In this multipart series we will cover surge and transient protection for all types of signal, control and power lines. My goal is to help the reader understand the basic principles and be able to assess the capabilities of the types of protection available on the market today. One of the most important skills you should learn from this series is the ability to ask the right questions and evaluate the answers received. In part 2 of this series we looked at the nature of surges and their impact on equipment and systems. In this and the next several segments we will look at the basic operation of surge arresters and TVSSs and the individual components they employ.

Basically, all surge and transient protection in use today is one of two types. These two types of devices are called clamps and crowbars. Clamps limit the voltage across the device and dissipate some of the surge energy in the process. Clamps include metal oxide varistors (MOVs) and silicon avalanche diodes (SADs). SADs are also called transient voltage suppressors (TVS) by some manufacturers. Crowbars shunt the vast majority of the surge current to either the grounded (neutral) or grounding conductors for dissipation in the earth. Crowbars are also frequently used in telephone protection circuitry as either three element gas tubes or carbon blocks. Crowbars include gas tubes, spark gaps, thyristors, and carbon blocks.

We will look first at the clamp type devices beginning with MOVs. Like other forms of voltage clamps there is a significant temperature rise when the MOV is conducting. Because the duration of transients and surges is, as we discussed in segment 2, very short we can safely dissipate the energy of a rather large current pulse without damaging the clamping device. The amount of power that these devices can handle is dependent on the volume of the device and construction. MOVs can generally be expected to handle larger currents than SADs.

MOVs are commonly used protection devices for power circuits. MOV material is made up of zinc oxide and other materials in granular form. This material is compressed and heated (sintered) to form a ceramic cylinder in one of several different diameters. These cylinders are sliced into discs. The thickness of the disc determines the voltage that may be applied before the clamping action will begin. The volume of the disc will determine the energy that the disc can dissipate. The ceramic material will have a multitude of boundaries between the grains. It is at these points where the nonlinear voltage-current relationship occurs. A metallic coating is applied to both sides of the disc to improve the distribution of currents through the disc and minimize hot spots.

As voltage across the MOV terminals increases beyond a threshold the device begins to conduct more rapidly. Ohm's law states that resistance is equal to voltage divided by current thus as the current increases through the device and voltage across the device remains the same the resistance of the device must decrease. The power dissipated in the device is equal to the current times the voltage. It is this nonlinear action that limits the peak voltage from terminal to terminal. This action initiates very rapidly on the order of at most a few nanoseconds. The little time delay present is due to junction propagation delay and is determined by the number of intergranular boundaries that must begin to conduct.

In our next segment we will talk about some of the important and not so important specs of MOVs and how they relate to SADs.

Ed Roberts

ed.roberts@efrobertsassoc.com

Lightning and Transient Protection, Grounding,
Bonding and Shielding Education
www.efrobertsassoc.com
Copyright © 2005 by E. F. Roberts and Assoc.

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