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ITFrequently Asked Questions · STS
What is a Static Transfer Switch (STS)?
An STS automatically transfers loads from the main source to the backup source when the main one fails, within a few milliseconds, a gap server power supplies ride through without interruption. It has overload and short-circuit protection.
An STS (Static Transfer Switch) is connected to two independent sources, for example two UPS units or a UPS and the mains, and feeds the load from the preferred source. If that source goes out of tolerance, it transfers the load to the second one using electronic power switches (thyristors) instead of mechanical contacts. The transfer is completed within a few milliseconds, a time that server and network equipment power supplies ride through without noticing; that is why in practice it is considered uninterrupted.
The most common use is in server rooms and racks, where devices with a single power supply need to gain the benefits of dual feeds. This way, a fault or maintenance on one UPS does not interrupt operation.
For correct operation, the two sources should have the same voltage and, ideally, be phase-synchronised so that the transfer is smooth. The total load must not exceed the capacity of either source, since each must be able to carry it alone.
A common mistake is feeding both STS inputs from the same line or the same UPS. That throws away exactly the redundancy you wanted to achieve.
