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UPS Overview

Uninterruptible Power Supplies (UPS) are electronic based power backup systems, capable of providing short term supply of high quality and stabilized electrical power, without interruption to operations.

There are various types of UPS available, the vast majority of which use a battery string to feed an inverter which takes the DC voltage, inverts the voltage to AC, and then manufactures and regulates an output voltage, independent from the input mains.

UPS equipment is very economic for relatively short periods of time which generally allow for sufficient time to ride our any short term disturbances or interruptions, and where required, allow for a proper and orderly shutdown of the connected critical connected equipment.

Where extended backup times are required, this can be provided through over sizing the UPS, adding additional battery packs, or engineering appropriately rated battery strings of sufficient ampere hour capacity, although to support equipment for longer autonomy periods, it can often be more economical to provide a generator, to be used in conjunction with the UPS.

UPS provide 2 main advantages:

• They filter many unwanted electrical disturbances from the mains power supply

• They supply an uninterrupted power to the connected loads in the event that the mains power supply fails.

But also be aware of their disadvantages, as UPS are often given a greater kudos, than what they can actually provide!

It is commonly misunderstood that Uninterruptible Power Supplies provide the highest possible level of surge protection available, to connected critical loads from power surges including lightning transients, where in reality, this is far from reality.

Whilst it is true that some UPS equipment provides a degree of isolation and surge filtering, this is typically limited to internal RF filtering components which are unlikely to attenuate transients that are brought about through lightning activity which are very high dV/dT and dI/dT, and are of a completely different bandwidth to power frequencies

Lightning, for example, is typically between 5 to 50 kHz.

Online UPS are themselves highly susceptible to damage caused by lightning transients, due to the inherent fact that UPS systems are themselves heavily semi conductor based, and are therefore subject to the same damaging and deleterious effects as brought about by lightning transients, as are the critical loads they are often charged with protecting.

UPS systems where never intended to be subjected to the level of transient energy contained in lightning currents, which are only addressed by using specific surge protection componentry such as MOVS followed by L-C wired filter components. Most well respected UPS manufacturers actually state in their product specifications (if at all) a maximum Rectifier input surge rating of ANSI C41.62 Cat B, which is 6kV 1.2/50us: 3 kA @ 8/20us.

Lightning transients regularly exceed these levels.

What is more disturbing is that UPS systems often include an internal static bypass as standard inclusion in their design, which automatically transfers the critical load over to raw mains, in order to protect the UPS in the event of a fault conditions, depleted battery autonomy, or during repair or annual maintenance requirements.

In mission critical environments, this is a major weakness, as the UPS can and will transfer the critical load over to unprotected raw mains, which usually occurs during times of questionable conditions, such a thunderstorms. This clearly is the time of greatest risk to the critical loads, and is exactly the time when the critical loads should have maximum amount protection. By the provision of the bypass, the UPS shows its true colours as a surge protector, by removing all protection from the critical load for no other reason but to protect itself.

UPS systems also fail/transfer to bypass when:

• Mains power is removed periodically or completely, causing battery life to be exhausted, where the unit will then shut down automatically, transferring the load to unprotected raw mains.

• Power system faults may cause the UPS to transfer to bypass, in order to protect itself, thereby transferring critical load to raw mains and removing any protection placing equipment on raw mains..

• Lightning transients often damage the rectifier/charger component, thereby transferring the load to the bypass, further requiring UPS to be put in maintenance bypass, or even completely removed from the circuit for off site repair. No protection is provided to the critical loads in either static or maintenance bypass modes.

It is our recommendation that ALL UPS equipment be powered from a suitably rated Surge Reduction Filter, where the UPS will be offered the protection it requires, and secondly where the UPS fails, or is removed for maintenance, or goes to bypass, the critical loads will subsequently still be protected from mains borne transients.