A transfer switch changes the power source of a load, with automatic switches having logic controls. A power backup system requires a transfer switch, which can be open or closed transition. Soft loading prevents overloading, and a generator transfer switch switches between generator and commercial power.
A transfer switch, which can be an automatic or manual transfer switch, is an electrical switch that changes the power source of a load. Transfer switches are usually electromechanical, although electronic switches also exist. Automatic transfer switches have logic controls that make decisions based on the nature of the power outage.
To demonstrate the transfer switch, a power load such as a laptop charger can be connected to a commercial power source. When commercial power goes out for some reason, a person can plug the charger into a portable generator running on gasoline. The action of transferring the power plug from the commercial power supply to the generator outlet is the transfer, while the person is the automatic transfer agent. The system can be wired using a double throw double pole switch, which can be switched in one position and then in the other to transfer the load from one power source to another. This is the equivalent of a transfer switch.
A power backup system usually requires a transfer switch. When commercial power goes out for some reason, the transfer switch changes to receive power from the generator. The generator takes some time to start up and there is a few minutes delay before a transfer. Other systems have a battery backup where direct current (DC) power is supplied uninterrupted.
Open transition transfer switches open the circuit from the previous power source before connecting to the new power source. The closed transition creates a new circuit before interrupting the other. The choice between open and closed is the short to long term characteristic of the load and the nature of the alternating current (AC) or DC power considered as alternative power sources. For example, an air conditioning unit may be left without power for a few minutes, while a mission critical computer server may not be left without power for any length of time.
Soft loading prevents overloading of the backup power source. Instead of moving the entire load to backup, soft loading can leave parts of the load de-energized. It can also delay the energization of the entire power load.
A generator transfer switch (GTS) usually switches the load between generator output and commercial power. It takes a few minutes for a generator to start up and stabilize its output voltage and frequency, so the GTS relies on logic control circuitry to assert the correct voltage and frequency before switching. Generators can be connected in parallel if there is a way to synchronize the phases of the output voltage.
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