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Grounding riddle No.20 - Grounding of electro-static-precipitator
Why do we ground positive terminal of electro-static-precipitator (ESP)?
Author : Yogi - From: India
 
#1
Thu, July 23rd, 2009 - 13:41
Negative grounding:

In most types of electronic equipment the negative terminal of the dc power supply is intentionally connected to the metal frame or housing of the electronic equipment. The frame is also connected to the earth or ground (sometimes using the green wire in the power cord). A power supply typically uses a transformer and various electronic rectifiers and filters to convert 110 volt alternating current power into a direct current at a constant voltage (typically 5 volts) needed by the electronic equipment. It would be more descriptive to call this device a “power converter” instead of a power supply. In these applications, the reason for connecting the negative voltage to the frame or housing and grounding it is partly like the reason in a vehicle: the frame carries current instead of using additional insulated wires. Another reason is that the grounding of the housing helps to shield the electronic devices inside from electromagnetic waves (radio and television signals or electromagnetic waves arising from electrical equipment in the vicinity such as the motor that operates an elevator). Incidentally, in some very few older types of electronic equipment (using so-called PNP transistors) the positive terminal of the power supply was connected to the frame, housing and earth.

Positive grounding:

The reason for grounding the positive terminal of the power supply in some electrical system such as telephone outside plant wiring and probably your concerned subject (electrostatic filters) is to minimize the amount of corrosion on the copper wire. Corrosion effects related to the grounding of the power supply would not occur if the insulation around the wire were perfect (no leakage current whatever) and bare (non- insulated) parts of the wire were never exposed to moisture or chemically reactive molecules in the air. It would also be nice, to absolutely prevent corrosion if the insulation around each wire was a perfect barrier to the passage of metal atoms or ions. If this ideal situation were to exist, then we could ground either terminal of the battery and not worry about corrosion. If absolutely all of the current flows from the + to the - pole of the battery only via the wires , then we would not be concerned about the current removing or placing (electroplating) copper atoms off of or onto the wires. No current would leak through the insulation and return to the other battery terminal via the conductive earth. That would be nice but this desirable situation is not practically possible.
Plastic insulation used today is much better -- lower leakage current -- than paper pulp or silk or cotton fiber insulation used in the past. But even with very good insulation, corrosion can be accelerated because the insulation is not perfect and some "leakage" current gets through the insulation and some metal or oxygen molecules can move through imperfections in the insulation. The leakage current is typically only in the range of 1 to 10 microamperes, but over a long time it could cause some serious changes in the wire diameter if we made the wrong design choice regarding battery grounding.
If we connect the terminals of a dc battery to two pieces of metal and then place them in a conductive liquid, like water with a little bit of acid or a soluble mineral added to it, electric current will flow through the liquid. Furthermore, positive metal ions (atoms with one or more electron removed) will move through the liquid from the positive terminal to the negative terminal. If the positive terminal in the liquid is gold and the negative terminal is a “base” metal like lead, gold atoms will coat the base metal. When the gold ions reach the surface of the negative terminal in the liquid, electrons from the negative terminal of the battery will meet these positive ions and electrically neutralize them.
When leakage current flows from one wire to another via microscopic passages through the insulation and via wet earth or via a very moist atmosphere, positive metal ions (atoms with one electron removed) leave the positive wire and move by an available path through the wet earth toward the negative wire or negative voltage nearby objects. If we made the wrong design choice and grounded the negative battery terminal, then all the wet earth in the vicinity of the positive wire would be a convenient nearby place for these positive ions to go. We would then continuously "de-plate" copper atoms (the reverse of the process of electroplating) off of the negative wire and deposit them in the earth.
We would also electro-plate some copper and some copper oxide and/or copper carbonate onto the negative wire. Copper oxide and carbonate form because the atmosphere provides oxygen and carbon dioxide that lead to these chemical products. The dangerous situation occurs because the positive copper wire in this situation is continually getting thinner and thinner, so its electrical resistance is increasing and it may eventually break due to mechanical stress.
Incidentally, the verdigris coating that forms on copper or its alloys (brass, bronze, etc.), composed of copper oxide and copper carbonate, is called verdigris from the Latin words "verdi" (green) and "gris" (grey), which describes the approximate color of this material.
If we ground the positive battery terminal, then even when the earth is wet, the earth is at a positive voltage just like the positive (tip wire or green insulation) wire. Therefore, there is little or no voltage difference between that + wire and the + voltage earth, and there is much less leakage current and much less "de-plating" of the wire. There may still be some copper plating or a build up of copper oxide or copper carbonate onto the negative (ring or red insulation) wire if it is physically close to the wet earth or in a wet corrosive atmosphere.
If we have made a secure metal- to-metal contact of the wire to a screw terminal (or a type-66 stake-down electrical connection block that cuts through the insulation with two tinplated metal prongs and grips the copper wire), with no gaps between the wire and the metal terminal, there should not be a problem with corrosion forming between the wire and terminal. If we made a loose "sloppy" connection, then there will probably be a problem with corrosion forming in the gap between the negative wire and the terminal (but not so much corrosion forming on the positive wire). In that case we would need to occasionally clean the wire and reconnect it, a definite maintenance problem.




 
Author : Hamid - From: Iran
 
#2
Wed, July 8th, 2015 - 19:17
Please easily explained the answer. 
Author : arun kumar sahu - From: indian
 
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