Sometimes on a larger electrified railway, tramway or trolleybus system, it is necessary to power different areas of track from different power grids, without guaranteeing synchronisation of the phases. Long lines may be connected to the country's national grid at various points and different phases. (Sometimes the sections are powered with different voltages or frequencies.) The grids may be synchronised on a normal basis, but events may interrupt synchronisation. This is not a problem for DC systems. AC systems have a particular safety implication in that the railway electrification system would act as a "Backdoor" connection between different parts, resulting in, amongst other things, a section of the grid de-energised for maintenance being re-energised from the railway substation creating danger.