Introduction

In dual‑supply power designs, the device that decides which source feeds the load is as critical as the UPS itself. Static transfer switches and automatic transfer switches solve the same problem with different technologies and response times. This page explains how to choose and deploy them safely.

What a static transfer switch does

An STS uses solid‑state devices to select between two synchronized AC sources and transfer the load between them within a very short window. Because it is electronic, it does not rely on mechanical movement and can react quickly under normal conditions.

STS vs ATS, practically

An ATS uses electromechanical contacts to move the load from Source A to Source B. It is robust and widely used but inherently slower. STS is preferred for sensitive IT and electronic loads that benefit from fast source transfers within a few milliseconds when sources are healthy and synchronized. In mixed or harsh conditions, an ATS may be the correct, simpler option.

Where STS fits in a UPS design

In A‑B power architectures for racks, labs, and small rooms, an STS sits downstream of two conditioned sources—often two online UPS or a UPS and a conditioned bypass—and feeds single‑corded loads. Dual‑corded loads typically do not need an STS; they can take A and B feeds directly.

Sizing and installation notes

Size the STS in kVA/kW to match your peak load with adequate headroom. Consider inrush currents of attached equipment and the available fault levels. Coordinate protection, earthing, and cabling. Keep source synchronization and phase rotation correct to minimize nuisance transfers or blocked transfers.

Safety and commissioning

Provide a maintenance bypass path for safe service. Verify earthing continuity, transfer logic, and alarms. Document a commissioning procedure: source checks, functional transfer tests under load, and rollback steps. Train operators on normal and emergency use.

FAQs

Is an STS the same as a maintenance bypass? No. A maintenance bypass allows manual isolation of UPS for service; an STS automatically selects between two sources for the load.

Will an STS always transfer without interruption? Transfers rely on source health and synchronization. Under faults or out‑of‑tolerance conditions, a transfer may be inhibited to protect the load.

Can I feed an STS from a generator? Yes, but ensure proper frequency and voltage stability and follow the vendor’s limits for synchronization windows.

Do I still need dual UPS if I install an STS? For true A‑B redundancy, each source should be independently capable. An STS cannot compensate for insufficient capacity upstream.