Surge protection device basic knowledge

Think of surge protection as a bouncer at a nightclub. He may only let certain people in and quickly tosses the troublemakers. Getting more interesting? Well, a good whole-house surge protection device does essentially the same thing. It allows in only the electricity your home needs and not the unruly over-voltages from the utility—then it protects your devices from any trouble that can occur from surges inside the house. Whole-house surge protective devices (SPDs) are typically wired to the electric service box and located nearby to protect all the appliances and electrical systems in a home.

80 percent of surges in a home we generate ourselves.

Like many of the surge suppression strips, we’re used to, whole-house surge protectors use metal oxide varistors (MOVs), to shunt power surges. MOVs get a bad rap because in surge strips one surge can effectively end the usefulness of a MOV. But unlike those used in most surge strips, the ones in whole-house systems are built to shunt large surges and can last for years. According to experts, more homebuilders today are offering whole-house surge protection as standard adders to help differentiate themselves and help protect homeowners’ investments in electronic systems—especially when some of those sensitive systems can be sold by the homebuilder.

Here are 5 things you should know about whole-house surge protection:

1. Homes are in more need of whole house surge protection today than ever.

“A lot has changed in the home over last few years,” says our expert. “There are many more electronics, and even in lighting with LEDs, if you take an LED apart there’s a little circuit board there. Washers, dryers, appliances also have circuit boards today, so there’s a lot more today to be protected in the home from power surges—even the home’s lighting. “There’s a lot of technology that we’re plugging into our houses.”

2. Lightning isn’t the biggest danger to electronics and other systems in the home.

“Most people think of surges as lightning, but 80 percent of surges are transient [short, intense bursts], and we generate them ourselves,” says the expert. “They’re internal to the home.” Generators and motors like those in air conditioning units and appliances introduce small surges into a home’s electrical lines. “It’s rare that one large surge will take out appliances and everything at one time,” explains Pluemer, but those mini-surges over the years will add up, degrade the performance of electronics and cut short their useful lifespans.

3. Whole house surge protection protects other electronics.

You may ask, “If most of the harmful surges in a house come from machines like AC units and appliances, why to bother with whole-house surge protection at the breaker panel?” The answer is that an appliance or system on a dedicated circuit, like an air conditioning unit, will send the surge back through the breaker panel, where it can be shunted to protect everything else in the home, the expert says.

4. Whole house surge protection should be layered.

If an appliance or device sends a surge through a circuit that’s shared among other devices and not dedicated, then those other outlets could be susceptible to a surge, which is why you don’t want it just at the electrical panel. Surge protection should be layered in the house to be at both at the electrical service to protect the whole home and at the point of use to protect sensitive electronics. Power conditioners with surge suppression capability, along with the ability to provide filtered power to audio/video equipment, are recommended for many home theater and home entertainment systems.

5. What to look for in whole house surge protection devices.

Most homes with 120-volt service can be adequately protected with an 80kA-rated surge protector. Chances are a home is not going to see large spikes of 50kA to 100kA. Even nearby lightning strikes traveling over power lines will be dissipated by the time that surge reaches a house. A home will likely never see a surge over 10kA. However, a 10kA–rated device receiving a 10kA surge, for example, could use up its MOV surge-shunting capacity with that one surge, so something in the order of 80kA will ensure it lasts longer. Homes with subpanels should have added protection of about half the kA rating of the main unit. If there’s a lot of lightning in an area or if there’s a building using heavy machinery nearby, look for an 80kA rating.

A load management system allows industrial management and facilities engineers to control when a load is added or shed from a power system, making paralleling systems more robust and improving power quality to critical loads on many power generation systems. In the simplest form, load management, also called load add/shed or load control, allows removal of non-critical loads when the capacity of the power supply is reduced or unable to support the entire load.

It allows you to determine when a load needs to be dropped or added again

If the non-critical loads are removed, critical loads can retain power under circumstances where they could otherwise experience poor power quality due to an overload condition or lose power due to a protective shutdown of the power source. It allows for removal of non-critical loads from the power generation system based on certain conditions such as a generator overload scenario.

Load management enables loads to be prioritized and removed or added, based on certain conditions such as generator load, output voltage, or AC frequency. On a multi-generator system, if one generator shuts down or is unavailable, load management enables lower priority loads to be disconnected from the bus.

It improves power quality and ensures that all loads are operational

This ensures that the critical loads are still operational even with a system that has an overall capacity lower than originally planned. In addition, by controlling how many and which non-critical loads are shed, load management can enable a maximum number of non-critical loads to be supplied with power based on the actual system capacity. In many systems, load management can also improve power quality.

For example, in systems with large motors, the starting of the motors can be staggered to allow a stable system as each motor starts. Load management can further be utilized to control a load bank so when loads are below the desired limit the load bank can be activated, ensuring proper operation of the generator.

Load management may also provide load relief so that a single generator can connect to the bus without being overloaded immediately. Loads can be added gradually, with a time delay between adding each load priority, enabling the generator to recover voltage and frequency between steps.

There are many instances where load management can improve the reliability of a power generation system. A few applications where the use of load management FAQ-surge-protection-device-4may be implemented are highlighted below.

  • Standard paralleling systems
  • Dead-field paralleling system
  • Single generator systems
  • Systems with special emissions requirements

Standard paralleling systems

Most standard paralleling systems have used for some type of load management because the load must be energized by a single generator before the others can synchronize to it and add power generation capacity. Further, that single generator may not be able to supply the power requirements of the entire load.

Standard paralleling systems will start all generators simultaneously, but they are unable to synchronize to each other without one of them energizing the paralleling bus. One generator is chosen to energize the bus so that the others can synchronize to it. Although most generators are typically synchronized and connected to the paralleling bus within a few seconds of the first generator closing, it is not uncommon for the synchronization process to take up to a minute, long enough for an overload to cause the generator to shut down to protect itself.

Other generators can close to the dead bus after that generator shuts down, but they will have the same load that caused the other generator to be overloaded, so they are likely to behave similarly (unless the generators are different sizes). In addition, it can be difficult for generators to synchronize to an overloaded bus due to abnormal voltage and frequency levels or frequency and voltage fluctuations, so the incorporation of load management can help bring additional generators online more quickly.

Provides good power quality to critical loads

FAQ-surge-protection-device-2A properly configured load management system will typically provide good power quality to critical loads during the synchronization process by ensuring that the online generators are not overloaded, even if the synchronizing process takes longer than expected. Load management may be implemented in a multitude of ways. Standard paralleling systems are often controlled by paralleling switchgear, this paralleling switchgear typically contains a programmable logic control (PLC) or another logic device that controls the sequence of operation of the system. The logic device in the paralleling switchgear can also perform the load management.

Load management may be performed by a separate load management system, which may provide metering or may use information from the paralleling switchgear controls to determine generator loading and frequency. A building management system may also perform load management, controlling the loads by supervisory control and eliminating the need for switches to interrupt the power to them.

Dead-field paralleling systems

Dead-field paralleling differs from standard paralleling in that all generators can be paralleled before their voltage regulators are activated and the alternator fields are excited.

If all generators in a dead-field paralleling system start normally, the power system reaches rated voltage and frequency with full power generation capacity available to supply the load. Because the normal dead-field paralleling sequence does not require a single generator to energize the paralleling bus, load management should not need to shed load during a normal system start.

However, as with standard paralleling systems, the starting and stopping of individual generators are possible with dead-field paralleling. If a generator is down for service or stops for another reason, the other generators may still be overloaded. Thus, load management may still be useful in these applications, similar to standard paralleling systems.

Dead-field paralleling is usually performed by parallel-capable generator controllers, but can also be performed by a paralleling switchgear installation. Parallel-capable generator controllers often provide built-in load management, allowing the load priorities to be directly managed by the controllers and eliminating the need for paralleling switchgear controllers.

Single Generator Systems

Single generator systems are typically less complicated than their paralleling counterparts. Such systems may use load management in the generator controller to control loads when subject to intermittent loads or load variations.


An intermittent load—such as chillers, induction ovens and elevators—does not draw continuous power, but can vary power requirements suddenly and significantly. Load management can be useful in situations where the generator is capable of handling a normal load, but under certain circumstances intermittent loads may increase the total load of the system above the maximum power capability of the generator, potentially hurting the power quality of the generator output or inducing a protective shutdown. Load management can also be used to stagger application of loads to the generator, minimizing the voltage and frequency variation caused by the inrush to large motor loads.

Load management may also be useful if local codes require a load control module for systems where the rated generator output current is less than the service entrance current rating.

Systems with Special Emissions Requirements

In some geographic areas, there are minimum load requirements for a generator anytime it is operating. In this case, load management could be used to keep loads on the generator to help meet emissions requirements. For this application, the power generation system is fitted with a controllable load bank. The load management system is configured to energize various loads in the load bank to maintain the generator system output power above a threshold.

Certain generator systems include a Diesel Particulate Filter (DPF), which typically needs to be regenerated. In some cases, engines will derate to 50% of rated power during a parked regeneration of the DPF, and could leverage the load management system to remove some loads during that condition.

Although load management can improve power quality to critical loads in any system, it may add delays before some loads receive power, increase the complexity of the installation and add a significant amount of wiring effort as well as parts costs, such as contractors or circuit breakers. Some applications where load management may be unnecessary are outlined below.

Properly Sized Single Generator

There is usually no need for a load management system on a properly sized single generator, as an overload condition is unlikely, and generator shutdown will result in all loads losing power, regardless of priority.

Paralleling Generators for Redundancy

Load management is generally unnecessary in situations where there are paralleling generators and the site power requirements can be supported by any one of the generators, as a generator failure will only result in another generator starting, with only a temporary interruption in the load.

All Loads are Equally Critical

On sites where all loads are equally critical, it is difficult to prioritize the loads, shedding some critical loads in order to continue providing power to other critical loads. In this application, the generator (or each generator in a redundant system) should be appropriately sized to support the entire critical load.

FAQ-surge-protection-device-12Damage from electrical transients, or surges, is one of the leading causes of electrical equipment failure. An electrical transient is a short duration, the high-energy impulse that is imparted on the normal electrical power system whenever there is a sudden change in the electrical circuit. They can originate from a variety of sources, both internal and external to a facility.

Not just lightning

The most obvious source is from lightning, but surges can also come from normal utility switching operations or unintentional grounding of electrical conductors (such as when an overhead power line falls to the ground). Surges may even come from within a building or facility from such things as fax machines, copiers, air conditioners, elevators, motors/pumps, or arc welders, to name a few. In each case, the normal electric circuit is suddenly exposed to a large dose of energy that can adversely affect the equipment being supplied power.

The following are surge protection guidelines on how to protect electrical equipment from the devastating effects of high-energy surges. Surge protection that is properly sized and installed is highly successful in preventing equipment damage, especially for sensitive electronic equipment found in most equipment today.

Grounding is fundamental

A surge protection device (SPD), also known as a transient voltage surge suppressor (TVSS), is designed to divert high-current surges to the ground and bypass your equipment, thereby limiting the voltage that is impressed on the equipment. For this reason, it is critical that your facility has a good, low-resistance grounding system, with a single ground reference point to which the grounds of all building systems are connected.

Without a proper grounding system, there is no way to protect against surges. Consult with a licensed electrician to ensure that your electrical distribution system is grounded in accordance with the National Electric Code (NFPA 70).

Zones of protectionFAQ-surge-protection-device-16

The best means of protecting your electrical equipment from high-energy electrical surges is to install SPDs strategically throughout your facility. Considering that surges can originate from both internal and external sources, SPDs should be installed to provide maximum protection regardless of the source location. For this reason, a “Zone of Protection” approach is generally employed.

The first level of defense is achieved by installing an SPD on the main service entrance equipment (i.e., where the utility power comes into the facility). This will provide protection against high energy surges coming in from the outside, such as lightning or utility transients.

However, the SPD installed at the service entrance will not protect against internally generated surges. In addition, not all of the energy from outside surges is dissipated to the ground by the service entrance device. For this reason, SPDs should be installed on all distribution panels within a facility that supply power to critical equipment.

Similarly, the third zone of protection would be achieved by installing SPDs locally for each piece of equipment being protected, such as computers or computer controlled devices. Each zone of protection adds to the overall protection of the facility as each helps to further reduce the voltage exposed to the protected equipment.

Coordination of SPDs

The service entrance SPD provides the first line of defense against electrical transients for a facility by diverting high-energy, outside surges to ground. It also lowers the energy level of the surge entering the facility to a level that can be handled by downstream devices closer to the load. Therefore, proper coordination of SPDs is required to avoid damaging SPDs installed on distribution panels or locally at vulnerable equipment.

If coordination is not achieved, excess energy from propagating surges can cause damage to Zone 2 and Zone 3 SPDs and destroy the equipment that you are trying to protect.

Selecting the appropriate Surge Protective Devices (SPD) can seem like a daunting task with all of the different types on the market today. The surge rating or kA rating of an SPD is one of the most misunderstood ratings. Customers commonly ask for an SPD to protect their 200 Amp panel and there is a tendency to think that the larger the panel, the larger the kA device rating needs to be for protection but this is a common misunderstanding.

When a surge enters a panel, it does not care or know the size of the panel. So how do you know if you should use a 50kA, 100kA or 200kA SPD? Realistically, the largest surge that can enter a building’s wiring is 10kA, as explained in the IEEE C62.41 standard. So why would you ever need an SPD rated for 200kA? Simply stated – for longevity.

So one may think: if 200kA is good, then 600kA must be three times better, right? Not necessarily. At some point, the rating diminishes its return, only adding extra cost and no substantial benefit. Since most SPDs on the market use a metal oxide varistor (MOV) as the main limiting device, we can explore how/why higher kA ratings are achieved. If a MOV is rated for 10kA and sees a 10kA surge, it would use 100% of its capacity. This can be viewed somewhat like a gas tank, where the surge will degrade the MOV a little bit (no longer is it 100% full). Now if the SPD has two 10kA MOVs in parallel, it would be rated for 20kA.

Theoretically, the MOVs will evenly split the 10kA surge, so each would take 5kA. In this case, each MOV has only used 50% of their capacity which degrades the MOV much less (leaving more left in the tank for future surges).

When selecting an SPD for a given application, there are several considerations that must be made:


Ensure that the SPD is designed for the zone of protection for which it will be used. For example, an SPD at the service entrance should be designed to handle the larger surges that result from lightning or utility switching.

System voltage and configuration

SPDs are designed for specific voltage levels and circuit configurations. For example, your service entrance equipment may be supplied three-phase power at 480/277 V in a four-wire wye connection, but a local computer is installed to a single-phase, 120 V supply.

Let-through voltage

This is the voltage that the SPD will allow the protected equipment to be exposed to. However, the potential damage to equipment is dependent on how long the equipment is exposed to this let-through voltage in relation to the equipment design. In other words, equipment is generally designed to withstand a high voltage for a very short period of time and lower voltage surges for a longer period of time.

The Federal Information Processing Standards (FIPS) publication “Guideline on Electrical Power for Automatic Data Processing Installations” (FIPS Pub. DU294) provides details on the relationship between clamping voltage, system voltage, and surge duration.

As an example, a transient on a 480 V line that lasts for 20 microseconds can rise to almost 3400V without damaging equipment designed to this guideline. But a surge around 2300 V could be sustained for 100 microseconds without causing damage. Generally speaking, the lower the clamp voltage, the better the protection.

Surge current

SPDs are rated to safely divert a given amount of surge current without failing. This rating ranges from a few thousand amps up to 400 kiloamperes (kA) or more. However, the average current of a lightning strike is only approximately 20 kA., with the highest measured currents being just over 200 kA. Lightning that strikes a power line will travel in both directions, so only half the current travels toward your facility. Along the way, some of the currents may dissipate to ground through utility equipment.

Therefore, the potential current at the service entrance from an average lightning strike is somewhere around 10 kA. In addition, certain areas of the country are more prone to lightning strikes than others. All of these factors must be considered when deciding what size SPD is appropriate for your application.

However, it is important to consider that an SPD rated at 20 kA may be sufficient to protect against the average lightning strike and most internally generated surges once, but an SPD that is rated 100 kA will be able to handle additional surges without having to replace the arrester or fuses.


All SPDs should be tested in accordance with ANSI/IEEE C62.41 and be listed to UL 1449 (2nd Edition) for safety.

Underwriters Laboratories (UL) requires certain markings be on any UL listed or recognized SPD. Some parameters which are important and should be considered when selecting an SPD include:

SPD Type

used to describe the intended application location of the SPD, either upstream or downstream of the main overcurrent protective device of the facility. SPD Types include:

Type 1

A permanently connected SPD intended for installation between the secondary of the service transformer and the line side of the service equipment overcurrent device, as well as the load side, including watt-hour meter socket enclosures and Molded Case SPDs, intended to be installed without an external overcurrent protective device.

Type 2

A permanently connected SPD intended for installation on the load side of the service equipment overcurrent device, including SPDs located at the branch panel and Molded Case SPDs.

Type 3

Point of utilization SPDs, installed at a minimum conductor length of 10 meters (30 feet) from the electrical service panel to the point of utilization, for example, cord connected, direct plug-in, receptacle type SPDs installed at the utilization equipment being protected. The distance (10 meters) is exclusive of the conductors provided with or used to attach SPDs.

Type 4

Component Assemblies -, the Component assembly consisting of one or more Type 5 components together with a disconnect (internal or external) or a means of complying with the limited current tests.

Type 1, 2, 3 Component Assemblies

Consist of a Type 4 component assembly with internal or external short circuit protection.

Type 5

Discrete component surge suppressors, such as MOVs that may be mounted on a PWB, connected by its leads or provided within an enclosure with mounting means and wiring terminations.

Nominal system voltageFAQ-surge-protection-device-6

Should match the utility system voltage where the device is to be installed


The Maximum Continuous Operating Voltage, this is the maximum voltage the device can withstand before conduction (clamping) begins. It is typically 15-25% higher than the nominal system voltage.

Nominal Discharge Current (In)

Is the peak value of current, through the SPD having a current waveshape of 8/20 where the SPD remains functional after 15 surges. The peak value is selected by the manufacturer from a predefined level UL has set. I(n) levels include 3kA,5kA, 10kA and 20kA and may also be limited by the Type of SPD under test.


Voltage Protection Rating. A rating per the latest revision of ANSI/UL 1449, signifying the “rounded up” average measured limiting voltage of an SPD when the SPD is subjected to the surge produced by a 6 kV, 3 kA 8/20 µs combination waveform generator. VPR is a clamping voltage measurement that is rounded up to one of a standardized table of values. The standard VPR ratings include 330, 400, 500, 600, 700, etc. As a standardized rating system, VPR allows the direct comparison between like SPDs (i.e. same Type and Voltage).


Short Circuit Current Rating. The suitability of an SPD for use on an AC power circuit that is capable of delivering not more than a declared RMS symmetrical current at a declared voltage during a short circuit condition. SCCR is not the same as AIC (Amp Interrupting Capacity). SCCR is the amount of “available” current that the SPD can be subjected to and safely disconnect from the power source under short circuit conditions. The amount of current “interrupted” by the SPD is typically significantly less than the “available” current.

Enclosure rating

Ensures that the NEMA rating of the enclosure matches the environmental conditions at the location where the device is to be installed.

FAQ-surge-protection-device-20Although often used as separate terms in the surge industry, Transients and Surges are the same phenomenon. Transients and Surges can be current, voltage, or both and can have peak values in excess of 10kA or 10kV. They are typically of very short duration (usually >10 µs & <1 ms), with a waveform that has a very rapid rise to the peak and then falls off at a much slower rate.

Transients and Surges can be caused by external sources such as lightning or a short circuit, or from internal sources such as Contactor switching, Variable Speed Drives, Capacitor switching, etc.

Temporary overvoltages (TOVs) are oscillatory

Phase-to-ground or phase-to-phase overvoltages that can last as little as a few seconds or as long as several minutes. Sources of TOV’s include fault reclosing, load switching, ground impedance shifts, single-phase faults and ferroresonance effects to name a few.

Due to their potentially high voltage and long duration, TOV’s can be very detrimental to MOV-based SPD’s. An extended TOV can cause permanent damage to an SPD and render the unit inoperable. Note that while ANSI/UL 1449 ensures that the SPD will not create a safety hazard under these conditions; SPDs are typically not designed to protect downstream equipment from a TOV event.

equipment is more sensitive to transients in some modes than othersFAQ-surge-protection-device-28

Most suppliers offer line-to-neutral (L-N), line-to-ground (L-G), and neutral-to-ground (N-G) protection within their SPDs. And some now offer line-to-line (L-L) protection. The argument is that because you don’t know where the transient will occur, having all modes protected will ensure no damage occurs. However, equipment is more sensitive to transients in some modes than others.

L-N and N-G mode protection is an acceptable minimum, while L-G modes can actually make the SPD more susceptible to overvoltage failure. In multiple line power systems, L-N connected SPD modes also provide protection against L-L transients. Hence, a more reliable, less complex “reduced mode” SPD protects all modes.

Multi-mode surge protective devices (SPDs) are devices which comprise a number of SPD components within the one package. These “modes” of protection can be connected L-N, L-L, L-G, and N-G across the three phases. Having protection in each mode provides the protection for the loads particularly against the internally generated transients where the ground may not be the preferred return path.

In some applications such as applying an SPD at a service entrance where both the neutral and ground points are bonded there is no benefit of separate L-N and L-G modes, however as you go further into the distribution and there is separation from that common N-G bond, the SPD N-G mode of protection will be beneficial.

While conceptually a surge protective device (SPD) with a larger energy rating will be better, comparing SPD energy (Joule) ratings can be misleading. MoreFAQ-surge-protection-device-6 reputable manufactures no longer provide energy ratings. The energy rating is the sum of surge current, surge duration, and SPD clamping voltage.

In comparing two products, the lower rated device would be better if this was as a result of a lower clamping voltage, while the large energy device would be preferable if this was as a result of a larger surge current being used. There is no clear standard for SPD energy measurement, and manufacturers have been known to use long tail pulses to provide larger results misleading the end users.

Because Joule ratings can easily be manipulated many of the industry standards (UL) and guidelines (IEEE) do not recommend the comparison of joules. Instead, they put the focus on the actual performance of the SPDs with a test such as the Nominal Discharge Current testing, which tests the SPDs durability along with the VPR testing that reflects the let-through voltage. With this type of information, a better comparison from one SPD to another can be made.