example is 'notching', which appears in circuits using silicon-controlled rectifiers (power thyristors). The switching of these devices causes sharp inverted spikes during commutation (transfer of conduction from one phase arm to the next). Figure 8.2 shows the typical waveform with this type of disturbance.
Figure 8.2 Waveform distorted by notching
Harmonics in supply system is yet another form of disturbance. This subject will be reviewed in detail later in the chapter. A typical waveform with harmonic components is shown in Figure 8.3.
Figure 8.3 Waveform distorted by harmonics
Switching of large loads in power circuits to which automatic data processing (ADP) loads are connected can also cause disturbances. Similarly, faults in power systems can cause voltage disturbances. All these distortions and disturbances can find their way to sensitive electronic equipment through the power supply mains connection and cause problems.
Apart from these directly communicated disturbances, sparks and arcing generated in power-switching devices and high-frequency harmonic current components can produce electromagnetic interference (EMI) in signal circuits, which will require to be properly shielded or screened to avoid interference. Figure 8.4 shows diagrammatically the reasons for noise from the equipment within a facility.
Figure 8.4 Noise emanating from electrical systems within a facility
The following general principles are applicable for reducing the effects of electrical noise:
- Physical segregation of noise sources from noise-sensitive equipment
- Electrical segregation
- Harmonic current control
- Avoiding ground loops which are a major cause of noise propagation (including measures such as zero signal reference grid, explained later in this chapter)
- Shielding/screening of noise sources and noise-susceptible equipment including use of shielded/twisted pair conductors.