UPS 101 - Knowledgebase
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What is a Manual Hard Bypass Switch and why are they necessary?
UPS installations often include a manual wrap-around hard bypass switch, also referred to as maintenance
bypass switch, or MBS. The manual bypass switch allows the UPS to be de-energized without interrupting
power to the critical load. The manual bypass switch provides a safe easy means for a trained service
technician to perform routine maintenance, inspections and repairs to the UPS. This is done by manually
transferring the load to the bypass power source using a make-before-break switch or breakers. Once the
UPS is successfully serviced, the load can be safely transferred back without any power interruption. All
manual hard bypass switches must be operated in the correct sequence or loss of load and damage to the
UPS can result.
What are the differences between an internal and external manual bypass switch?
Internal Manual Bypass Switch: The manually operated make-before-break bypass switch is located within
the main UPS cabinet. Typically the switch is a two position rotary switch with a “NORMAL” and “BYPASS”
switch position. Rotating the switch from normal to bypass will connect the load directly to the bypass power
source allowing the UPS to be shutdown for maintenance. Once maintenance is completed the bypass
switch is rotated back to the normal position reconnecting the critical load to the conditioned UPS power
Pros: The UPS can be easily bypassed and shut off for servicing by following the correct sequence.
Cons: Since the bypass switch is located inside the UPS cabinet, AC power is still present on some
components within the UPS. The UPS is not totally isolated from the AC power source and
cannot not be physically removed or replaced without interrupting power to the load.
External Manual Bypass Switch: The manually operated make-before-break bypass switch is located in a
separate enclosure from the UPS. Commonly, the separate enclosure is wall mounted or in a matching
sidecar cabinet to the UPS. Similar to the internal bypass switch, the UPS load can be seamlessly
transferred between the UPS and bypass power source. The External manual bypass switch totally isolates
the UPS from the AC power source. This allows the UPS to be de-energized for service and even physically
removed or reinstalled without interrupting power to the critical load. External rack mount manual bypass
switches are typically used on UPS and Inverters that are designed to also be mounted in racks.
Pros: Not only can the UPS be completely de-energized, but the UPS can also be physically removed for servicing. All
internal components are de-energized. This is the most desirable bypass configuration for service accessibility and safety.
Cons: Bypass Switch must be mounted externally and may require more floor or wall space.
Manual Bypass Switches