This paper reviews some of the conditions for electron MHD, and shows how this leads to anomalous resistivity, which plays an important role in the behaviour of plasma opening switches (POS are switches, based on the modification of the global resistance of a plasma flow, used to shorten the characteristic time of pulse power machines and increase their performances). Anomalous resistivity is build up from a transfer of momentum between the electrons and ion acoustic waves, which in turn modify the momentum of ions. One of the main consequences of EMHD is the diffusion of B in the plasma. A rough model envisions this diffusion as a propagation of nonlinear waves that may compete with convective transport.