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Everything You Need to Know About Type 3 SPD Surge Protection Devices


Surge Protectors, often errononeously referred to as “power strips”, is officially known as a Type 3 SPD (surge protection device).

These single outlet, point-of-use protection devices are the last line of defense against transient and voltage spikes that can damage your expensive electronic devices like LED TVs, stereo equipment, large and small appliances in a blink of an eye.

Due to sheer number of manufacturers, features and claims, most people don’t give much thoughts and grab something that seems to be most popular at the moment.

That is, until a lightning storm rolls through or when the power goes out during a storms and you have that kick-in-the-gut feeling like I did when my PCs fried during a nasty power surge.

I am going to try to cut through the confusion and BS claims to help you make an educated decision that is right for you.

In this Article You Will Learn


What is Type 3 Surge protection?

In an effort to consolidate confusing terminologies, Underwriters Laboratories (UL) decided in September 2009 to discontinue using the term “TVSS” and “Secondary Surge Arrester.”

allthumbsdiy-electrical-surge-protection-undewriters-laboratories-logo-flInstead, it implemented Surge Protective Device (SPD) to describe protection devices used on AC power systems rated less than 1000 VAC.

UL further classified SPD into four types: Type 1, Type 2, Type 3, and Type 4.

This classification is based on the application and the location where they are to be used.

Type 1 SPDs are permanently connected devices which bear a UL listing for installation at any location between the secondary of the utility service transformer and the service entrance primary disconnect. A Type 1 SPDs can also be installed anywhere on the load side of the service entrance and allowed to be installed anywhere on the low-voltage electrical system without requiring a dedicated fuse or

Type 2 SPDs are permanently connected devices which bear a UL listing for installation on the load side of the service entrance primary disconnect. Type 2 SPDs may, or may not require the use of a dedicated fuse or breaker.

Type 3 SPDs are installed at a conductor length of 10 meters (30 feet) or greater from the electrical panel. These devices are typically cord connected, direct plug-in, receptacle type SPDs installed at the load equipment being protected.

Type 4 SPDs are components and assemblies that do not have a UL Listing. They might carry the UL recognized mark. Examples are Metal Oxide Varistors, MOV and gas tubes.

The basic idea behind type 3 SPD is the same as Type 1 or Type 2 SPDs: to help protect electronic equipment (i.e. computers, televisions, home theater systems, game systems, large and small appliances, etc.) from electrical spikes and surges.

Spikes are usually caused by storms or problems on utility lines, resulting in overvoltages in a fraction of a second. Surges carry less voltage than spike but with longer duration. Most surges are caused by large appliances (i.e. regrigerator, HVAC, etc.) that draw a lot of power.

Type 3 SPD is designed to absorb and redirect damaging voltages caused by surges and spikes away from devices connected to it.

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History of the Power Surge Protectors


An examples of early power strips exist as far back as 1932 when Carl M. Peterson was granted U.S. Patent # 1,889,695 for his invention, “Table Tap”.

Carl Peterson created his invention with four (4) primary objectives in mind:

  • To provide a plurality of electrical outlets in a single unit which may be connected by a cord and plug to a wall receptacle base board outlet or the like to furnish ample plug-in facilities on a table where such electrical appliances as percolators, toasters, waie irons or the like may be used without the annoyance of entangling cords and a plurality of connectors.
  • To provide a device of this type which simple in construction, reliable in operation and which may be used either as a portable or as a fixed device. In its use as a portable device the same is properly weighted or balanced at its base so that it will occupy a relatively stationary position when in use, or, if desired, the same may be fastened to a table or other support in a concealed position, if desired, to permit of attaching the cords appliances thereto in a neat and attractive manner.
  • To provide a device which may be made from mold-able material, such for example, as Bakelite and which includes in its organization a pair of contact strips so arranged that any desired number of plugs may be connected with said strips. Furthermore, the arrangement is such that the contact strips will have sufficient spring or yield to always insure the prongs of the plugs making proper electrical contact with the strips.

One early iteration, called a “power board”, was invented in 1972 by Australian electrical engineer Peter Talbot working under Frank Bannigan, Managing Director of Australian company Kambrook. The product was hugely successful, however, it was not patented and market share was eventually lost to other manufacturers.[source: wikipedia)

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How Does a Surge Protector Work?

A surge protector, or suppressor, is a device that blocks a sudden damaging voltage increase by sending it away from your electronics and into the ground.

Sudden voltage increases are commonly referred to as electrical surges and spikes. They are fast and unpredictable increases in electricity that travel through the wiring in your home. Without a power surge protection system, both can cause substantial damage to any computer, appliance or piece of equipment that has electronic components.

The difference between an electrical spike and an electrical surge is that a spike lasts for 2 nanoseconds (billionths of a second) or less compared to a surge that by definition lasts for 3 nanoseconds or more. While longer exposure to increased electricity would logically cause greater damage, the force of the excess electricity is an equally damaging factor.

Most surge protectors have a component called a metal oxide varistors (MOV). The MOV, which forms a connection between the hot power line and the grounding line, has three parts: a piece of metal oxide material in the middle, connected the power and grounding line by two semiconductors.

allthumbsdiy-electrical-surge-protector-power-strip-vs-power-protector-disassembled-flThe two conductors have a variable resistance, which is dependent on the voltage.

When the voltage is correct, the MOV remain the same and does absolutely nothing. If the voltage falls below a certain level, the electrons in the semiconductors flow in a manner that creates high resistance.

When the voltage exceeds the correct level, the MOV acts like a pressure valve and conducts the excess current, which eliminates the extra voltage.

NOTE: point of use power strip surge protectors only work when plugged into grounded outlets.

Homes built without electrical grounding (i.e. outlets only accept 2-prong plugs) provide no path to ground for power strip types to dispense the excess voltage. Whole house surge protection devices on the other hand, still work at 100% efficiency in homes that were built without internal grounding. This is because a spike and surge protector that protects an entire home functions at the main circuit breaker box and utilize the much larger main grounding wires by NEC

Please note that only few “power strips” unction as the Point-Of-Use Surge Protection device so you need to examine the specifications carefully to make sure you are getting what you think are getting.

Plug this surge protection device into one power outlet and it passes the electrical current along to a number of electrical and electronic devices plugged into the power strip. A SPD provides the necessary protection from surges in power. If the voltage from the outlet spikes or the surge exceeds the accepted level, the surge protector diverts the overage into the outlet’s grounding wire. Some power strips (not all) include the surge protector feature.

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Features and Functions

Good vs. Not-So-Good SPDs

Warranty Claims

something happens due to a sheer amount of available products with wild performance claims.

Once the massive overvoltages have been taken off the line at the service panel, the more sensitive components of point of use protection can come into play.

A good to excellent surge protectors should be able to sacrifice itself to block lower level surges that are damaging to electronics, including large LED TVs, refrigerators, washers, etc.

The marketing folks from some of these companies want you to believe their products are so superior, they are willing to include a $250,000 warranty against damages to electronic devices cause by a surge.

A quick search on the internet and you will soon find out that with so many exclusion clauses in fine print, you will be lucky to get a replacement surge suppressor.

In my opinion, large assortment of surge protection devices on the market are simply junk and others a fire hazard so the moral of the story is take some to research and pay little more upfront and buy a good surge protection device.

Built in Features for a high Quality Surge Protection Device

Here are some characteristics that goes into making a quality SPD power strip:

  • Energy dissipation:[1] 400 joules or greater, this is the maximum amount of electricity it can handle before failing;
  • Clamping voltage:[2] 330 or less, also known as let-through voltage it’s the trigger point that activates dissipation to ground;
  • Underwriters Laboratories (UL) approval;
  • Low resistance to grounding;
  • Fast response time: [3] 1 nano second or less; longer response times equal more exposure and greater damage.
  • Automatic power disconnect if the unit fails;
  • Self-diagnostic feature (indicator lights and test buttons) to indicate whether the receptacle is properly grounded and the protection system is operating;
  • Contains high quality Metal-Oxide Varistors (MOVs);
  • Abundant receptacle slots;
  • Plenty of spacing between receptacles to accommodate bulky transformers;
  • Wall plug should rotate and have a flat profile;
  • Indicator lights: Even the best units are only able to suppress a certain number of spikes. This number depends on the severity of the spikes and their duration so it’s important to know that your equipment is always working correctly.

[1], [2], [3] These are no UL standards for these parameters so take them with a grain of salt

Of course not every feature can be found in a particular device but more the merrier.


Many manufacturers carefully word-craft their descriptions to make it appear that you are getting surge protection. In fact, you are getting nothing more than a very expensive extension cord with a cheap circuit breaker that won’t protect anything other than starting a fire and that’s not guaranteed either.

Often time’s power strips that do not provide any protection at all are erroneously referred to and used as “surge protectors” by unaware consumers. Don’t make this potentially costly mistake,

Three Specifications of a Good SPD

Three technical specifications to watch when buying a surge suppressor: joules, response time and UL certification. Joules isn’t a rating on protection from a single surge, it is a rating on how many surges the suppressor can handle. It’s sort of a life expectancy rating. The experts recommend looking for at least 600 Joules. This APC suppressor gives you almost double that amount at 1,080 Joules.

Response time is also important. This determines how long your equipment will be exposed to the surge before the suppressor kicks in and clamps the voltage down. For this APC suppressor, the response time is a very respectable 1 nanosecond. That’s tech speak for 1 billionth of a second. Surges take a few thousands of a second to reach their peak, so this response time should be more than adequate.

The UL certification standard for surge suppressors is UL 1449. UL provides safety related certifications, not performance related certifications. A UL certification basically means that the surge suppressor itself and the equipment connected to it are extremely unlikely to blow up and start a fire when it’s hit with a surge.

Ignore “clamping voltage”. For technical reasons, you really want to watch the “Let-Through Voltage”. UL certification does include the specification for Let-Through Voltage with levels of 300, 400 and 500. This APC unit has a UL certification with a maximum Transient Voltage Surge Suppressor (TVSS) Let-Through Voltage of 300 volts, which is the best rating that UL provides for surge suppressors. Other manufacturers often make a big deal out of clamping voltage but avoid mentioning that their UL Let-Through Voltage is 400 or 500 volts.

Finally, a huge feature in a surge suppressor is a “Fail Safe” function. When a surge suppressor reaches it’s maximum Joules rating it quits providing surge suppression. The question is, how do you know that has happened? Most surge suppressors have an green idiot light (LED) that shows that the unit is still providing surge suppression. If you happen to notice that this LED has gone dark then you’re in luck. Most people don’t. A Fail Safe feature shuts the surge suppressor down so it no longer passes current through to your equipment. So, if your fail safe unit quits providing surge suppression, your equipment shuts down and you know the instant the surge suppressor has reached its life span. Very few manufacturers include a fail safe protection, so you continue merrily on your way until your $1,000 television or laptop suddenly melts down during a thunderstorm. APC provides fail safe protection on all of their surge suppressors including this unit.


Monday 13th of March 2023

Thanks, didn't realize there was so much going on in that little quarter- sized MOV and why it was suggested to tuck into my home-built solid-state XCVR.


Saturday 8th of May 2021

Do they normally have security screws on the back

Pete maneri

Tuesday 28th of August 2018

When you talk about 400 joules minimum of surge protection, are you talking per leg? i.e. - 400 x 3 legs or 1200 min?