Transfer switches can generally be classified as manual, automatic, high-speed mechanical and static. Manual transfer switches require a human operator. As such, switching will generally require a minimum of several minutes to accomplish. These devices are relatively inexpensive, but may not provide adequate reliability to loads and processes vulnerable to extended interruptions. Automatic transfer switches have the ability to immediately sense loss of voltage on the primary feeder and switch over to the secondary feeder in a matter of seconds. Critical loads are often connected to more than one source through the use of a transfer switch. The load is normally connected to a primary feeder and is transferred to a secondary feeder if the primary feeder experiences an outage. Typically, transfer switches consist of two interlocked switches that prevent both switches from being closed at the same time.