Surge Suppressors - Surge Suppression, EMI, Phone, RF and TV filtering and UPS systems

Most modern electronic equipment is fairly sensitive to the power it receives. Fortunately, here in the United States, our electrical power is fairly consistent, preventing damage to our equipment. However, there are a large number of things which can happen; any one of which could cause our equipment serious problems or even permanent damage.



That’s why surge suppressors are so important. Amazingly enough, most electronic equipment is built with very little internal protection for electrical noise and voltage surges. The manufacturers are counting on the line voltage to be clean, consistent and spike free. While it usually is, we can’t always count on it staying that way. There are just too many things which can affect the voltage we are receiving from the power grid, along with things which can happen right in our own homes to create noise and spikes on the line.

Surge suppressors perform a number of tasks, including:


  • Increase number of outlets available
  • Ensure that the equipment is properly grounded
  • Overload protection
  • Surge and spike protection
  • Electro-magnetic interference (EMI)


Some also provide surge and spike protection for phone lines or TV antennas

Before we go any farther, let me clarify one little detail. Surges and spikes may seem like the same thing, but in fact there is a technical difference between them. Surges last longer than 3 nanoseconds (three-billionths of a second) and spikes are shorter than 3 nanoseconds. Either way, the voltage is higher than the 120 volts that you should have in your home’s electrical wiring.

Okay, so let’s take a look at those things and see how the surge protector takes care of them. The most obvious is the number of outlets. Most surge protectors are also power strips, giving you about six different outlets that you can plug into. With today’s equipment, this is an important function, as we usually have a lot of equipment plugged in together, especially with computer systems and entertainment centers.

Modern electrical outlets use a three-prong plug. The two flat vertical prongs are the hot and neutral liens, while the horseshoe shaped prong is the ground. Without a proper ground, electrical equipment is susceptible to electrostatic shock, as well as electrical noise and surges. The surge protector uses this connection to eliminate surges and spikes; so if it is cut, the surge protector can’t do anything. That includes inside the wall as well. If your home is improperly wired, so that the ground lug of the electrical connectors is not connected, your equipment will not be protected.

There is a simple device which you can buy in any home-improvement center or hardware store for checking the home’s wiring and the ground. It looks like a yellow electrical plug with some LEDs on it. Just plug it into the outlet and it will check to make sure that you have good wiring, including a good ground. If you own an older home, especially one that was built between the late 60s to the late 70s, when they were using aluminum wiring in homes, you might want to make this check.

Overload protection simply means that the surge protector will have a circuit breaker in it. If you connect too much equipment to the same surge protector, it will trip the breaker, turning everything off. Many surge protectors have these circuit breakers built into the switch.

Surge and Spike Protection

Okay, now let’s get down to the meat of the issue. The main purpose of connecting equipment through a surge protector is to protect it from electrical surges. This is accomplished by putting a series of MOVs in the device. MOVs are metal-oxide varisters. A varister is a variable resister. In other words, the amount of electrical resistance that it offers changes. In this case, the varister changes its electrical resistance by the line voltage.

The varisters themselves are connected between the hot line and ground as well as between the neutral line and ground. How many varisters a surge suppressor has will tell you a lot about its quality. As a general rule of thumb, the more varisters the better the surge protector. At a minimum, it needs to have at least three. You can see this listed in the specifications as “L-N, L-G, N-G.”

When the line voltage is normal (120 VAC) it has almost infinite resistance, preventing the electricity from passing through. When the line voltage rises above 120 VAC, the resistance of the varister lowers, allowing the extra voltage to be dumped to ground.

One really great thing about this sort of design is that the MOV works in such a way that it keeps the voltage going to the equipment at 120 VAC. So, even while it is dumping the excess electrical current to ground, it is still making sure that your equipment has enough power so that it can continue working.

So, where does this extra voltage come from? Typically, it’s some sort of natural event that is affecting the electrical grid. The most common is from lightning. If lightning strikes or even nearly strikes electrical power lines, it will put a huge spike in the voltage. While that dissipates over distance, it might affect your equipment if you are close enough. Another place that it can come from is EMPs (electro-magnetic pulses) from the Sun. We experienced this in the months of September and October, 2013.

One very important thing you need to know about MOVs is that they don’t last forever. Each time a MOV is activated, it reduces its strength. Eventually they reach a point where they no longer provide any protection whatsoever. Unfortunately, except for some very high-end units, which have self-test functions built in, there is no way of telling when these MOVs are losing their potency, short of a failure.

Electro-magnetic Interference (EMI)

A large number of things can cause EMI in your electrical power. The prime offender for creating EMI is electrical motors. Appliances, vacuum cleaners, well pumps, computer printers and fans all have electrical motors that create EMI. Another big offender is fluorescent lighting, specifically the ballasts that start the lights.

This EMI might manifest as static noise in audio and video. Better quality surge protectors eliminate this EMI as well. Filtering of this noise is usually accomplished by capacitors attached between the hot terminal and ground, as well as the neutral terminal and ground.

Phone and TV Filtering

Units that provide surge protection for phone lines and television cables do so in the same way that they do for the power lines. These lines can pick up power surges in the same way that power lines do. While it is possible to connect a television cable from a satellite dish to the surge protector, the main reason that these are provided is for people who have cable television. Those lines are much more susceptible to surges than the short line at your home.

Uninterruptible Power Supply (UPS)

An uninterruptible power supply is different than a surge protector in that it is a battery device, designed to provide power to a computer or other piece of equipment, in the case of a power outage. Simply put, they are a battery, with a charging circuit and a voltage inverter. When power is lost, the UPS turns on automatically, providing power to the equipment.

The reason I am making mention of the UPS in this article, is that most UPSs will include a surge suppressor in the design. While this may not be clearly stated, you can check easily enough by looking at the specifications. Look for the same code letters in the specifications that I mentioned to prove that a surge protector has at least three MOVs.


There are a few basic specifications which are important for a surge protector. They are:

Clamping voltage – The clamping voltage is the voltage at which the MOVs start working. Ideally, the clamping voltage should be as close to 120 VAC as possible.

Peak Surge Current – This is the maximum voltage that the surge suppressor can withstand, without damage to the MOVs. The higher the peak surge current, the better.


Response time – The response time refers to how fast the MOVs start reacting. The shorter the time, the better, as it provides the protection more instantaneously.  

 Rich Murphy