What are the Components of a UPS?

Ever wondered what are the main components of a UPS system?

Here Power Continuity engineers explain the UPS components.


This part of the UPS converts the incoming alternating current (AC) supply to direct current (DC).
Provides charging current for the battery and the inverter.


In a number of UPS systems, separate battery chargers are used due to the nature of their design.
This is common on large UPS systems and is less common on small sub 3kVA units.


This is the electrical power storage unit, mostly VRLA batteries.
Keep all batteries charged. By the rectifier or a separate charger.
Storage capacity= AH (ampere hour).
That’s the number of amperes of current that the battery can supply for an hour.
Autonomy is the length of time in minutes of available electrical power storage ready for use.
Simply explained is the length of time the VRLA battery/batteries will support the UPS load.
There will be a number of limiting factors depending upon the application.
The batteries can be discharged at a lower level. (should the design call for a shorter autonomy)
In the case of emergency lighting, where there is a requirement for a long autonomy.
These conditions help to allow the maximum design life of the battery.


This is part of the Uninterruptible power supply provides an AC output to the load.
Typically the UPS inverter is always in phase with the input mains supply.
Due to the number of conversions from AC to DC and back to AC, this filtering is termed as a ‘clean supply’.

UPS component capacitors

Static Switch : this circuit is a relay.

More commonly achieved by using thyristors, a key UPS component.
Importantly it’s purpose is to switch the load between the inverter and the utility mains supply.
At all times, the output of the inverter is always in phase with the utility mains supply.
Therefore any switching between the two will be virtually seamless.
This arrangement ensures that if the inverter experiences an overload situation, circuits it will transfer the load to the more resilient mains supply.


A typical example:
An inrush current occurs when the server rack is switched on.
Depending upon the size of your uninterruptible power supplies, the load transfers to the utility mains supply.
Once switched on the load will transfer back to the inverter providing the inverter has sufficient capacity to support the load.
Any 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.

External Maintenance UPS Bypass

Wall mounted or wrap around bypass.
External maintenance bypass allows the UPS to be removed/replaced without interruption to the load.
As well as, if the actual maintenance bypass is fed from a separate supply, it can allow UPS load testing.
Typically engaged during a repair and/or checking of the autonomy under simulated load conditions.
Ensure site load is supported. By the external maintenance bypass circuit.
More commonly, this is used when allowing the UPS System to be bypassed onto standby diesel generator power.
Alternatively, when batteries are replaced and removed, can only be done by placing the uninterruptible power supply into external bypass.

UPS Maintenance Bypass

More typically found in small power UPS systems.
This arrangement allows the load to be transferred under controlled conditions to the utility mains. The UPS to be shut down without loss.
Normally carried out for routine UPS maintenance or UPS repair.

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