Power supply (PSU) is the device that transfers electric power from a source to a load using electronic circuits. A typical application of power supplies is to convert utility's AC input power into regulated DC voltage(s) required for electronic equipment.
Depending on the mode of operation of power semiconductors PSU can be linear or switching (SMPS). What is SMPS? SMPS stands for switch mode power supply. In this supply power handling electronic components are continuously switching on and off with high frequency in order to provide the transfer of electric power via energy storage components (inductors and capacitors). By varying duty cycle, frequency or a relative phase of these transitions average value of output voltage or current is controlled . The frequency range of an SMPS is from 20 kHz to several MHz 

Static Switch : this circuit can be as simple as a relay or more commonly using thyristors, its purpose is to switch the load between the inverter and the utility mains supply. As has been previously mentioned the output of the inverter is in phase with the utility mains supply so that any switching between the two will be virtually seamless. This arrangement ensures that if the inverter experiences an overload situation, due to its extremely fast over-current detection circuits it will transfer the load to the more resilient mains supply. A typical example would be when a server rack is switched on, the inrush current, depending upon the size of your uninterruptible power supplies , could cause the transfer to the utility mains supply, and once switched on the load will transfer back to the inverter providing the inverter has sufficient capacity to support the load. Also a fault on the inverter will cause the load to be transferred, again virtually seamlessly; it would be unusual for any loss of load to occur during these conditions.
 
Maintenance Bypass : more typically found in UPS systems with a capacity of 6kVA or greater. This arrangement allows the load to be transferred under controlled conditions to the utility mains and the UPS to be shut down without loss. Normally carried out for routine UPS maintenance or UPS repair.
 
External Maintenance Bypass : the installation of an external maintenance bypass can allow the UPS to be removed/replaced without interruption to the load, also, if the actual maintenance bypass is fed from a separate supply it can allow load testing in the case of a major UPS repair and/or checking of the autonomy under simulated load conditions while the site load is being supported by the external maintenance bypass circuit. This is often used when allowing the UPS System to be bypassed onto standby diesel generator power. Alternatively, when batteries are replaced and removed, it can only be done by placing the uniterruptible power supply into external bypass.