We present an efficient procedure for the calculation of the characteristic impedance matrix of a lossless multiconductor transmission line with one or more conductors periodically grounded. This matrix can be used to properly simulate power transmission or distribution lines with shield wires periodically grounded and, also, distribution lines with grounded neutral. The presented procedure can be applied to an arbitrary number of ungrounded and grounded conductors. Numerical results are shown for practical cases. Finally, an equivalent circuit of the impedance matrix is derived for the simulation in the time domain.