Transfer Switch - Basic Properties, Terminology and Theory
What is a Transfer Switch?
Transfer Switch is a critical component of any standby power system. When the normal (preferred) source of power is lost, the transfer switch transfers from the normal source of power to the emergency (alternate) source of power. This permits critical loads to continue running or begin running again, once the transfer is made. After the normal source of power is once again available, the transfer switch transfers back to the normal source from the emergency source.
Operation of the transfer switch from normal to emergency and back to normal can be manual operation of automatic. It depends on the type or configuration of the transfer switch equipment. If loads are very critical or no operator is on site to make a manual transfer, then an automatic transfer switch (ATS) is used to ensure the fastest possible transfer.
There are four basic types of transfer switches: Non-Automatic Transfer Switch (Manual), Automatic Transfer Switch (ATS), Bypass Isolation, and Basic Transfer Switch. Each of these devices performs a well-defined function. All transfer switches do not necessarily perform these basic functions in the same manner.
Non-Automatic (Manual) Transfer Switch
A non-automatic transfer switch (manually operated) consists of Power switching device to shift the load circuits to and from the source of power and a manual operating handle. Manually operated devices (Non-Automatic Transfer Switch) can transfer power. This switch is ideal if operating personnel are readily available and the load is not of a critical nature requiring immediate restoration of power. An example of this is a refrigeration plant. While the plant could not operate in an extended power outage, a brief downtime while power is transferred manually could be tolerated.
There are two types of non-automatic transfer switches:
- Manually Operated
- Manually Initiated, Electrically Operated
Manually operated non-automatic transfer switches provide all the mechanics to effect the transfer from source to source. The actual transfer of power, however, is accomplished by true hand operation of the transfer switch mechanism.
Manually initiated, electrically operated non-automatic transfer switches are similar to the manually operated version except that an electrical operation feature is added to the switch. Power source monitoring and automatic transfer control are not included. The switch electrically transfers power when a pushbutton, usually mounted on the switch's enclosure, is pushed. If necessary, the switch can be operated manually.
Automatic Transfer Switch (ATS)
There are two types of non-automatic transfer switches:
- Power switching device to shift the load circuits to and from the source of power
- Transfer logic controller to constantly monitor the condition of the power sources and provide the control signals to the power switching device
- Control power source to supply operational power
The automatic transfer switch includes controls (logic) to detect when a power failure occurs, and triggers other controls (logic) to start the engine when the emergency power source is an engine generator. When the generator reaches the proper voltage and frequency, the switch transfers load circuits from the normal source to the emergency source. When the normal source is once again ready to supply power, the switch retransfers the load circuits to the normal source. It also triggers controls (logic) to shut down the engine generator. The standard complete operation handled by the automatic transfer switch each time there is a power failure and power restoration is:
- Engine starting
- Transfer to generator
- Retransfer to normal
- Engine shutdown
Bypass Isolation Switch
A bypass isolation switch is actually a manually operated non-automatic transfer switch in parallel with an electrically operated automatic transfer switch. The bypass isolation switch is also referred to as a maintenance bypass switch. They both operate perform a similar isolation function yet; these switches were designed to perform unique functions.
The isolation of the automatic transfer switch is permitted for periodic maintenance or repair while power to the load from either power source is maintained. Because the bypass switch must carry the same load as the automatic transfer switch, they must have the same ratings and with the same withstand and interrupting ratings.
This type of equipment is often required in critical life support systems and standby power situations that need safe system maintenance without a power disruption.
Basic Transfer Switch
A basic transfer switch is designed for use with customer furnished controls. It is similar is design to the automatic transfer switch except intelligence and supervisory circuits are omitted. This type of switch is not necessarily available from all manufacturers. It is intended for use by customers with the expertise to complete the design, and with a desire for additional value added.
Transfer switches range from 16-100 amps, with 600VAC maximum. Work with a transfer switch specialist to determine the best product for your application.