A power source does not only supply a single load. Connecting to several subsystems and several loads is common. When one of the loads is increased or the device is applied, the bus voltage properly drops because of the sudden increasing current. Especially for loads such as motors, the voltage drop is more obvious. Generally, the current DC-DCs can cope with the input deviation of voltage drop. However, if a supply interruption situation occurs, all connected devices would shut down. The standard of EN 50155 – Railway Applications – Rolling Stock – Electronic Equipment describes two conditions: supply interruption, and change-over, in which the input voltage drops to zero for 10ms to 30ms. The interruption of the input power source will make any DC/DCs stop operating temporarily even for a short period of 10ms to 30ms, and all connected loads will lose their functions, then the equipment will face several seconds to several minutes to restart. The process of shutdown and restart may be hazardous for a moving train. Therefore, there are also clear regulations in EN 50155 that supply interruption and change-over class of S2, S3, and C1 must comply with criteria A; C2 must comply with criteria B. (Criteria A means that while the condition occurs, the device still maintains its function. Criteria B means that temporary performance degradation is allowed).
