Abstract: A protection system for an electrical power network uses a method of determining the times of capture of current signals obtained by protection devices (3A, 3B) at first and second spaced points along a power line. Measurements are captured at the first device (3A) and sent to the second device (3B), which captures a second measurement and sends a reply to the first device. The timing of both measurements is synchronised using a GPS signal, and the total propagation time tp1+tp2) of the outward and return signals (S1, S2) is stored in a memory. In the event that the GPS signal is lost, the stored propagation time is used to calculate the actual time of the second measurement relative to the first measurement.