The Types of Surge Suppressors

All surge protectors can be put into two main categories based on the way they operate: shunt mode and Series Mode®. The accompanying diagrams show one example of each type: an MOV (Metal-Oxide Varistor) based surge protector as an example of shunt mode, and SurgeX as an example of Series Mode.


Shunt Mode Protection

The most common type of surge protector at this time is shunt mode. Most of these rely on MOVs connected in such a way as to shunt surge energy to ground. An MOV is a semiconductor device which "turns on" when the voltage across it exceeds a certain value.

MOVs are about the size of a quarter, and cost about a quarter (in high volume). When a surge appears at the terminals of an MOV, its resistance drops dramatically as the voltage across it exceeds its threshold, and it conducts the surge current, diverting the surge to the neutral wire or the ground wire. Shunt mode protection is therefore diversion technology - the surge is diverted from the hot wire to the neutral wire or the ground wire.

MOV-based surge protectors have been in use since the 1970's but have limitations that must be understood if they are to be used reliably. Of prime concern is the fact that MOVs have a lifetime "joule" rating. The joule is an engineering term for energy (energy can be measured in joules). The joule rating for an MOV means that when a particular MOV has received the specified amount of energy, it no longer works! This could be the result of one or two large surges or several small surges. Therefore, for MOVs to be used reliably, a strict replacement schedule is necessary. It is like not having a gas gauge in your car - you never know when you are going to run out of gas so you have to keep putting more gas in your car! Lack of replacement can have very serious results - MOVs have been known to cause fires.

Shunt mode protection has another aspect to it that should be considered when purchasing a surge protection device. That is, that the surges are not handled or contained in any way, they are simply diverted to the neutral wire or the ground wire. If shunt mode protection is working properly, it can, subject to limitations of its let-through voltage, protect a single piece of equipment that does not contain a switching power supply (see section 5. switching power supplies) or a bank of equipment which is all powered from the same 110v outlet. However, because shunt mode diverts surges to ground, it can cause damage to interconnected equipment that is plugged into a different 110V outlet. The reason for this is that the safety ground that is used as the ground reference in audio/video systems is the same ground onto which surges are dumped by shunt mode protection! The surges then travel along the interconnecting cables! All wires including building wiring and interconnect wiring have impedance, which will generate a substantial voltage when the thousands of amps of surge current are suddenly dumped onto such wiring. This is nothing more than Ohms Law. One of the worst case examples of this problem is that of interconnected computers, because computers are often interconnected by long datacomm cables, and plugged into 1l0V outlets at very different parts of a building.

The important points for shunt mode protectors are:

  • Cheap and simple to produce
  • Limited lifetime
  • Contaminate ground wire

Series Mode Protection

Series Mode protection was originally developed in the mid 1980's to meet the more demanding requirements of industry. As its name suggests, series mode protection operates in series with the hot wire and works by blocking the surge voltage rather than by diverting the surge current (which could be thousands of amps) to ground. This has the dual advantages that the surge protector does not have to handle thousands of amps of surge current and, most importantly, ground is not contaminated.

So, how does Series Mode protection actually work? The accompanying diagram shows how SurgeX surge protection works. The first line of defense in the SurgeX system (and key to SurgeX protection) is the massive surge reactor which is in series with the hot wire. (It is the weight of the surge reactor that is the reason SurgeX products weigh much more than other surge protectors.) The surge reactor filters out small transients, slows larger surges (surges and transients are very fast), and limits the surge current. All current which passes to equipment connected to a SurgeX unit passes through this filter. The second part of the SurgeX system is an electronic circuit which tracks, clamps and contains the surge energy after it has been slowed by the surge reactor. This energy is then released slowly to the neutral wire, not the ground wire. In this way, the safety ground is never contaminated, and interconnected equipment is not put at risk when a surge comes in on the power lines feeding the equipment.

SurgeX has no sacrificial components (such as MOVs) and can withstand thousands of the largest surges normally found on 110v wiring (6000V, 3000A). The SurgeX system also has superior clamping because it tracks the power wave and instantly clamps transients typically to within only 2 volts of the ac power wave! No MOV-based product can claim such excellent clamping. An oscilloscope connected to the output of a SurgeX product barely shows any measurable voltage rise when a surge is injected into the unit, whereas MOV-based products have a very measurable voltage rise. SurgeX products are not available as whole-house protectors because it is important to protect equipment from transients produced within a building as well as surges entering from outside. The best protection is provided by installing a surge protector for each piece (or bank) of equipment rather than installing a whole-house protector.

Of course, the all-important question is: does Series Mode protection really work, and how well does it work? Series Mode protection has been used in the most demanding industrial and commercial environments with exemplary performance. It has protected equipment when there have been direct lightning hits on buildings, and has also protected equipment from data errors caused by transients produced inside factories. Furthermore, all SurgeX products meet the highest level of testing that is available from UL - the buyer's absolute guarantee of performance. This will be discussed in the next section.

The important points for Series Mode protection are:

  • Unlimited lifetime
  • No sacrificial components
  • Unsurpassed voltage clamping
  • Safety or reference ground is not contaminated