Harmonic State Estimation (HSE) integrates measurements and simulations to fully determine the voltages and current harmonics of the network. However, the measurement uncertainty may affect significantly the results of the HSE. This paper analyses the influence of the typical uncertainty of PQ instruments on the results of HSE for transmission networks. The results show that PQ instruments according to IEC 61000-4-30 class A which can measure the harmonic phase angles referred to an absolute time reference, provide accurate HSE results for those harmonic orders with relevant disturbance levels.