Circuit Tripping

It sounds like phase angle crossing where the transfer is taking place so fast that the sources are out of phase, you can either slow down the transfer and let the load decay or use an in-phase monitor to prevent out of phase transferring. If you go with an in-phase monitor be sure it has a dead bus feature or it won't operate on a power failure. Its not a big problem.

dg set nuteral need to conect earth

check governors...and go with no.2...slow down your load transferring..

Check the DG set panel program. Cheek the earthling. Check the load distribution .

check the earthing of the your DG system and recheck the earth fault relay settings

It may be out of phase as said by norton850, as RCCB tripping means there is large out of phase you have to slow down the transfer rate considerably to avoid such a situation, also check the Dg earthing if the earthing is having a leak some where. if you are having an auto changeover system, do the changeover manually and check the voltage in each phase and then allow the changeover, by doing this way we can examine the exact areas as where is the real problem of the system.

your synchronising panel also to be checked for exact settings.

first, Make sure that all the DG alternators have their neutrals to earth, separately (different eart pits...)

2nd, Check that in your wiring system, none of the neutral wires is leaking to earth (damaged insulation and contacting metal bodies that will be earthed...).

3rd, If you have a lot of surge suppressors connected in your circuitry on electronic dc supplies, some have capacitors connected to the earth and either the neutral or phases. If discharged, when energising, they draw a substantial current to ground and may trip the RCCB. All depends on the ratings and the current surge.

I did check the earthing of neutral of our main Dg ie 125 KVA one.As such there are no loose connections.But when i measured the earth resistance value, it came to a very high one ie 12.5. Can this be a major cause for the tripping of RCCB? Thanks in advance

If this is "On-load" changeover, then there could be out-of-balance currents due to motor inrush currents.