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The Static Transfer Switch is a critical component for increasing resiliency and minimizing downtime in any critical electrical installation. It is frequently used in the data centre environment with 2N configuration for provision of redundancy for devices without dual power supplies. Normally fed from two independent UPSs or a group of UPSs they seamlessly transfer the critical load from one UPS to the other for maintenance purposes or in case of complete loss of one of the supplies. They can economically provide the next level of redundancy for critical installations where 2N configuration is not available and infrastructure changes would be prohibitively expensive and disruptive to the operation of the business.
They are available in different formats from small DIN-rail mounted units, single-phase bypass switches for industrial application, rack-mounted single and 3 phase static switches for telco and data centre applications, to wall-mounted and freestanding units in single and 3 phase for protection of whole departments or complete critical infrastructure.
An STS – static transfer switch has no mechanical switching devices (SCRs used for switching) and on manual command, transfers your critical load at the point of zero current in the AC waveform. As such this is at a point where there is no net-flow of power to the load and is, therefore, the best opportunity to undertake the change-over. In case of loss of the primary source, transfer is done at any point in the waveform to avoid any break of supply to the load. Transfer time is 0ms as long as both sources are synchronized. SCRs are liberally rated with large safety margins and power supplies are redundant to ensure long life and reliable operation.
ATS – Automatic Transfer Switch, claim to do this also, however, these do not protect your load under all conditions. Transfer time is typically less than four milliseconds vs zero milliseconds for Solid-State Transfer Switch. The relay types can cause arching and sparking across the gaps of the contact of the internal relays that are used to undertake the change-over. However, they are typically intended for smaller loads (16-20A) and in principal, switch-mode power supplies in the loads are capable of handling 2-4ms breaks. They also protect the loads from brownouts or overvoltages.