  We investigate the abrupt breakdown behavior of coupled distribution grids
under load growth. This scenario mimics the ever-increasing customer demand and
the foreseen introduction of energy hubs interconnecting the different energy
vectors. We extend an analytical model of cascading behavior due to line
overloads to the case of interdependent networks and find evidence of first
order transitions due to the long-range nature of the flows. Our results
indicate that the foreseen increase in the couplings between the grids has two
competing effects: on the one hand, it increases the safety region where grids
can operate without withstanding systemic failures; on the other hand, it
increases the possibility of a joint systems' failure.
