Ode to Those Unsung Heroes of Surge Protection

Updated June 21, 2014.

I always thought this site may begin dabbling in hardware but I had no idea it would start like this. Summer is around the corner for you northern hemisphere people and that means thunderstorms. Fun stuff, but if a destructive power surge makes its way into your house and kills your computer or worse, you’re gonna have a bad time.

Surge protection usually makes people think of lightning but that’s actually one of the rarer causes of power quality problems for electronic equipment. More often would be an overcurrent swell resulting from an electrical fault somewhere outside your house, for example, due to digging or debris across power lines. Faults can happen indoors as well. Some reasons are old wiring that’s falling apart at the connections and damage from rodents, earthquakes, fire and water.

But the most common source of power trouble is load switching at any point in the local power system, including inside your house. You’ve seen this when a high-current motor or transformer turns on and momentarily dims the lights or distorts audio or video signals. These surges can come from your air conditioning, vacuum, washing machine or refrigerator, or power source switching such as when a backup generator kicks in. Load switching from nearby buildings can even affect your power quality. Also when the power goes out and resumes again, whether a flicker or a 9 hour outage, surges are part of the package.

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If power surges are of concern to you, proper surge protection is the only real solution and it’s a quick & easy one if you just want a “buy this, install that” directive. As for the why, well that’s not quick. Nor easy. So on the menu for today, we’ll dive deep into the topic of surge protection and how to best apply it to protect your building’s tender electronic insides. By building, I mean something residential in size and power draw like a house, workshop, pool shed, solar panel tree, etc. This is the not so quick, not so easy route but later I give some general recommendations for different price points and protection levels.

One thing though. This site has many readers around the world and while the concepts and best practices mentioned would be valid anywhere, this article is intended specifically for North America. There are some fundamental differences in power delivery, wiring and testing standards between there and other countries so, just know that not everything below may be applicable to you.

What Is a Surge Protector Really?

A “surge protector” is a general term used for one of three main types of surge protective devices (SPD) as defined by the American National Standards Institute and Underwriters Laboratories standard 1449, 3rd edition. UL is one of several American Nationally Recognized Testing Labs (NRTL) which conducts safety tests for electrical devices. Some other NRTLs you may encounter are Intertek Testing Services, the Canadian Standards Association and TÜV Rheinland.

Relevant to us here are ANSI/UL1449 type 1, 2 and 3 devices. Type 3s, also called point-of-use or plugin devices, are the iconic surge protector strips we’ve all seen before—not to be confused with plain power strips which look identical. Other forms of type 3s are wall-mount multi-outlet plugs and outlet receptacles themselves can contain surge protection.

Type 1 and 2 SPDs resemble type 3s in function but aren’t confined to points of use. They’re often referred to as service entry or whole house protectors and just as your router’s firewall guards your entire local network, a service entry SPD is the perimeter device for your house’s electrical system. They come in various forms and many are UL listed jointly as type 1 and 2, meaning they underwent stricter testing and can be used outdoors. Some type 2 devices are an integrated circuit breaker/SPD combination but more common is a small box mounted next to, on to, or inside your main electrical panel.

Types of Power and Surges

Phases and modes are two words you’ll frequently see in SPD specs. Residential AC power is usually called split phase power, meaning there are two feeder wires (2 phases of the voltage cycle) coming from the pole, L1 & L2, along with a neutral wire (N). Confusingly, split phase is technically a form of single phase power but can also be called two phase or single phase 3-wire. Three phase power (L1, L2 & L3) is normally for big loads needed in commercial and industrial facilities but it can be found in residential too. Needless to say, a type 1 or 2 SPD must be matched to the building’s power delivery.

There are two modes which describe current pathways between line, neutral and ground. Normal mode describes a voltage difference across any or all hot lines and neutral (L-L & L-N), also called differential, symmetrical or metallic mode. Then there is common mode, also called longitudinal or asymmetrical mode. This describes voltage across each hot line or neutral to ground (L-G, N-G). Some SPD specs say “all mode” and this means both normal and common.

Normal and common mode power fluctuations are constant occurrences but at such low voltages they amount to noise interference. Here’s an example: see figures 1 and 2 in this test paper by Juice Goose where they record 48 hours of noise across both modes at their office in Houston, Texas. This can be tamed by SPDs with built in EMI/RFI filtering but that still leaves you vulnerable to surges in the passthrough frequencies.

Surges originating upstream in the power system will arrive to you as normal mode surges. Some scenarios are a lightning strike on a pole, recloser operation and external load switching; all SPDs for AC power have at least normal mode protection. Common mode surges are usually localized to your building’s electrical system and while seldom damaging, they can cause signal disruptions in electronics using the building’s safety ground (a 3 prong, grounded plug). Destructive common mode surges can be an electrical fault inside the building, or induced onto wiring or even entire grounding systems by large motors and lightning strikes.

Electromagnetic Noise Filtering

Also referred to as sine wave tracking, EMI/RFI filtering is fairly widespread (to various capabilities) among even midrange SPDs. Most filter-type protectors operate entirely on this basis. To a point, noise in a power system is normal if for no reason other than wire impedance. Good noise filtering improves the lifespan of MOV and SAD based protectors while reducing the low energy, high frequency impulse and ringwave transients [p.2] which don’t harm equipment, but can upset data signals (remember the recordings from Juice Goose). An example of this would be TV picture quality degradation by a running vacuum cleaner from an outlet on the same circuit.

Eaton’s Guide to Surge Suppression discusses EM filtering and specifies a signal attenuation of 50 dB @ 100kHz “for premium performance” [p.15] based on the MIL-STD-220 filter performance testing standard, but expect to find around 40dB, even for quality protectors. SPDs with noise filtering are often listed as both a UL1449 Surge Protective Device and a UL1283 Electromagnetic Interference Filter. Nearly all point-of-use surge filters are strictly UL1283 listed.

Enclosure Rating

For a service entry SPD, the enclosure should be NEMA or IEC rated. That will indicate its overall build strength and if the protector can be used outdoors. Should something crazy happen, plastic or fiberglass don’t dissipate heat and contain fireworks as well as steel. I personally prefer metal enclosures, even for type 3s and even though they’re more expensive. If a load center’s SPD cannot double as a SWAT battering ram, it has no business protecting my Ninja Blender. There’s my bias.

If you decide on a protector with a composite body, you can buy a NEMA or IEC rated steel enclosure, powder coated with knockouts, and mount the SPD inside. On the other hand, if you choose a protector that isn’t weatherproof and want to use it outdoors, there are both composite and steel enclosures for that as well. These are available from electrical supply stores for about $20 in the size you’d need.


Normally I place little confidence in warranties but an exception must be made for service entry SPDs. Some manufacturers include full replacement warranties with their products, meaning that if the SPD or its modules were to die, be it due to a destructive surge—including lightning—or old age, they replace it and the cost to you is only shipping. The warranty life varies by manufacturer and this is a non-transferrable warranty so it applies only to the homeowner at the time of installation. A free replacement warranty for an SPD is a hallmark of high-end service entry protectors.

On the other hand, connected equipment replacement warranties are mostly bullshit, intentionally crippled with loopholes and contingencies. You won’t find these from any service entry SPD manufacturer worth considering but they do plague nearly all type 3 protectors. Sure this isn’t universal among manufacturers, but you’re wasting your time if shopping for plugin protectors on the premise that you’ll get $30k for a new home theater because the $30 surge protector supposedly didn’t do its job. Do as you will, but I suggest that resources are more wisely spent on quality surge protection and grounding from the beginning.

What About Lightning?

Telco towers, rocket launch facilities and high rise buildings handle direct strikes just fine but their protection infrastructure is far beyond what’s found residentially. For homeowners and lightning, it all comes down to your Earth grounding but more on that later. The average ground lightning stroke is around 20kA but recorded strokes range anywhere in the area of 5kA to 220kA and higher in extreme record cases.

Along with fault current, lightning is the most abuse your local utility line or antenna could see, but your surge protectors won’t ever meet such high energy, only the remaining transient. Here are two quotes by Schneider Electric (the parent company of APC and Square D) from a whitepaper titled Choosing the Right Protection. Peak Ampere Surge Current Ratings of SPDS [p.2-3].

Annex A of IEEE C62.41.1-2002 also provides information demonstrating the physical impossibilities for high surge currents making it to the point of the SPD or into a building. Since the basic insulation level (BIL) of a typical electrical system is approximately 6,000 volts, the high voltages that accompany high surge currents will flash over prior to reaching the SPD.

From these scenarios it is clear that higher-rated surge current (kA) devices offer no real benefit over lower-rated devices when comparing their abilities to shunt large amounts of current. This is due to limiting factors within the distribution system that keep the SPD from experiencing extreme currents. The advantage of the higher-rated device is the ability to handle more transient surge events over time, not necessarily larger individual transient surge events.

Here is another quote from Eaton’s Guide to Surge Suppression. [p.11]

It is physically impossible to have the energy associated with a lightning stroke travel down the AC power conductors.

A building’s electrical system can’t conduct more than 6,000 volts at 3,000 amps. The full energy of a lightning stroke won’t enter the building on the AC feeder lines because it will arc from panel busbars to the panel enclosure itself, both of which are connected to Earth ground. While the outcome wouldn’t be pretty, if such a high current tries to enter elsewhere (like a hit to your satellite TV dish) that results in heat, melting, maybe fire, explosion and/or arcing to more convenient grounds than air. Such damage arises from a vastly improper Earth grounding (or in the case of most satellite mini-dish installations, zero Earth grounding).

The basic insulation level is one rationale for using the IEEE based C1 impulse wave (6kV, 3kA 8×20) in UL1449 3rd ed. testing as designation for low exposure conditions. But lightning greatly magnifies differences in ground potential. Even two different NEC compliant Earth electrodes, when unbonded, can create 120,000 volts with just a 5kA lightning stroke (Ohm’s law). You can get away with an insufficient Earth ground when fielding minor surges but the thunderbolt will be the great equalizer.

When Decisions Are No Fun

This section is for the people who would rather just know a model number, buy and move on with life. However, treat these recommendations like you would default settings—suitable for most uses but may or may not be your exact fit. SPDs at the service entries for power, signal and data are the most cost-effective solution to surge protection. Even if you do nothing more than this, you’re still massively improving your situation. If you’re unable to install at the service entry because you don’t own the building or have very old wiring, then type 3 SPDs (and possibly even normal-mode-only type 3s) are your only hope so make your money count.

I recommend these devices not based on personal use, but by datasheet specifications and having spoken with the companies about the SPDs’ internals. Do not equate these recommendations to any kind of partnership or means of sales generation. These are not sponsored listings, these are protectors I would comfortably spend my own money on but it’s entirely possible these change in the future. I offer four different AC power service entry SPDs to choose from with all-mode protection and exceptional warranties. To order one of these, you must call the manufacturer directly or search their website and they’ll refer you a distributor. These models are also available outside North America.

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