If synchronization doesn’t occur within that time span, some transfer switches have the ability to default automatically to a delayed transition that serves as a failsafe. In-phase transitions are typically completed in 150 milliseconds or less to ensure that inrush current is equal to or less than the normal starting current of the inductive load(s). Unless some type of stored energy system, such as an uninterruptible power supply (UPS), is located downstream of the transfer switch, loads will experience a brief interruption in power during the transition delay period. A transfer between power sources can be initiated automatically or manually.Operation is independent of electrical synchronization between both sources of power.Inrush current can occur when an inductive load is rapidly reconnected to a non-synchronized power source. It can be an issue in applications where a residual voltage is briefly maintained at the load due to the generator effect created by a rotating motor or by the stored energy released from a transformer’s windings or core. Building a delay into the transition process can prevent higher than normal electrical current ( also known as “inrush current”) from developing.That delay typically lasts either a specific, pre-set amount of time or however long it takes the load voltage to drop below a prespecified level. In an open delayed transition, the transfer switch pauses in-between disconnecting from one power source and connecting to the other. For some period of time between disconnection and connection, neither the normal power source nor the emergency source is providing electricity to downstream loads. That is, the transfer switch breaks its connection to one power source before making a connection to the other. Some of the standard transfer switch diagrams (left: Transfer between 2 sources – 1 busbar right: Transfer between 2 sources – 2 busbars where sources are usually 1 transformer and 1 genset, and loads split between critical and non-critical)Īn open transition is a “break before make” transfer. The specific functions performed by a given load and the importance of those functions to safety or security play an important role in determining which kind of transition is required. Transfer switches can transition loads between normal and emergency power sources in two basic ways:
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