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For many years, some Manufacturers have incorporated discrete devices into their circuit and board designs in an attempt to mitigate the effects of electrical power disturbances of various kinds. Some of those devices include:

  • Zener diodes – A semi-conductor that becomes conductive and begins to act like shunt resistor when circuit voltage approaches a design threshold. Zener diodes are often applied at a value around twice the nominal circuit operating voltage. They typically fail when operated above their design current, which can be quite small or quite large. They may fail open or shorted.
  • Spark-gap capacitors – These devices are usually employed at terminal attachment points. They absorb transients up to a set point and then discharge across a fixed gap to ground. These devices can fail open or shorted.
  • MOV [Metal Oxide Varistor] – These are resistors that are analogous to Zener diodes in their mode of operation in that they become conductive above a certain voltage threshold and typically shunt excess voltages to ground. These devices often fail shorted, creating ground faults in systems that supervise for grounded conductors.
  • Transorbs – These are semi-conductor devices that operate to shunt excess current when the applied voltage exceeds their avalanche breakdown potential. They are clamping devices that suppress over-voltages above their breakdown voltage, and automatically restore, if not destroyed by the transient. Transorbs operate significantly faster than gas tubes or MOVs.
  • Ferreroresonant [aka constant voltage] transformers – These large, relatively heavy, expensive transformers have long been used in mission-critical power supplies, high-end UPS systems, and Line Conditioners. Ferroresonance is the property of a transformer design in which the transformer contains two separate magnetic paths with limited coupling between them. The output contains a parallel resonant tank circuit [third set of windings and large capacitors] and draws power from the primary to replace the power delivered to the load. “Resonance” in ferroresonance is similar to that in linear circuits with series or parallel inductors and capacitors, where the impedance peaks at a particular frequency. In a non-linear circuit, such as a ferroresonant transformer, “resonance” is used to reduce the changes in supply voltage and provide more constant output voltage. A magnetic device is non-linear. Its reluctance changes abruptly above a certain magnetic flux density. At this point, the magnetic device is defined as being in saturation. The design of the ferroresonant transformer allows one magnetic path to be in saturation, while the other is not. As a result, further change in the primary voltage will not translate into changes in the saturated, or secondary voltage, and voltage regulation results.

    Ferroresonant-based power supplies and power conditioners are designed to protect or isolate AC load side equipment from all disruptive and destructive AC input related power problems [other than the complete loss of AC power]. They can tightly regulate output voltage, they provide superior noise attenuation, and are generally rugged enough to withstand the harshest electrical environments. Because they lack moving parts, ferroresonant transformers and power conditioners are virtually maintenance free. Although these devices are typically able to attenuate voltage surges at around a 2000:1 ratio, larger units tend to produce a good deal of heat and an audible hum that some find distracting or intrusive. Typical efficiency is in the 85%+ range when operated at or near full load. Interestingly, when their output is shorted, the output voltage drops to zero, which effectively makes them inherently current limiting. They are restricted to use in single phase applications, and they cannot correct frequency fluctuations. When wired appropriately, ferreroresonant-based UPS and Line Conditioners can qualify under the NEC as separately derived [electrical power supply] systems.

    These devices are not designed or suitable for protection of low voltage input or output circuitry. As noted, their only function is to separate or isolate sensitive AC loads from disturbances on the supply side, a task they perform reliably and extremely well. I have seen such devices still functioning normally 40 or 50 years after initial commissioning.

Many modern TVSS devices employ a series or parallel-wired, “hybrid” approach where several of the above individual components and\or other types of components, are assembled into a packaged unit that permits input and output wiring connection directly to the device. Again, these devices rely on an effective ground to absorb or re-direct charges beyond the internal capacity of the components to “clamp” transients at some desired level. Some contain replaceable fuses, and others do not.

Some of the newer devices available employ an hybrid approach that differs significantly from all previous designs because they do not rely on, or even require a ground connection to function as designed or desired. These devices are filled with a special heat-absorbing epoxy. The device components convert spikes and transients to heat, and dissipate that heat internally. They either fail open, or automatically self-restore. Some have pilot LED indicators for easy diagnosis. It should be noted that the manufacturers of these devices will often replace a TVSS device that failed or was damaged in service, for free, if the old part, or what’s left of it, is returned to them.

Next Time: The Paths to Critical Systems and Equipment that Must Be Protected