Below is a reference table that can be updated based on the project-specific requirement. There should be isolation between each type of earthing mentioned above. If there is no isolation which means that the purpose of the above earthing requirement will not be fulfilled.
Control panels inside the control room fall under the Indoor type. Within the Cabinet, the Instrument Earth bar shall be isolated from the Safety Earth bar by mounting the Instrument Earth bar on insulating buses. This common Instrument Earth bar shall be further connected to the nearest Instrument earth grid using copper cables. Star connection or series connection. This common Safety Earth bar shall be further connected to the nearest safety earth grid using copper cables.
The Main Earth grid conductor size can be calculated based on the desired fault current within the required time to be earthed. The surrounding soil of the earth pit must be kept moist. Instrument Earth all individual shields Screen and over all shields single or multi pair cable shall be isolated from electrical earthing due to avoiding interference and terminated different bus bar.
Instrument bus bar will be connected to grounding dispatcher by 25 sqmm green- yellow cable. Grounding dispatcher to earth pit is connected to 70 sqmm green -yellow cable. Electrical Earth: Electrical earthing also called as dirty earth or protective earth. It is used to protect the power system,electrical equipment and personnel from electric shock. Instrument Earth is also called as Reference earth.
Instrument Earth all individual shields Screen and over all shields single or multi pair cable shall be isolated from electrical earthing due to avoiding interference and terminated different bus bar. Instrument bus bar will be connected to grounding dispatcher by 25 sqmm green- yellow cable. Other than that what are the main influences from an EMC point of view? Thanks in advance There are some technical reasons for the four different techniques mentioned by SL And there are misconceptions and there are myths.
As I see it, it started with audio amplifiers and microphones more than sixty years ago. Microphone signals were and are low level and the amplifiers had lots of amplification. So any "hum" picked up used to be very disturbing. Protective earth was not very common in those days and the result was hum pick up in the microphone cable.
Screening helped to some extent, but if the screen was connected to earth at the microphone end, stray currents from amplifier through screen to earth induced hum in the cable. Isolating the far end of the screen helped a lot. That is how the first rule: "Connect screen in one end only" came about. And also how the dreaded "hum loop", which many people still refer to as if it was an eternal truth - valid in all installations.
Which it is not. Second scenery: Digital communication like RSC and other crude techniques made its debut. These signals had immunity levels around a couple of volts and the "hum" was not really a problem any more. But transients and HF pollution were. Transients are HF, so the technique where the screen was grounded wherever possible started to be used.
So, "ground whenever you can" started to be a sucessful strategy. The earlier communication lines were mostly intra-building and seldom more than twenty or thirty metres. Short-haul modems remember the KM-1? When field buses made their entry, longer distances could be covered. Often between buildings. It then happened that the supply system for one building was not the same as for the other building. A quite common situation, actually.
The result was that current was flowing in the screen if it was connected to the different grounds in the buildings. And if there was an earth fault in one building, the potential rise forced large currents through the screen and it very often scorched the screen and cable so that equipment failure resulted.
That is when users started thinking. The thinking resulted in different ways of solving the problem. There are simple means like grounding via HF capacitors, using carrier and isolating transformers, using opto-couplers, systems with enhanced common-mode range like the RS and also bonding of the different buildings to a common potential.
Every manufacturer and every communication standard now have found their preferred way of handling the problem with connecting screens and these installation are often very reliable. It is in special situations and when equipment from suppliers with different philosophies shall be connected together that problems start to pop up.
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