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[nextpage title=”Introduction”]

Most users want (and should) protect their valuable equipment. Surge suppressors, which are also known by other names such as surge protectors, noise suppressors, transient filters, line filters and TVSS (Transient Voltage Surge Suppressor), protect your equipment by removing noise and spikes coming from the power grid – and sometimes also from the telephone line and from the satellite or cable TV line, if you have a suppressor with this option. But how do they work? What components do they have inside? How can you differentiate a good product from a bad product? Read on.

Besides the noise and spike removal, surge suppressors have other three basic goals. First, and the most obvious, to expand the number of AC outlets you have available near your computer or audio/video equipment – yes, surge suppressors can be used with any kind of equipment, and audio and video are the most common ones besides computers, as any noise coming from the power grid can appear as an audio noise or a video noise.

Second, to make sure that all your equipment is properly grounded. For grounding to work, you can’t cut the ground pin from the power cord of your equipments (including the surge suppressor itself) and you also must have proper grounding on your AC outlet – i.e., having an AC outlet with three pins and making sure that the third pin (the grounding pin) is correctly grounded. If you don’t have correct grounding the filtering circuit will not work correctly, as it basically re-route all voltage excess to ground.

Third, overload and short-circuit protection. All surge suppressors feature a circuit breaker that will shut power down if the total current pulled by your equipment is beyond its rated current. 15 A is the most common value. On several units the circuit breaker is built together with the on/off switch, so if you need to reset the breaker you will have to move the power switch to the off position and then to the on position again. On some other units the circuit breaker isn’t built together with the on/off switch and you will find a separated reset switch. Of course you need to check first why the circuit breaker activated, otherwise it will activate again as soon as you reset it.

Before disassembling some units to show you how their internals, let’s talk a little bit more on the aesthetics side of surge suppressors.

The most common problem nowadays is that we have several devices that may have a transformer attached to their AC plug – broadband modems, routers, printers and external hard drives are some typical examples. Also don’t forget about devices that you want to have on your desktop like battery chargers for you digital camera, cell phone or generic rechargeable batteries and may have to connect them to your surge suppressor because you may not have enough AC outlets near your desk.

The problem with these transformers is that they so big that they typically obstructs the outlet right next to it (and sometimes more than one, see Figure 2). So you may want to choose a surge suppressor that has “special” outlets for transformers.

Surge SuppressorFigure 1: Transformers like this one block the AC outlet right next to it.

Surge SuppressorFigure 2: Example of this transformer taking three AC outlets.

The suppressor portrayed in Figure 2 actually has one “special” transformer outlet, as you can see in Figure 3 (the one on the far right). The circuit breaker of this surge suppressor is together with its on/off switch, so you won’t find a reset switch on this model.

Surge SuppressorFigure 3: Surge suppressor with one transformer outlet.

In Figure 4, you can see a fancier surge suppressor with four transformer outlets. Of course you can still use these outlets with regular power plugs. The circuit breaker of this surge suppressor is separated from its on/off switch, so it has a reset switch (on its lower left corner in Figure 4). It also has a LED that indicates if grounding or the component used for peak protection (MOV) isn’t good, another LED that indicates that the circuit breaker is active (overload situation) and also cable/satellite filter and phone line filter. We will talk more about this feature later.

Surge SuppressorFigure 4: Fancier surge suppressor with four transformer outlets.

Some manufacturers solved this transformer problem by putting the AC outlets on cables instead of putting them on the surge suppressor housing, as you can see in Figure 5.

Surge SuppressorFigure 5: You won’t have problems attaching as many transformers as you want with this surge suppressor.

Now that we’ve talked a little bit about layout, let’s talk about what is most important: filtering.[nextpage title=”AC Filtering”]

There are two kinds of filtering that a surge suppressor needs to deal with: electromagnetic interference (a.k.a. EMI) and spikes coming from the power grid. Both can be caused by several means, the first being typically produced by motor-based appliances, like blenders, washers, microwave ovens and even your printer. Spikes are short moments where the voltage peaks to values way above the normal, which can literally burn your equipment.

The main problem with surge suppressors is that the majority of surge suppressors available on the market don’t have all filtering components necessary for the good EMI and spike filtering. Some cheap “suppressors” don’t have any filtering component at all, so they work just like a regular cord extension with multiple outlets. These “suppressors” are now being labeled as “power strips” to let you know that they don’t do any filtering at all. Don’t buy or use this kind of “suppressor” as they add no filtering at all to your equipment (even though they provide a circuit breaker and helps grounding all your equipments). The “suppressor” we have shown before on Figures 2 and 3 is one of these no filtering devices. We show it completely opened in Figure 6 so you can see by yourself what we are talking about.

Surge SuppressorFigure 6: This “surge suppressor” has no filtering components at all.

Typical components found on real surge suppressors include:

  • MOV (Metal Oxide Varistor): This is the heart of a surge suppressor, in charge of holding voltage peaks coming from the power grid. All surge suppressors mu
    st have at least one MOV, connected between hot and neutral wires from the power grid. Good models must have at least three MOV’s, one between hot and neutral, one between hot and ground and one between neutral and ground. MOV’s don’t last forever. After some years they stop working correctly (the exact life expectancy depends on some factor like, for example, how many times the MOV entered in action – a typical value is 1,000 surge protections), and some surge suppressors have a LED to indicate that their MOV’s aren’t working right. On surge protectors without this LED you will think that your surge protector is still protecting your equipment when their MOV’s have gone bad, since there still voltage on the AC outlets, while in fact your surge suppressor is working just like a power strip – i.e., cord extension. Every MOV is rated with a maximum absorption energy level, given in a unit called joule. If the amount of energy delivered to the MOV’s is higher than their ratings, they will burn – protecting your equipment, at least. It is important to know that MOV’s only holds overvoltage peaks. If a voltage or current higher than the maximum the MOV can handle is applied during a sustained period of time, the MOV will not only burn, but may pass overvoltage to your equipment, creating a risk of fire. Physically MOV’s are disc-shaped components with two terminals, usually blue in color.
  • X Capacitor: In charge of EMI filtering, this capacitor has two terminals and is usually squared and yellow in color.
  • Y Capacitors: In charge of EMI filtering, these capacitors always come in pairs. They have two terminals each, one capacitor is connected between ground and “hot” wires coming from the power grid and the other is connected between ground and neutral wires. Since these capacitors are connected to ground if you don’t have correct grounding this filtering won’t work. Physically they are disc-shaped and usually light blue, being smaller than MOV’s. When opening surge suppressors some people confuse Y capacitors with MOV’s. When you are doubt whether the blue disc in front of you is a MOV or a Y capacitor, keep in mind that Y capacitors always come in pairs and one of their terminals are always connected to ground.
  • Ferrite coil: Another component in charge of EMI filtering, it is a coil made from copper wire with a ferrite bar or ring in the middle.

Cheap surge suppressors don’t have the EMI filtering components, even though they may have at least one MOV. Good suppressors will have both MOV and EMI filtering components.

On Figures 7 and 8 you can see the surge suppressor from Figure 4 opened (a unit from APC that costs around USD 30.00, by the way). The components found in the middle of the unit in Figure 7 are from the phone and cable/satellite filter, which we will discuss in the next page, while on the left-hand side you can see the printed circuit board from the AC filter.

Surge SuppressorFigure 7: Inside a good surge suppressor.

In Figure 8, you can see the AC filter from this unit in details. As you can see, this unit doesn’t have one MOV but seven! This unit does not have Y capacitors, but it has two thermal fuses, that act just like a circuit breaker but without a reset switch – i.e., it shut downs the circuit if its internal temperature reaches a certain “trigger” value, meaning that the circuit is overloaded or there is a short-circuit. The smaller components at the bottom are used to control the two available LEDs.

Surge SuppressorFigure 8: Inside a good surge suppressor.

High-end surge suppressors have filtering also on the load side, preventing your equipments from generating noise to your power line.

Now let’s talk about the importance of the phone and cable/satellite filtering and how this filtering should be done.

[nextpage title=”Phone and Cable/Satellite Filtering”]The main goal of phone and cable/satellite filtering is preventing that abnormal high voltages reach your modem, computer, TV, decoder, phone set, etc coming from your phone line, cable TV or satellite cable. This kind of situation is more common than you think. If a lightning strikes a pole where your phone line and/or your cable TV line are located, the lightning will reach your equipment and literally fry it. In fact, the lightning doesn’t need to reach the pole: striking nearby is enough to induct high voltage on your phone or cable TV line.

This circuit can be based on MOV’s or on a component called gas arrestor, where the circuit discharges the lightning, throwing it to ground. Surge suppressors using this component are not so common. Gas arrestors have life expectancy lower than MOV’s but on the other hand can handle higher voltages without self-destructing.

On Figures 9 and 10 you can see the circuitry used on our APC surge suppressor. As you can see, there is no communication between the phone/cable/satellite filter and the AC line filter, except for grounding. The same goes for the phone and cable/satellite sections: they don’t share any components. The phone filter is based on four MOV’s and four capacitors (see Figure 9) while the cable/satellite filter, which is located inside the metallic protection surrounding the two coax connectors, is based on a transient voltage suppression diode (transorb) – a semiconductor that acts similarly to a MOV but is faster –, also having one fusistor and one small coil.

Surge SuppressorFigure 9: Phone filtering section.

Surge SuppressorFigure 10: Cable/satellite filtering section.

Both phone line and cable/satellite filters work by throwing the excess voltage to ground. Thus, once again, if you don’t have proper grounding at your home or office these filters won’t work correctly and you won’t be protected.

[nextpage title=”What Surge Suppressor Should I Pick?”]

If you are really worried about protecting your equipment you should buy a decent Surge Protector, a not the cheapest unit around. The problem with cheap units is that they will give you a false safety sensation, while they are in fact not protecting anything at all. As we shown, really cheap units don’t even have a MOV, working just like a regular power cord extension.

From what we explained, you should choose a surge suppressor that has a least three MOV’s. How do you know it? If on the product box or specs page something like “L-N, L-G, N-G” shows up, this means that the surge suppressor has at least three MOV’s.

Then look for the unit with the lowest clamping voltage (330 V is the minimum nowadays). Clamping voltage is the voltage at which the MOV’s will start working. We want that they to start working as soon as a voltage peak occur, right?

Then we have response time, which is the delay the surge suppressor has between a peak occurring and the protection kicking in. Of course we
want this number as low as possible, zero if possible.

We also have the peak surge current, which is the maximum peak current the MOV’s can handle without burning. The higher, the better.

And finally, we have the energy absorption level, given in joules. This is how much peak energy the unit can handle before burning its MOV’s. The higher, the better: the probability of your surge suppressor being burned by a voltage peak will be smaller.

We also recommend extra features like phone line protection and cable/satellite protection and a LED indicating if the ground and/or MOV’s are working properly.

Of course there are much more we could say about surge suppressors, we wanted to keep it simple.

If you want to test your knowledge on surge suppressors, we recommend you to take our Surge Suppressors Quiz. This is a terrific way to evaluate what you have just learned by reading this tutorial.