While the UPS inverter is unsynchronised from the bypass line it will run to its own clock frequency, usually derived by an internal crystal-oscillator (50/60Hz). During this period of unsynchronisation any transfers between the inverter and bypass will be inhibited by the UPS logic to prevent any damage to UPS components or potential loss of the critical load.

@@comfortablynumb6862 understands that..but recently one of our clients explained a situation, where the operator turned on the manual bypass switch directly and he then switched it off suddenly when he heard an alarm. The power module was found faulty. Is it really possible ?

@@jinusivan3452 It sounds like the manual bypass switch was operated without first transferring the system to static bypass via the static switch. This is unfortunately a common occurrence with operators who are not familiar with UPS systems and yes it can cause significant damage to the UPS. In order to transfer the system to "maintenance bypass" the system must be first transferred to "static bypass". "Static bypass" is where the load is fed via the bypass line through the bypass static switch. Depending on the system this may be initiated by stopping the inverter or commanding a transfer via an HMI, for example. Only at this point may the manual bypass switch be used otherwise you are paralleling the bypass line with the inverter output and bad things happen! I responded to a recent event where an operator had did just that on a large industrial UPS. They lost their critical load and could not return the UPS to normal operation. Fortunately in this instance no damage had occurred to the UPS inverter or static switch components, only the inverter output fuses had blown.

@@comfortablynumb6862 that explanation was so helpful.. Let's say if we want to know why it occurs.. is it because of sync issues between the power supply from the bypass line and inverter output ?

@@jinusivan3452 Do you know what system model you are referring to? There are some commercial systems that do utilise a bypass switch which can solely be actuated for maintenance bypass transfer. These systems inform the UPS control logic via auxiliary contacts that the system has been switched to bypass and at the same time activate the static switch/shut down the inverter. Would be easier to comment if I knew what type of UPS you are referring to.