Transfer Switches
Overview - Automatic, Manual and Static Transfer Switches
Most common application of transfer switches is to transfer to and from standby power sources such as generators. These are almost always automatic transfer switches (ATS). Some critical power systems use a manual transfer switch (often a "Safety Switch") as a UPS catastrophic bypass system. Transfer switches are generally break before make designs which briefly interrupt power even when transferring between live power soutces. Thus these type of transfer switches are usually ahead of UPS suystems. ATS can be designed as open or closed transition. A closed transition is an overlapping transfer which does not interrupt power when both sources are live and synchronized. Due to lethal risks in operation of closed transition switches, their application requires significant design co-ordination.
Static transfer switches (STS) are solid state transfer switches which permit extremely fast transfer between two live power sources. They have no moving parts. The are subcomponents within quality on line UPS designs. Also, stand alone STS are used on the load side of multiple independent UPS systems to allow switching of IT load from one source to another fast enough that IT loads do not drop. STS can transfer within 4 milliseconds which is well within the 16 to 20 millisecond requirement of modern IT equipment. When IT equipment is not dual powered by separate sources STS can be applied to minimize dropping of loads through UPS failure and human interface error at the UPS level. STS range from rack mounted size for small or individual loads through over 1000 amp for large IT loads.
Similar to rack mounted STS but slower are contactor based transfer switches. These are designed with a slower transfer time because physical contacts must operate internally to cause the transfer as opposed to solid state switching in a STS. They are lower cost and often will meet the needs of IT equipment.