Transient Voltage Surge Suppression


Surge suppression is something that everybody knows about, but it is very misunderstood.  Everybody seems to know that they should have a plug in surge suppressor on their computer, but the prices are all over the spectrum, and the ratings do not seem to make much sense.

A surge suppressor is a device that is used to divert excess voltage during a momentary surge.  This device should mitigate the effects of many surges that enter your facility.  There are some crucial things to consider though.

Types of TVSS

Surge Suppressors can be of a few different designs, but the most common are MOV (Metal Oxide Varistor)  based or Silicone Avalanche Diode based. You will also see some manufacturers using selenium diodes, or inductors and capacitors in their surge suppression design.  As I said, the two most common are the MOV based and the Silicon Avalanche Diode based. Basically what a surge suppressor does is mitigate the effects of a large tidal wave of voltage coming in on your sinewave as it enters your building.  The surge can be up to 6000 volts or more.

As we explore the different TVSS types available, you will learn that marketing spin is ever present when looking at the TVSS manufacturers.  The one thing I learned is that none of them are 100% right and none of them are 100% wrong.  I will give you the basics to help you decide.


The MOV based TVSS is a good design as long as you know the limitations.  The MOV based unit is affordable and if properly sized and installed, it will do what it was intended to do. The biggest benefit of the MOV based TVSS is that it can handle huge amounts of energy.  The other side of the issue is that the MOV based technology is a bit slow, and lets a fair amount of energy through before it clamps to ground.  MOV's do degrade when they are hit with big surges, so you should be aware that these are considered a wear item.  Especially when dealing with Class A Plug and Play Surge Diverters, which tend to degrade quickly.  Cheap MOV based systems have been known to catch on fire, and many fire inspectors will look closely to make sure the plug in tvss units meet the safety requirements, and that they are properly utilized.

The MOVs are a great design as long as you are using a well engineered system, and you know what you are buying.  Make sure that the TVSS you buy is UL 1449 - 2 certified.  The rev 2 of UL1449 raises the safety standards of the old UL 1449.

Silicon Avalanche Diode Based TVSS

The SAD based TVSS is almost the exact opposite of the MOV based, and the manufacturers both claim that the other is worse.  The SAD units have some great advantages.  The best advantage is the speed of the reaction.  The SAD's are fast and they divert current much quicker than MOV's.  They cannot handle as much energy though, and a large fast surge could destroy some of the diodes.  Because of this the manufacturers tend to use redundant diodes that raises the cost of the units.  A large transient can destroy the unit.  These do tend to be of a more modular design which allows the unit to be repaired easily if it is damaged.

Classification of TVSS

Category A Category A surge suppressors are for use on long branch circuits, or for units that are more than a 30 foot run from category B units.  These are the typical Plug and Play boxes that are seen at many retail locations.
Category B Category B surge suppressors are for distribution panels or short branch circuits that are less than 30 feet from the distribution panel.
Category C Category C surge suppressors are for the service entrance of the building, or for outside applications. 


A properly designed system does not just use one approach.  The best way to handle a surge is using a standard cascade design.  Use a Category C TVSS on your service entrance.  This will knock a 6000 V surge down to a more manageable level as it works into your building.  The residual amount that slipped through is still damaging to equipment, The next step is to put a Category B TVSS in the main distribution panels that feed any circuits that have critical equipment on them.  This will again knock down the 1000 to 2000 Volt surge that could have gotten through the Service Entrance unit. The final step is to use a good Category C TVSS for any sensitive loads that are plugged into walls or hard wired into long branch circuits. 

By following a proper cascade design you will be better prepared than most businesses for any transients that could enter your building.

Caveats Of TVSS Installs

Now that you have an idea of the equipment that you need to really secure your building from surge, you think that you are done.  This is a major mistake that most business owners and managers make.  The big Caveat is that your TVSS will only function as well as your grounding system. 

Most Power Quality issues seen today are the result of building wiring or grounding issues.  Many buildings grounding systems do not even meet the NEC.  We like to suggest that buildings be wired to the IEEE Emerald Book standards which are much tougher than the NEC.

Before you trust your sensitive equipment to your TVSS, be sure to measure your ground resistance, and make sure that your grounding is in good condition, and you have no ground loops or other grounding issues that can cause major problems.


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This site was last updated 02/06/08

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