DC Earthing and AC Earthing?

I had one strong doubts Regarding Earthing. I am seen several devices. Earthing is Provided in AC Voltages only. My Doubt is Earthing Factor will comes only in AC Devices or DC Volatage are Required Earthing. If So why we are mention only Positive and Negative Terminals there is No Earthing Terminal. But In AC There will be Phase, Neutral and Earth Terminal?

It really does not matter whether it?s ac or dc, the earth-wire concept is to provide local reference ..
Nowadays, we don?t use dc to transmit electricity and any electrical circuit that uses voltages below 48Vdc ? 24Vac ? or so ? is recognized as safe, so in those low voltage situations there is no real need for an extra earth wire ..
However, when you go above the values mentioned above, the earth reference becomes a legal issue and therefore .. bla .. bla .. bla .. you have to have it ..

IanP

DC (about 800V) is still used to carry energy for some subway trains and some city tramcars. And they are not safe at all if good precautions are not taken.

High voltage DC supplies are also used in railroad cars internal power distribution and in electric cars. They usually have protective earth connections as with AC power, if not provided by the metallic chassis.

Could U explain y DC to AC converter earthing was not availble. the Output of DC to AC converter was 230 V AC. So we required earthing? iF not necessary means could u explain why its not necessary ? I am not seen any DC-AC converter earthing wire? Input -48 V DC and Output 230 V AC. This Procedure is correct?

If correctly designed, the converter provides safe isolation for the AC output similar to a safety transformer. In this case, earthing of the connected equipment isn't necessarily required, if only a single device is powered by the converter. If multiple devices are supplied, their protective earth must be connected to a local earth. It's the same when operating an AC generator.