The unit number four of Tarbela power station has been tripped causing a deficit of 175 megawatts in Wapda power grid, sources said. According to Wapda sources Unit-IV of Tarbela power station has been closed due to technical fault causing a shortfall of 175 megawatts in power generation. According to Wapda sources emergency has been declared at the power station and repair work of the unit has been started. The nature of the fault yet to be traced.