Can a UPS Effectively Do a SPD’s Job?


AC Edge, SPD

Some time ago while presenting a grounding and protection symposium and explaining the differences between an uninterruptible power supply (UPS) system and a surge protector, an engineer in the back of the room yelled, ”A UPS is not a surge suppressor.”  While I gladly accept help from the audience and generally agree with his statement, some types of UPS systems provide isolation from harsh AC or grounding. However, how effective are they really?
Generally the difference between a surge suppressor and a UPS or conditioner is pretty straightforward. A conditioner interacts with the service power to maintain stable current and voltage, and often features battery back-up to allow time for systems to power down or to be switched to an alternative power source.  A surge suppressor or surge protection device (SPD), on the other hand, is a passive circuit that begins to conduct energy when there is a rapid spike in nominal line voltage.  Such spikes, also referred to as transient overvoltages, are common yet very destructive to switching and logic control circuits.  Hence, the SPD is critically important because many of today’s UPS systems are anything but uninterruptable.
UPS, SPD, AC Edge, ALPUUntil a few years ago most UPS systems were virtually uninterruptable ferroresonant transformers that converted electrical energy from one circuit to another through inductively coupled conductors. Large changes in the input voltage resulted in little or no change in the output. In recent years, however, most modern UPS systems have evolved into systems that afford much less isolation from utility power. Both line interactive and standby types mostly utilize complicated solid state switching schemes, voltage inverters, and battery back-up to manage AC line voltage dropout. One system design attempts to regulate output voltage by turning the onboard inverter on to convert battery DC to AC. Another concept employs utility power to feed the loads until battery back-up is needed, then transfers to batteries through a DC-to-AC inverter until the power is restored or a separately derived power source comes on line.  A third popular design option, commonly referred to as double conversion, takes the incoming AC voltage, converts it to DC to charge the batteries, then converts the DC power back to AC through a second inverter to feed the loads.  Of all previously described designs, however, only the original ferroresonant UPS is truly separately derived from the utility. This design provides excellent protection, isolation from harsh electrical environments, and battery backup.
While most manufacturers claim to include surge protection in these newer versions of UPS systems, it is often insufficient for critical loads.  Mostly located on the input power, these surge protectors are unable to effectively capture switching transients generated by the system’s internal circuitry.  Typically these systems are sufficiently powerful, lighter, smaller, and cheaper than ferroresonant UPS systems, but they are not uninterruptable, separately derived power sources. They can pass-through overvoltage events and even be the source of such. 
Numerous papers and power line quality studies suggest that surges and sags are becoming much more frequent as demand goes up.  Equipment sensitivity is rising, and smaller circuits require less power to do more functions.   IEEE and PFL studies on the effects of lightning on low-voltage distribution show that typical events are between 1500V to 2500V, and can be as high as 6000V.  Several years ago, Bell Labs showed in a two-year study that most locations in the United States experience approximately 25 major power line disturbances annually. Among these, 87% are sags below 96 volts! In addition, two separate IBM studies indicate that the average business experiences more than 50 significant surges per month. These numbers are poised to only go up.
While a ferroresonant UPS is a great power quality product, it does not guarantee effective protection against transient overvoltages.  IGBT and PWM, technologies used in most modern systems, are useful for brown-outs and temporary outages, but not for transient blocking.  An effective power quality plan should include a temporary backup battery system, or ferroresonant isolation if needed, as well as quality voltage limiting surge protection devices. As above-mentioned data suggests, investing in the right product for the right purpose will prove invaluable in the end. Even if an SPD is only called upon one time it will pay dividends.   
Reid Harlocker 
LMR Business Development for Transtector Systems