At times the customer requires PDUs for special applications. In addition to the four major PDU categories (basic, metered, monitored and switched) Tripp Lite offers a complete line of PDUs for special applications. If the customer wants to hot-swap a UPS system (remove it for maintenance while continuously powering connected devices), they should use a Tripp Lite Hot-Swap PDU. Hot-Swap PDUs feature an external manual bypass switch that allows the entire UPS system to be removed for maintenance while the Hot-Swap PDU remains installed, continuing to power connected equipment as long as utility power is present. If the customer wants to provide two redundant power sources for a device with a single power cord, then a Tripp Lite ATS PDU is the choice. ATS PDUs have two input cords (which can be plugged into two separate power sources). If the power fails on the primary input, the PDU automatically switches over to the secondary input. If the customer wants to provide two redundant power sources for devices with two power cords, they should use the Tripp Lite Dual-Circuit PDUs. Dual-Circuit PDU models feature two separate PDUs in a single housing.