In view of the fact you have taken some liberties with the identification of certain information, I find it necessary to make a couple of assumptions in responding to this question.  First of all, I will assume we are discussing a centrifugal pump with a design capability of 2 cubic meters/hour at a Total Dynamic Head of 20 m.

Secondly, by the 'actual head at plant site' I will assume that is the Discharge head on site that the pump is operating against.

With these assumptions it would appear that the pump is operating well out on its performance curve at a capacity well in excess of its design of 2 cubic meters/hour, because the TDH of 8 m. is much lower than the design TDH of 20 m. This means that the pump will draw a much higher amperage than it should, thus causing the motor to trip out.

In other words, there's nothing wrong with the pump, it's the system that is the problem. Try throttling back on the discharge side to bring the flow rate back in line with the original design, and the tripping should stop.