GROUNDING (OR EARTHING)
This is one of the simplest but most efficient methods to reduce interference. Grounding can be used for three different purposes:
Provides protection for the operators from dangerous voltages. Widely used on mains-operated equipment.
The ground is used as a conductive path for signals.
Example: in asymmetrical cables screen, which is one conductor for the signal, is connected to the ground.
Used to provide a neutral electrical path for the interference, to prevent that the interfering voltages or currents from entering the circuit. In this chapter we will only consider the third aspect. Grounding of equipment is often required for the cases 1 or 2 anyhow, so that the screening ground is available "free of charge". Sometimes the grounding potential, provided by the mains connection, is very "polluted". This means that the ground potential itself already carries an interfering signal. This is especially likely if there are big power consumers in the neighbourhood or even in the same building. Using such a ground might do more harm than good.
The quality of the ground line can be tested by measuring it with a storage scope against some other ground connection, e.g. a metal water pipe or some metal parts of the construction.
It might contain strong interference, Because it carries the load current of all electrical consumers. The grounding can be done by single-point grounding or by multi-point grounding. Each method has advantages which depend on the frequency range of the signal frequencies. All parts to be grounded are connected to one central point. This results in no "ground loops" being produced. This means the grounding conductors do not form any closed conductive path in which magnetic interference could induce currents. Furthermore, conductive lines between the equipment are avoided, which could produce galvanic coupling of interference. Central grounding requires consistent arrangement of the grounding circuit and requires insulation of the individual parts of the circuit. This is sometimes very difficult to achieve. A system using the single-point grounding.
In multi-point grounding all parts are connected to ground at as many points as possible. This requires that the ground potential itself is as widely spread as possible. In practice, all conductive parts of the chassis, the cases, the shielding, the room and the installation are included in the network.
When considering the effect of electrical and magnetic fields, we have to distinguish between low and high frequencies. At high frequencies the skin effect plays an important roll for the screening. The penetration describes the depth from the surface of the conductor, where the current density has decayed to 37% compared to the surface of the conductor.
When signal lines run close to interference sources or when the signal circuit is very sensitive to interference, screening of signal lines will give an improvement. There are different ways of connecting the cable screen:
Three different ways of connecting the cable screen. Cable screen not connected. This screen will not prevent any interference, because the charge on the screen, produced by interference, will remain and will effect the central signal line. Also, the current induced by interference in the line will flow through the sink, effecting the signal. Cable screen grounded on one side only. This screen will only prevent interference at low frequency signals. For electromagnetic interference, where the wavelength is short compared to the length of the cable, the screening efficiency is poor. Cable screen grounded on either sidet is effective for all kinds of interference. Any current induced in the screen by magnetic interference will flow to ground. The inner of the cable is not affected. Only the voltage drop on the screen will affect the signal in the screen. type of grounding is
- Ensure proper and careful connection of the screens.
- Use suitable plugs in connection with the cable screen.
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