FAQs - Static Transfer Switches

Static transfer switches and how they can be used to provide A and B power sources to critical loads.

Static Transfer Switches typically have an instantaneous transfer time between power sources but with Automatic Transfer Switches the transfer time can be 2-10ms or greater. In a generator installation, critical loads are generally connected to an uninterruptible power supply and battery power us used to cover the transfer time of the ATS device. Most servers can withstand an ATS switch over time if it is less than 10ms due the store electrical capacitance within their switch mode power supplies. Very sensitive loads, including medical apparatus may require the type of break-free supply provided by a static switch.

Yes some STS and ATS devices have a front panel key switch to allow the user to manually switch between power sources. This can be useful during commissioning or a preventative maintenance visit. Where the transfer switch is connect to a remote management system a signal contact or network command may be issued to initiate a transfer.

A 3-pole circuit breaker has a connection for the three wires of a three phase system (phase 1: brown, phase 2: black, phase 3: grey) with a 4-pole having an additional connection for the neutral (blue) of the supply. Old installations may still use red, yellow and blue cable colours.

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