1. A method performed by data processing apparatus, the method comprising: receiving, by a data processing apparatus, signal characteristic data that specify signal characteristic values for signals that are received over each of a plurality of communications channels of a power line communications network; determining, by the data processing apparatus, that the signal characteristic values for the signals that are received over at least one of the communications channels are outside of a baseline signal value range; identifying an endpoint that communicates over the at least one communications channel; determining that a set of the signal characteristic values matches one of a plurality a grid event signatures for the identified endpoint, each of the grid event signatures being indicative of a particular grid event corresponding to an impedance change; and providing data that identify the endpoint and the particular grid event for the grid event signature that is matched by the set of monitored signal characteristics.