Can a UPS Effectively Do an SPD's Job?

January 1, 2011

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 straight forward. A conditioner interacts with the service power trying to maintain stable current and voltage and often has 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 Uninterruptible Power Source systems (UPS) are anything but uninterruptable.

Until a few years ago most UPS systems were virtually-uninterruptable, ferroresonant transformers that convert 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 and therefore provides excellent protection, isolation from harsh electrical environments, as well as, battery backup.

While most manufacturers claim to include surge protection into these newer versions of UPS systems, it is often insufficient for critical loads. Mostly located on the input power, these surge protectors are also unable to effectively capture switching transients generated by the systems internal circuitry. While these systems are usually sufficiently powerful, lighter, smaller, and cheaper than ferroresonant UPS systems, 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, 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. Already 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, 87% are sags below 96 volts! In addition two separate IBM studies indicate that the average business experiences over 50 significant surges per month. These numbers are only poised to 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 back up battery system, or ferroresonant isolation if needed, as well as quality voltage limiting surge protection devices because 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
Global Business Manager for Smiths Protection Technology Group
Certified Communications Site Inspector – Contributor to Motorola R56 1995, 2005 and 2010

Gene McCurry
Reid this is a GREAT ARTICLE! You hit the nail right on the head with this....GREAT JOB Thanks for passing it along
1/11/2011 10:04:24 AM

Mark H. Viness

Very well stated. R56 calls for separate and distinct surge suppression. Any Engineer that believes his scheduled UPS Unit will protect his customer's expensive elctronic gear, whatever it might be, is doing a disservice to all of us in my opinion! Thanks for your article and please keep up the good work.

Best Regards,

Mark H. Viness
Motorola Solutions-National Site Design Team
1/11/2011 9:47:21 AM

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