There are three grounding systems float,-ve and +ve.Now my question is that why +ve grounding is carried out and what are the advantages and disdavantages of +ve grounding as compare to other two system?
[This message has been edited by luckyali (edited 07-08-2004).]
I mean that in DC system we ground the negative or positive or none of both,so my question was,why positive grounding carried out and what are the advantages and disadvantages as compare to others two?
"+ve" and "-ve" are commonly used abbreviations for positive and negative in parts of the world which came under British influence.
Ali, There are various reasons for adopting positive or negative grounding on DC systems, probably the most well-known one being the electrolytic action which can be set up, resulting in the erosion of conductive materials.
That's one reason why almost every telephone system in the world works with a DC supply which has the positive pole of the battery grounded.
When it comes to supplies for feeding certain electronic equipment, there is also the question of whether the primary devices used in the equipment require a supply line positive or negative with respect to earth.
From an historical viewpoint, valve/vacuum tubes operate with a positive supply line, and thus have a negative-ground supply, whereas much early transistor equipment used PNP transistors which required a negative line and thus a supply with its positive side grounded.
I believe that pauluk gave the primary reasons for grounding the positive terminal on a DC system.
With respect to electronic equipment on DC supplies, I would like to expand a bit. Clear the supply to the devices requires both positive and negative, but especially with respect to 'three terminal devices' (vacuum tubes, transistors, etc) you see a useful asymmetry: The control signal to the device (gate drive, base drive, etc) is referenced to one of the terminals. If you ground that terminal, then the voltages associated with the gate drive are greatly reduced. For example, an N channel MOSFET is controlled by a gate voltage relative to the source terminal of a few volts, even for MOSFETS (or IGBTs) rated for drain to source voltages of thousands of volts. In circuits that use these devices, it generally makes sense to ground the source terminal. (Note that many interesting circuits use MOSFETS in ways where at least some devices can not be grounded.) Some devices connect the source lead to the metal body of the device; if the source lead is at ground potential, then you can ground your heat sinks, a very attractive design feature
I believe that you left out one additional sort of grounding used in DC systems, at least in small signal circuits, and in large VSD DC links: neutral grounding, where you have +Ve and -Ve, and ground '0' in between the two. Common power supplies for things like op-amps are +-15V, and in most VSD systems you have a 'ungrounded' DC bus supplied by rectifying the supply AC...but the supply system is grounded, so the net result is a DC bus which is balanced about ground potential.
Good point about a balanced DC supply with grounded neutral Jon. The op-amp is an excellent example.
There are plenty of other electronic applications of DC with positive and negative (with respect to ground) supplies of vastly different levels.
In high-power tube transmitters, for example, it's quite common to need a supply with a positive (B+) line of hundreds or even thousands of volts, while at the same time you need a supply which is negative with respect to ground (C-) of maybe 30 to 100V (used for grid bias).
In DC power systems there are also examples of some strange unbalanced-about-ground systems.
The London Underground (subway system), for example, has a 630V DC supply using a four-rail system (two running rails, plus two completely separate and isolated conductor rails). Apparently they have the supply arranged so that the outer rail is at about +420V to ground and the central rail at -210V.
I can answer your question in more depth if you are still interested.
As Paul pointed out it has to do with electrolytic action. In the early days of telephone the outside plant cables were lead sheathed and referenced to the battery positive terminal (-48 VDC systems). This kept the lead from erroding and leaching into the soil.
On the other hand radio transmission (microwave/sat) equipment uses +24 VDC with the negative terminal referenced to ground.
These two schemes exist today. You are correct neither polarity has to be referenced to ground, but it is still done as to facilitate simple and econnomical overcurrent protection systems. Otherwise you would need to use ground fault protection/detection, and use shunt type breakers on each polarity in primary and secondary circuits for protection. Hope that helps.