With a basic understanding of today's UPS technology for online UPS and offline UPS under our belts, let’s look at some common situations and how each backup power source handles them. 

In most cases, an online uninterruptible power system UPS offers many advantages over its less expensive cousin when more sophisticated power supply problems exist.

In our examples we’ll assume that the incoming line voltage is nominally 120V - but the scenarios could be presented for any line voltage, including 220, 240, 480, etc. 

UPS Emergency Power Supply Solutions For...

Power Outages

A power outage is perhaps the most common reason why companies install UPS systems - they want to be protected in case of an emergency.  That is why Fonar - a maker of the world’s first stand-up MRI - uses a UPS for their patient table. 

So what happens when the incoming line voltage drops?  With an offline uninterruptible power supply UPS the batteries switch on, providing power to the protected load. The time provided by the batteries is dependent upon such factors as how many batteries are available (or how the UPS is sized relative to the protected load), and the condition of the batteries. 

In the Fonar example, for instance, if there was a power outage while a patient was undergoing an MRI, there is sufficient power to retract the table from the enclosed MRI area.  Without this measure in place, the patient might be stuck inside the tubular area - which can add even more stress to this uncertain situation.

Under-Voltage Sags

While power outages are the reason that most businesses install UPS systems, they are perhaps the least occurring incidents.  A far more common instance is “sag” - so named because the incoming voltage sags below normal range.  Unlike the outage, there is still voltage to the system - but it is below normal, acceptable levels. 

With an offline UPS, a sag can also cause the batteries to switch on, depending upon the severity of the sag.  If incoming live voltage is 120V, and a sag occurs which causes the line interactive UPS to switch to batteries, the power output is not the 120V that the system is designed for, but usually a lesser figure - maybe 100V.  While this keeps the downstream equipment powered up, it may not be performing optimally.  Depending upon the protected load, this voltage shortage may be enough to cause irregularities such as dropped or corrupted data, overheated transformers and other maladies. 

Keep in mind that typical utility power is supplied at plus or minus 10% of nominal - which means that for a 120V line a variation of 108V to 132V is within standards.  If variations in voltage are frequent, the line interactive UPS may be reverting to battery power often - resulting in premature battery failure.

The same scenario with an online uninterruptible power supply (aka  double conversion UPS) would yield different results.  Because this system is creating a clean, stable sine wave continuously, it can handle sags without even going to battery.  Note that when subjected to the same voltage fluctuation which caused the line interactive UPS to switch to battery power, the online UPS is able to produce a steady 120V of power output.

Even locations which have fairly consistent power are subject to sags due to local usage patterns, for instance, in a manufacturing facility where equipment is powered up all at once.  Because the demand has gone from near zero to near max, there is a temporary sag in the utility voltage until the supply can catch up to the new demand.  Similar instances occur in office buildings where most employees start work (and fire up their computers, lights, etc.) at the same time.

Over-Voltage Swells

On the opposite side of the spectrum from low voltage situations, or sags, are high voltage situations or “swells” (so named because the voltage “swells” above normal levels).  Recall that typical utility voltage is supplied at +/- 10% - meaning that incoming line voltage can be as much as 10% higher than anticipated.  So 120V line could really be sending 132V through to the UPS.  Whether this happens with regularity or only infrequently, this situation is hard to identify because, like sags, swells present few symptoms. 

While swells occur for various reasons, one common occurrence is a sudden drop in demand for power - such as at the end of the day when a lot of computers, lights, and production equipment are being powered down for the day. 

Swells are problematic for line interactive uninterruptible power supply (UPS) technology because there is nothing within the technology to combat this situation.  Therefore, increased voltage is merely passed through the UPS and into the protected load.  What makes this most precarious is that there are few symptoms presented - other than shortened service life of the equipment, and potentially blown fuse and tripped circuit breakers. 

Online UPS technology, however, can accept this situation without difficulty because of the double conversion process.  Recall that regardless of the input voltage, the UPS is generating a clean, steady, stable sine wave - allowing it to pass through 120V power to the protected load.

Harmonic Power Quality

Harmonic power quality is another variable which is worth considering when discussing the merits and drawbacks of UPS technology.  Harmonics are very prevalent and can be found in locations from manufacturing sites to accounting firms. 

Harmonics is a term used to explain currents and voltages that have multiplied within an electrical system.  A harmonic spectrum can exist from the 3rd through the Nth order.  A harmonic order is a specific measurable amplitude existing within this spectrum.  

The presence of harmonics can cause serious distortion of the sine wave, which shows both a normal sine wave, a 5th order harmonic wave and a sine wave distorted by the presence of 5th order harmonics.

Common causes of harmonics include:

• adjustable speed drives
• variable frequency drives
• electronic welding equipment
• transformers and generators
• medical imaging equipment
• lighting controls & dimmers
• computers
• copiers and scanners, and more. 

These may cause problems such as interference with telephones and communications systems, overheated conductors, bus bars and switch-gear, tripped or arcing circuit breakers, and overheated motors.

Harmonics can also cause wires to become hot (leading to its premature failure), insulation to break down and reduced equipment life. 

So what do harmonics have to do with UPS technology?  The answer might be “plenty” depending upon what type of UPS is selected. 

Offline UPS technology has no mitigating effect on harmonics.  If harmonics are present on the incoming voltage, they will be passed through in the battery-boosted voltage output - and their detrimental effects will also be passed along.

Online UPS technology creates a clean, stable sine wave - this technology does have a mitigating effect on harmonics.  If there are harmonics present on the incoming voltage, this will removed from the output voltage - saving downstream equipment from the harmful effects. 

Summary - Don't Buy Solely on Price

There are countless industries and applications which may benefit from the protection that a UPS provides. 

While cost is surely a factor - the initial purchase price advantage enjoyed by line-interactive UPS technology may be offset with reduced equipment life and other downstream equipment costs which may not be readily apparent. 

In short, the lowest initial cost approach may not yield the greatest economy in the long term. 

There are many applications which can suffice very well with the less expensive offline, single-conversion UPS technology (line-interactive variety), but there are also applications which would benefit (operationally and economically) from the use of online, double-conversion UPS technology.

One key to making a good decision is gathering pertinent information and weighing appropriate options.  One size does not fit all.  But a sound decision can provide years of positive returns on the initial investment.