Metal oxide varistors (MOVs) are widely used for surge protection on the AC mains ports of electronic products. They are also commonly used in external surge protectors intended for use on the AC mains. MOVs have been very popular for many years due to their low cost and their remarkable ability to handle large surge currents.
Under existing safety standards in the U.S. and the European Union (EU) that are based on IEC 60950-1, Information Technology Equipment, the requirements for MOVs have been fairly stable for several years and are relatively modest. However, in both the U.S. and the EU, safety standards based on IEC 60950-1 are currently scheduled to be withdrawn in December 2020, and will be replaced by new standards that are based on IEC 62368-1, Audio/Video, Information Technology and Communication Equipment.
The new MOV requirements in IEC 62368-1 are more challenging to meet, so manufacturers need to prepare for the transition. Simply complying with the earlier MOV requirements based on IEC 60950-1 will not guarantee compliance with new standards based on IEC 62368-1.
Unfortunately, the new MOV requirements in IEC 62368-1 have caused much confusion due to the wording of Annex G.8 for MOVs. The intent of this article is to clear up this confusion to the extent possible. Note that this is just the author’s interpretation of IEC 62368-1, and other interpretations are possible.
What is an MOV?
An MOV is a 2-terminal device that has bidirectional electrical behavior similar to a configuration of back-to-back avalanche diodes. For example, an MOV rated to stand off 400 Vdc will typically conduct less than 1 mA at 400 Vdc, but it will conduct progressively more current at higher applied voltages.
The current might reach 1 A at 600 Vdc, 100 A at 700 Vdc, and 1000 A at 900 Vdc. There is not a sudden turn-on threshold. Rather, the conducted current increases exponentially as the applied voltage increases, and decreases as the applied voltage is reduced. Figure 1 shows some representative MOVs.
Alternatives to MOVs
Another common surge suppression component that can survive very high surge currents is the gas discharge tube (GDT). GDTs achieve their impressive current handling ability by having an abrupt trigger voltage at which they switch into a conducting state that is almost a short circuit. This is sometimes referred to as a “crowbar” characteristic. Note that when a two-terminal surge protection component is conducting, the instantaneous power dissipation in the component is:
Power Dissipation = (voltage across component) x (current passing through component)
So, unlike an MOV, the GDT tolerates large surge currents because, when triggered, the voltage across it drops to a low value. This keeps the instantaneous power dissipation low. The result is that for the same surge current capability, a GDT can be physically smaller than the corresponding MOV.
Once triggered into the conducting state, a GDT requires that the current drop to nearly zero to reset the GDT to the off condition. For applied 60 Hz AC waveforms, the current drops to zero twice per cycle, corresponding to every 8.3 ms, so this would appear to provide the required turn-off opportunity. Unfortunately, if an overvoltage condition keeps a GDT in the conducting state for more than a few seconds, the accumulated heat in the GDT will prevent it from turning off in response to the very short zero crossings of a 60 Hz AC mains waveform. So GDTs, by themselves, are not well suited as a substitute for MOVs connected to the AC mains.
Since an MOV does not have a crowbar characteristic, it must tolerate the simultaneous presence of high voltage and high current. This leads to very high instantaneous power dissipation on the MOV, but it provides reliable turn-off when the AC mains voltage returns to normal after the surge.
Key MOV Electrical Parameters
While the general behavior of an MOV resembles that of two back-to-back avalanche diodes, the amount of surge current that an MOV can survive is considerably higher. Since the turn-on characteristic of an MOV is gradual rather than abrupt, it is difficult to define a specific voltage at which the MOV is considered to be off. To address this soft turn-on characteristic, the industry assigns an MOV voltage rating that corresponds to the maximum AC rms voltage that the MOV will withstand continuously without conducting significant current. Both IEC 60950-1 and IEC 62368-1 require that the rated voltage of the MOV be at least 125% of the rated voltage of the equipment. So, for a 240 V rms mains circuit, the rated voltage of the MOV must be at least 300 V rms.
At normal applied mains voltages of 240 V rms, an MOV rated at 300 V rms will conduct less than 1 mA. However, if a surge is applied across the MOV, the MOV will momentarily conduct far more current and will limit the surge to typically less than 1 kV. Given that lightning surges on the AC mains can exceed 6 kV and can have peak currents that exceed 3000 A, an MOV’s ability to limit such surges to less than 1 kV without damage is a very helpful first line of defense for surge protection.
While the nominal voltage rating for an MOV is usually the first parameter that a design engineer selects, the current handling capability is an often-overlooked consideration. In Figure 1, all of the MOVs shown are rated to continuously withstand 300 V rms (424 V peak). All of them have a nominal 1 mA threshold of 470 V dc. The difference in physical size (and cost) is that the larger MOVs can handle higher surge currents without damage. And, for any given surge current, the larger MOVs will have a lower clamping voltage.
When tested with a standard 8/20 us impulse surge, the smallest MOV in Figure 1 can handle 100 surges of 1000 A without being damaged, while the largest MOV can handle 100 surges of 3000 A. Furthermore, when tested with a 1000 A surge, the smallest MOV will limit the surge voltage to 1200 V, while the largest one will limit the surge voltage to 900 V.
So, in addition to selecting MOVs with suitable turn-on thresholds, designers must also consider both the size and number of surges to which the MOV is likely to be subjected in its lifetime. While a physically smaller 300 V rms MOV costs less than a larger one, it may not hold up well in real-world applications. Some of the requirements and tests in IEC 62368-1 are specifically designed to prevent designers from using undersized MOVs. To comply with the surge tolerance requirements in IEC 62368-1, most AC mains applications will require an MOV with a minimum disc diameter in the range of 14 mm to 20 mm.
MOV Failure Modes
As noted above, MOVs have a soft turn-on characteristic, and typically conduct a small amount of leakage current even at applied voltages that are well below their nominal threshold voltage. If an MOV is subjected to surges beyond its rating, permanent damage can occur that causes the leakage current to increase. In some applications, leakage currents of only a few mA can present a shock hazard.
Furthermore, if this leakage current gets high enough, there will be self-heating inside the MOV. As noted earlier, the instantaneous power dissipated in an MOV is the applied voltage times the current through the MOV. When an MOV is connected continuously across the AC mains, this self-heating can create positive feedback where higher leakage current leads to higher self-heating, which leads to even higher leakage current.
Subsequent surges can further accelerate this failure mode. At some point the MOV will go into a thermal runaway mode that generates considerable heat and destroys the MOV. In some situations, the heat produced by the MOV can cause nearby materials to catch fire. Figure 2 shows an example of damage from an overheated MOV.
Why are MOV Requirements Being Changed?
The evolving safety requirements for MOVs are directed at addressing two distinct aspects of the above MOV failure modes:
- Electric shock
- Fire hazard
If an MOV is connected between the AC mains and a reliable protective earth, an increase in the MOV leakage current will simply result in more leakage current being conducted to earth ground. However, if the earth ground connection is not reliable and is somehow not connected to earth, the leakage current through the MOV will allow conductive parts connected to the earth ground to rise to the AC mains voltage. This can present an electric shock hazard.
Regardless of whether the equipment is properly grounded, an MOV will typically generate a lot of heat once it goes into thermal runaway. Under certain conditions, this excessive heat can start a fire. So, safety standards are evolving to address the fire hazard as well as the risk of electric shock.
Requirements that Address Electric Shock Hazards
Most ordinary office and household equipment powered by the AC mains use the familiar plug that is inserted to a standard wall outlet. Equipment that uses this type of AC mains connection is referred to in safety standards as “Pluggable Type A” equipment.
Type A plugs can have either two or three contact pins. The 2-pin version only connects to the AC mains, and is used when the equipment does not require a ground connection to ensure safety. The 3-pin version adds a ground pin for connection to earth ground. Often the ground connection for such equipment consists solely of this third pin on the AC mains plug.
It is known that sometimes the ground connection in the wall outlet is not properly connected to earth ground. Furthermore, some users will use a “cheater adapter” to connect a 3-prong plug to a wall outlet that accepts only 2-prong plugs. For these reasons, the earth ground connection achieved with a Pluggable Type A plug is not considered to be reliable.
As noted previously, if an MOV is connected from the AC mains to protective earth, the leakage current through the MOV will simply flow to protective earth. However, if the intended earth connection is not made, steps need to be taken to prevent possible electric shock to users when they touch conductive parts that are supposed to be grounded.
For Pluggable Type A equipment, the most common solution for preventing the MOV leakage current from becoming an electric shock hazard is to place GDT in series with the MOV. A GDT has almost no leakage current until the applied voltage gets close to the GDT turn-on voltage. So, if the GDT has a nominal turn-on voltage of 300 V rms, no significant leakage current will flow for an applied AC mains voltage of 240 V rms.
Some readers may wonder why it is not possible to simply use a GDT by itself without the MOV. As noted earlier, GDTs have difficulty turning off after a sustained surge event that has caused heating of the GDT electrodes. For situations where there is always 240 V rms across the protection arrangement, the GDT needs some help from the MOV to turn off.
Requirements that Address Fire Hazards
To limit the risk of fire, several options appear in the existing standards:
- Put a simple fuse in series with the MOV to limit the maximum possible self-heating;
- Couple a thermally-activated series fuse to the MOV package;
- Keep the MOV sufficiently far away from any combustible material; or
- Put the MOV in a fire enclosure.
Note that Option 1 has some practical limitations because any surge currents that the MOV is intended to handle must pass through the fuse. It is very difficult to design a fuse that will open at a sufficiently low current to control self-heating in the MOV, while also not fusing open for the typical surge currents that the MOV is intended to conduct.
Option 2 is an increasingly common compliance method. Several vendors of MOVs offer thermally protected MOVs (TP-MOVs) that have a thermally activated thermal fuse coupled to the body of the MOV. Figure 3 shows two examples of TP-MOVs. The slight bulge on the side of the red TP-MOV is the thermal fuse. In the black TP-MOV, the MOV and an external thermal fuse are closely coupled within the enclosure.
Options 3 and 4 are commonly used in products where there is adequate space to accommodate them.
Key Exemptions from Certain MOV Requirements
The above discussion has addressed the most common configuration where a Pluggable Type A mains connector is the sole means of grounding, and the MOV turn-on thresholds are set to just above the peaks levels of the AC mains voltage.
The need to control MOV leakage current with a series GDT is significantly reduced if the equipment has a reliable ground. The requirement for a reliable ground can be met in several ways, such as requiring that a trained professional install the equipment, or by using an industrial Pluggable Type B mains connector.
For limiting the risk of fire, some standards will exempt an MOV that has a turn-on threshold far above the normal AC mains voltage. The rationale is that, if the likelihood of surge damage to the MOV is low, the risk of fire is adequately addressed. Using this option typically requires the MOV to have a nominal turn-on threshold that is above 1500 V rms, which has the effect of potentially limiting the usefulness of the MOV surge protection.
These exemptions will not be discussed further here, but they are worth investigating if the intended application might qualify.
Brief Review of Standards Development
The International Electrotechnical Commission (IEC) is a standards organization that includes over 100 member countries worldwide. The IEC publishes a wide range of reference standards including IEC 60950-1, Safety of Information Technology Equipment, and IEC 62368-1, Audio/Visual, Information and Communication Technology Equipment.
By themselves, these IEC standards have no regulatory power. However, member countries typically use the IEC standards as the basis for their national regulatory standards. This is how we end up with EN 60950-1 and EN 62368-1 in the EU, and UL 60950-1 and UL 62368-1 in the U.S.
IEC member countries typically adopt the latest version of an IEC standard after a delay of one or more years, since this requires a national technical review and sometimes legislative action to give the national standard the force of law. In adopting an IEC standard, some member countries make minor revisions to the IEC text. So, while all national versions of a given IEC standard are substantially identical, there can be minor differences among them.
The resulting differences in adoption dates and technical content make it difficult to make general statements about what is required in a specific country on a specific date. To keep things simple, this article will discuss only the IEC versions of 60950-1 and 62368-1. It turns out that even this apparent simplification does not completely resolve the problem.
Evolution of IEC 60950-1
The First Edition of IEC 60950-1 was originally published in 2001. The First Edition contained no explicit requirements for the use of MOVs in AC mains circuits.
A Second Edition was published in 2005. In the 2005 Second Edition, the use of MOVs in AC mains circuits was greatly restricted. MOVs were permitted to be placed across the AC mains, provided that a fuse was connected in series with the MOV. For MOVs connected between AC mains and earth ground, the earth ground connection had to be “reliable.”
Note that the requirement for a reliable earth ground ruled out the use of MOVs connected from AC mains to the ground pin on the AC mains plug of ordinary Pluggable Type A office and household products.
The 2005 Second Edition was amended in 2009 to allow GDTs to be placed in series with MOVs when used with reliably grounded equipment, but the amendment did not allow this series combination to be connected from the AC mains to the ground pin of Pluggable Type A equipment.
A second amendment in 2013 allowed an MOV-GDT series combination to be used between the AC mains and the ground pin of Pluggable Type A equipment, provided that the GDT alone could pass the following tests:
- The GDT had to pass the electric strength test for basic insulation; and
- The GDT’s external construction had to meet the creepage and clearance requirements for basic insulation.
For a typical application in a 240 V rms mains circuit, the requirements for basic insulation in IEC 60950‑1 include an electric strength test of 1500 V rms, a creepage distance of at least 2.5 mm, and a clearance distance of at least 2.0 mm.
So, it was finally allowable to connect a series MOV-GDT from the AC mains to ground of ordinary Pluggable Type A equipment, but the electric strength test for the GDT meant that the GDT had to have a breakdown threshold that exceeded 1500 V rms.
Evolution of IEC 62368-1
The intent behind IEC 62368-1 is to eventually replace IEC 60950-1 (as well as IEC 60065, which will not be discussed here) with a standard that approaches safety compliance using a different conceptual framework. IEC 60950-1 is generally regarded as a “prescriptive” standard that presents product designers with a set of prescribed design rules.
In IEC 62368-1, an attempt has been made to simply identify a set of known safety hazards, and then give designers the choice of several options for providing suitable protection from these hazards. This is why IEC 62368-1 is commonly referred to as a “hazards-based” standard. Designers who are only familiar with applying IEC 69050-1 will likely have some difficulty adjusting to the hazards-based framework of IEC 62368-1.
The First Edition of IEC 62368-1 was published in 2010, and a Second Edition was published in 2014. A Third Edition has been finalized for 2018 and is likely to be issued by the time this article goes to press.
In the 2010 First Edition of IEC 62368-1, the requirements for MOVs used in AC mains circuits were more extensive than those in IEC 60950-1. Specific surge tests were added to ensure that the MOV could tolerate expected lightning surge currents of 8/20 us duration, and long-term overloads of several hours.
And, for the first time, detailed requirements were added to address the risk of fire from an MOV. Designers were provided the options of using a protective fuse of 10 A maximum, a restricted area surrounding the MOV, or a fire enclosure. For MOVs rated at voltages less than twice the maximum rated voltage of the equipment, a series of long-term overload tests were applied for up to four hours at voltages of twice the rated voltage of the equipment. For this long-term overload test, the circuit had to respond by becoming an open circuit.
In the 2014 Second Edition of IEC 62368-1, the requirements for MOVs were further expanded. One change was to require that any MOV connected between mains and earth ground of Pluggable Type A equipment must have a GDT connected in series with the MOV. The series GDT had to meet the requirements of basic insulation with regard to external creepage and clearance distance. Furthermore, the GDT had to meet the electrical strength test for solid insulation used in basic insulation.
This had the effect of increasing the required electrical strength of the GDT to 1768 V rms, as opposed to the 1500 V rms test that had applied under IEC 60950-1. It is not clear whether this change was intentional.
A second change expanded and clarified the long-term overload tests. A third change was the addition of a temporary overvoltage test of up to five seconds. For this test, it was allowable, but not required, for the circuit to fail open.
In the 2018 Third Edition, much of the wording of Annex G.8 has been revised, but the underlying requirements are mostly the same. Some of the specific component-level requirements allow the use of the 2017 edition of IEC 61643-331 as an alternative to IEC 61051-2. For the temporary overvoltage test, reference is now made to the 2011 edition of IEC 61643-11, rather than specifying all the test details directly within Annex G.8
In countries that have adopted a national version of IEC 62368-1, the current national version is generally based on the 2014 Second Edition of IEC 62368-1. This will begin to change after the 2018 Third Edition is issued.
Currently, manufacturers have the option of using national versions of either the Second Edition of IEC 60950-1 (as amended) or national versions of IEC 62368-1. At some point, presently scheduled for December 2020, IEC 60950-1 will be withdrawn and manufacturers will no longer have the option of using the standard to demonstrate safety compliance for new products.
It is a common misconception that any product that complies with IEC 60950-1 will also comply with IEC 62368-1. While this was a general goal for the First Edition of IEC 62368-1, some conflicts were present even in the First Edition. Considerably more conflicts were introduced in subsequent editions of IEC 62368-1. So, as the date approaches when IEC 60950-1 is withdrawn, it is important to be prepared to meet the requirements of IEC 62368-1.
Representative Protection Circuits Containing MOVs
The discussion that follows will compare representative AC mains protection circuits that comply with IEC 60950-1 (Second Edition, as amended through 2013) and representative circuits that comply with IEC 62368-1 (Third Edition, 2018). The discussion will be limited to Pluggable Type A equipment. Figure 4 shows four different protection circuits that illustrate the range of possible solutions.
Note that all the circuits in Figure 4 are for an AC mains voltage of 240 V rms. For a design that supports only the 120 V rms AC mains used in North America, some of the stated component voltage ratings could be lower. Also note that the stated voltage ratings for both the MOVs and GDTs are the AC rms stand-off voltage. While MOVs are commonly rated using an AC rms voltage, GDTs are commonly rated for a DC trigger voltage, so designers must keep this distinction in mind when selecting components.
Circuit A shows a very simple circuit that is permitted under IEC 60950-1. Note that since there are no MOVs connected between the AC mains and ground, there is no issue with electric shock hazards created by the MOV. An MOV connected across the AC mains can have high leakage current without presenting a risk of electric shock.
In IEC 60950-1, the fuse is apparently called out because of a desire to limit the risk of fire. However, the requirements for this fuse are not specified other than the requirement that the fuse must have “adequate breaking capacity.” It appears that the term “adequate breaking capacity” might refer to the rated current of the product, rather than any specific characteristic of the MOV.
Circuit B shows how Circuit A would typically be altered to comply with the Third Edition of IEC 62368-1. The key difference is that the MOV has been replaced with a TP-MOV. The use of a TP-MOV is not explicitly required in IEC 62368-1, but the tests that are used to evaluate the risk of fire are very difficult to pass without using a TP-MOV.
It is important to note that while Circuit A and Circuit B are shown being used with an ungrounded mains plug, they can also be used with equipment that has a grounded mains plug. The key limitation for these two simple circuits is that there cannot be any MOVs connected between the AC mains and the ground pin of the AC mains plug.
Circuit C shows the much more complex configuration typically used to comply with IEC 60950-1 when there are MOVs connected to earth ground. Compared to Circuit A, the first change is that fuse F3 has to be added to limit the current through MOV-3. In addition, GDT-1 and GDT-2 have been added to block the MOV leakage current that could otherwise lead to electric shock if the equipment’s ground connection is missing.
Circuit D shows how Circuit C would typically be modified to comply with IEC 62368-1. The key change is that the three MOVs in Circuit C have been converted to TP-MOVs. Another change is that the AC rms stand-off voltage of the GDTs has been increased.
It is useful to note that the more complicated arrangements of Circuits C and D are not necessarily required for adequate surge protection. Due to other requirements for safety isolation, most mains-connected power supplies have an isolation barrier rated at 3000 V rms that separates the power supply output circuits from the AC mains inputs. It is usually not difficult or expensive to ensure that this isolation barrier can withstand common mode surges (surges applied between the AC mains and earth ground) of 10 kV peak.
A surge tolerance of 10 kV peak for common mode surges is usually sufficient to eliminate the need for having surge protection components connected between the AC mains and earth ground. In most cases, the cost of upgrading the isolation barrier to withstand 10 kV peak will be far less than the cost of adding surge protection components between the AC mains and ground.
Note that most applications will still require some form of surge protection across the AC mains, similar to what is shown in Circuits A and B. This is because the input circuits of most power supplies connected to the AC mains have active electronics connected across the AC mains, and these electronics need to be adequately protected from differential surges across the AC mains.
Some Comments about MOV Requirements in IEC 62368-1
The MOV requirements in IEC 62368-1 are only a few pages long, but they are difficult for ordinary design engineers to understand. While the two main issues that the MOV requirements are trying to address are electric shock and risk of fire, it is not always clear which of these two hazards a given requirement is trying to address.
Indeed, a key goal of this article is to help readers of IEC 62368-1 understand the underlying safety concerns that the new requirements are trying to address. This understanding can be quite helpful when attempting to interpret the stated requirements. In particular, knowing the underlying concerns is helpful for understanding the various exemptions and alternate solutions that are permitted.
Another problem with the current version of the MOV requirements in IEC 62368-1 is an inconsistent use of terms and definitions. For example, certain tests that apply to an MOV may not apply to a TP-MOV or to an MOV connected in series with a GDT. It would be helpful if the term “MOV” could be clearly distinguished from “a protection circuit that contains an MOV.”
In some other safety standards, a clear distinction is made between a surge protection component (a single component) and a surge protection circuit (two or more components connected together). This distinction is not always clear in the Third Edition of IEC 62368-1, and it creates room for different interpretations of the applicable requirements.
As a result, designers who expect to make the transition to IEC 62368-1 should plan to spend some time trying to understand how to comply with the MOV requirements, and should be prepared for possible differing interpretations among safety experts and test labs.
Summary
For several years, safety experts have been increasingly concerned with the possibility that overstressed MOVs could lead to electric shock hazards and also the possibility of fire. Starting with the 2005 Second Edition of IEC 60950-1, successive safety standards have expanded how these two MOV safety concerns are addressed.
At present, most designers of information technology equipment (ITE) have the option of using national
standards that are based on the 2013 version of IEC 60950-1, or alternate national standards that are based on either the 2014 or 2018 versions of IEC 62368-1. However, at some point in the future, most national standards based on IEC 60950-1 will be withdrawn, and the only applicable standards for ITE will be based on IEC 62368-1. In the EU and the U.S., the withdrawal of EN 60950-1 is presently scheduled for December 2020.
The important thing to keep in mind is that just because an existing design complies with IEC 60950-1 does not necessarily mean that the same design will comply with IEC 62368-1. The differences described here regarding MOVs are just one example of changes that will be required when transitioning to national standards based on IEC 62368-1.
