UPS/SurgeProtector FAQ/Information

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[H] Admin
Staff member
Aug 29, 2004
The level of protection you need will determine which of the following topologies is most suited to your needs.The main UPS topologies are :


Your computer is running at standard mains via a small limited AC Filter under normal circumstances (some systems do not have filters at all). After a power failure this type of UPS system switches, via a small relay, over to a simple inverter to provide power to allow you to save your open files and shutdown your computer.

The backup time is normally short and will depend on the VA rating (see definition above) of the UPS and the amount of power your equipment draws from it. This type of UPS will give you very limited or no protection against power surges, spikes and sags. Typical backup time = 5 - 10 Minutes maximum. The backup time cannot be extended.

This type of UPS System is aimed at the SOHO & small business market and is mainly used when you have repeated, short power failures but a reasonably good mains supply. This type of UPS system is not recommended for critical applications or areas where there is bad or high fluctuating mains supply.

Cost : Very Low


Your computer is running at standard mains via a small AC Filter under normal circumstances. After a power failure this type of UPS system switches, via a small relay, over to a simple inverter to provide power to allow you to save your open files and shutdown your computer. The system also normally includes a limited AVR (Automatic voltage regulator) which controls incoming mains fluctuations to a small degree.

The backup time is normally short and will depend on the VA rating (see definition above) of the UPS and the amount of power your equipment draws from it. This type of UPS will give you very limited protection against power surges, spikes and sags. Typical backup time = 5 - 10 Minutes maximum. The backup time cannot be extended. Our range is intelligent (RS232) but the automatic shutdown software and serial cable are not supplied as this is seldom required.

This type of UPS System is aimed at the SOHO & small business market and is mainly used when you often experience short power failures but a have a reasonably good mains supply. This type of UPS system is not recommended for critical applications.

Cost : Low


This type of UPS system will provide backup to enable you to save your work and shutdown your computer after a power failure and is also designed to regulate the output voltage and prevent, to a large degree, any spike, sags and blackouts from reaching your sensitive equipment (ranging from computers and telecommunications systems to computerised instruments).

Once again, the backup time will depend on the VA rating of the UPS (see VA rating definition above) and the amount of power your equipment draws from it. Typical backup time = 5 - 10 Minutes. With the relevant model, the backup time can be extended to several hours. Our range is intelligent (RS232) and comes with automatic shutdown software and serial cable.

Recommended for semi-critical applications and small to medium sized businesses. Only recommended for semi-critical applications if the UPS output is pure sinewave (as line interactive UPS systems are also available in stepped squarewave or modified sinewave).

Cost : Medium


This type of UPS system will provide protection and backup on power failure for some applications. Your computer system is always running via a constant voltage transformer which basically regulates the output voltage and also filters, to a large degree, spikes, sags, etc.

The single conversion and sinewave output provides a reasonable sinewave and in most cases, adequate protection for computers and any some other load. It is sometimes less efficient and older technology.

Typical backup time = 5 - 10 Minutes. This unit can be intelligent (RS232) and sometimes automatic shutdown software is available

This type of UPS System is aimed standard computer equipment where total protection is not necessary.

Cost : Medium high


This type of UPS system will provide total protection and backup on power failure for critical applications. Your computer system is always running on electronically produced power and it is not directly connected to the utility supply. The double conversion, double isolation and the pure sinewave form output ensure the cleanest, most compatible AC output for computers and any other critical load.

Typical backup time = 5 - 10 Minutes. Plug-in external battery packs are available to extend the backup time to several hours. This unit is intelligent (RS232) and comes with automatic shutdown software and serial cable.

This type of UPS System is aimed at the professional, medical/industrial and upper business market and is recommended when you have a very dirty, noisy & unreliable mains supply. It is also recommended for all critical Computer and medical applications. This type of system has the highest protection level available.

Cost : High


Ice Czar said:
and for those Inquisitors that are photochopped challenged :p


here you see why the Onlines are a different "Class"

Black is the "normal" path and grey the "fail" path
Onlines fail over to the mains and become just surge protectors
normally they are mixed with backup generators

the opposite of the other two that fail over to the batteries
onlines are constantly on and only "fail over" when they actually fail
the other two switch over in the event the mains fail


[H] Admin
Staff member
Aug 29, 2004
All surge protectors can be put into two main categories based on the way they operate: shunt mode and Series Mode

MOV Based Shunt mode Surge Supression


The important points for shunt mode protectors are:

* Cheap and simple to produce
* Limited lifetime
* Contaminate ground wire

Series Based Surge Suppression

The important points for Series Mode protection are:

* Unlimited lifetime
* No sacrificial components
* Unsurpassed voltage clamping
* Safety or reference ground is not contaminated

MOVs: Metal Oxide Varistors are commonly used for mains protection applications and are available in a wide range of clamping voltage and peak impulse current variations. MOVs are extremely cost effective components and have been proven by time to work efficiently in many applications. There are three main drawbacks in the use of MOVs namely their tendency to degrade with use, a relatively high terminal voltage when clamping high current impulses and a response time which could be considered slow when compared with silicon clamping technology. Despite these weaknesses, the MOV is perfectly suited to a number of applications.

* The ‘aging’ characteristic describes the degradation of the micro-granular structure of the material comprising the MOV. The result is an increase in leakage current drawn by the MOV at normal system voltages. If this current increase is suitably high, it is liable to drive the MOV into thermal runaway – a condition where the device is absorbing energy more quickly than it can be dissipated and it heats up to dangerous temperature levels. This hazard is effectively countered with the use of an effective thermal disconnect device which disconnects the MOV from the supply if thermal runaway occurs and should visually indicate the failure on the device on the outside of the casing. MOVs should, therefore, be chosen so that they will be exposed to surges which are well within the limits of their specified energy handling capability and should, like any other piece of industrial equipment, be checked and replaced if found to be faulty. I this is done correctly, the service lifetime of MOVs is increased dramatically and they provide a very cost effective surge protective solution.
* There is often confusion surrounding the quoted clamping voltage of a MOV and the fact that during a surge, the voltage across its terminals may reach three or four times this value. The clamping voltage is the voltage at which the MOV will begin to lower its impedance and begin drawing surge current. As this current flowing through the MOV increases, it raises the voltage across its terminals (the MOV is after all just a non-linear resistor with a very small but finite resistance and as the current through it increases, so must the voltage across it!). The fact is that in most mains applications, the equipment input is quite robust and will more than adequately be protected by a correctly chosen MOV product even if the terminal voltage does exceed the clamping voltage by 3 or 4 times for a short duration.
* The manufacturer quoted response time of the MOV is usually of the order of tens of nanoseconds which is once again more than adequate for most mains applications. Response time becomes critical when the surge is applied across sensitive silicon-based inputs which react quickly to a change in terminal voltage. The speed of response of the MOV material is almost instantaneous, however, and it is largely the inductance in the leads connecting the surge protector to the supply which causes a delay in response.


[H] Admin
Staff member
Aug 29, 2004
Metal Oxide Varistor Degradation
by Kenneth Brown

The purpose of this document is to provide an overview of the degradation process that can occur in metal oxide varistors (MOVs). MOVs are variable resistors primarily consisting of zinc oxide (ZnO) with the function of limiting or diverting transient voltage surges. MOVs exhibit a relative high energy absorption capability which is important to the long term stability of the device. The growing demand of ZnO varistors is due to the nonlinear characteristics as well as the range of voltage and current over which they can be used. This range is far superior to devices composed of other materials that were used prior to the development of MOVs.1

If MOVs are used within their well-defined specifications, degradation due to the environment is not likely. However, the environment that MOVs are used in is not well-defined. Low voltage ac mains are subject to lightning strikes, switching transients, voltage swells/sags, temporary overvoltages (TOVs) and other similar disturbances. Due to the variety of disturbances that MOVs are exposed to, degradation or failure are possible in many applications.

MOVs perform their intended function reliably and experience low failure rates when applied within their specified limits. For an MOV to operate without failure or degradation it must quickly dissipate absorbed energy and return to its standby operating temperature. The ability to dissipate energy to the environment will depend on the design of the environment itself—ambient temperature, ventilation, heat sinking, other component population and density, proximity of heat sources, weight of PCB conductor traces, presence of thermal cutoff devices, etc. Degradation and catastrophic failures may occur if an MOV is subjected to transient surges beyond its rated values of energy and peak current.

The life of an MOV is defined as the time required reaching a thermal runaway condition. The relationship between ambient temperature and the life of an MOV can be expressed by Arrhenius rate equation,

t = t0exp[Ea-f(V)]/RT

where :

(t) = the time to thermal runaway,

t0 = constant,

R = constant,

Ea = activation energy,

T = temperature in Kelvin,

and f(V) = applied voltage.

Most Arrhenius rate models impose increased voltage and/or elevated temperature to accelerate the reaction rate (i.e., degradation or time to thermal runaway) and do not adequately address the detrimental effects of surge history.2 Surge history, especially transient surges beyond rated maximums, are perhaps the greatest single contributor to reductions in varistor voltage, increased standby leakage current, and ultimate thermal runaway. When increased voltage is applied for durations longer than microseconds, physical and chemical changes occur within the many boundary layers of a multi-junction MOV device. As with single-junction semiconductor devices, these changes occur on electronic and atomic scales at rates determined by the diffusion rates of structural defects—electrons, electron holes, and interstitial vacancies and ions. The MOV’s joule heating increases rapidly and exceeds the MOV’s ability to dissipate heat causing a thermal runaway condition and ending the MOV’s effective life.

Metal Oxide Varistors Description
MOVs are bipolar ceramic semiconductor devices that operate as nonlinear resistors when the voltage exceeds the maximum continuous operating voltage (MCOV). The term varistor is a generic name for voltage-variable resistor. The resistance of an MOV decreases as voltage magnitude increases. An MOV acts as an open circuit during normal operating voltages and conducts current during voltage transients or an elevation in voltage above the rated MCOV.

Modern MOVs are developed using zinc oxide due to their nonlinear characteristics and the useful range of voltage and current is far superior to silicon carbide varistors. The characteristic feature of zinc oxide varistors is the exponential variation of current over a narrow range of applied voltage. Within the useful varistor voltage range, the voltage-current relationship is approximated by the expression:2


I = current in amperes,

V = voltage,

A = a material constant, and

a = exponent defining the degree of nonlinearity.

MOV Failures
MOVs have a large, but limited, capacity to absorb energy, and as a result they are subject to an occasional failure. The significant MOV failure mechanisms include: electrical puncture, thermal cracking, and thermal runaway, all resulting from excessive heating, in particular, from non-uniform heating. Non-uniform joule heating occurs in MOVs as a result of electrical properties that originate in either the varistor fabrication process or in the statistical fluctuations of properties that generally occur in polycrystalline materials.6

There are three basic failure modes for MOVs used within surge protective devices.3

1. The MOV fails as a short circuit.

2. The MOV fails as an open circuit.

3. The MOV fails as a linear resistance.

Note: Small-diameter MOVs that initially fail short circuit are likely to fail as an open circuit due to the absorption of large continuous current within the MOV.

The short-circuit failure of an MOV is usually confined to a puncture site between the two electrodes on the disk. Large fault current can create plasma inside the ceramic, with temperatures high enough to melt the zinc oxide ceramic. This failure mode can be caused by long-duration overvoltage, such as switching from a reactive load or thermal runaway of the MOV connected to the ac mains.

Open circuit failures are possible if an MOV is operated at steady state conditions above its voltage rating. The exponential increase in current causes overheating and eventual separation of the wire lead and disk at the solder junction.

Degradation of MOVs
It is well-known that MOVs experience degradation due to single and multiple current impulses. The test results documented in Mardira, Saha and Sutton show that MOVs can be degraded from an 8/20us surge current at 1.5 times the rated MOV surge current. A 20 mm MOV with a 10 kA surge current rating will be degraded if a 15 kA single pulse surge current is applied.5

When MOVs degrade they become more conductive after they have been stressed by either continuous current or surge current. MOVs generally experience degradation due to excessive surges exceeding the MOV’s rating while in operation. However, many MOVs show no signs of degradation when operated below a specified threshold voltage. The degradation of MOVs is primarily dependent on their composition and fabrication, as well as their application or duty.

Degraded MOVs were found to have smaller average grain size and change in the diffraction peak position compared to a new sample.5 The non-uniform temperature distribution in the material is due to the development of localized hot spotting during the current impulse and the dissolving in some other phases.

In high current conditions the zinc oxide junctions of the MOV begin to degrade resulting in a lower measured MCOV or turn-on voltage. As the degradation continues, and the MOV’s MCOV continues to drop until it conducts continuously, shorting or fragmenting within several seconds.

One of the key parameters related to measuring degradation of a varistor is leakage current. Leakage current in the pre-breakdown region of an MOV is important for two reasons:

1. Leakage determines the amount of watt loss an MOV is expected to generate upon application of a nominal steady-state operating voltage.

2. The leakage current determines the magnitude of the steady-state operating voltage that the MOV can accept without generating an excessive amount of heat.

The total leakage current is composed of a resistive current and a capacitive current. The resistive component of current is thermally stimulated and is significant, since it is responsible for the joule heating within the device. The capacitive current is a function of the MOV’s capacitance value and the applied ac voltage. If an MOV is subjected to an elevated voltage at a specific temperature, the internal current increases with time. Conversely, if the MOV is subjected to an elevated temperature at a specific applied voltage, the internal current increases with time. This phenomenon is accelerated by higher operating stress, and is further aggravated by elevated temperatures. The life of an MOV is primarily determined by the magnitude of the internal current and its increase in temperature, voltage, and time. As the current increases, the amount of heat (if not allowed to dissipate) can rapidly raise the temperature of the device. This condition may result in thermal runaway that can cause destruction of the MOV.

Tests were performed to induce thermal runaway. Photo 1 is a 40 mm MOV with an MCOV rating of 130 volts ac. During the test 240 V ac were applied at 15 amps and the MOV ignited.

MOVs exhibit greater power dissipation at higher temperatures given a fixed voltage. This characteristic can lead to thermal runaway. If the increase in power dissipation of the MOV occurs more rapidly than the MOV can transfer heat to the environment, the temperature of the MOV will increase until it is destroyed.

MOVs degrade gradually when subjected to surge currents above their rated capacity. The end-of-life is commonly specified when the measured varistor voltage (Vn) has changed by + 10 percent.4 MOVs usually are functional after the end-of-life, as defined. However, if an MOV experiences sequential surge events, each causing an additional 10 percent reduction of Vn, the MOV may soon reach a Vn level below the peak recurring value for the applied Vrms. When this state is reached the MOV draws in excess of 1 mA of current during each half-cycle of the sine wave voltage, a condition tantamount to thermal runaway. In nearly all cases, the value of Vn decreases with exposure to surge currents. The degradation manifests itself as an increase in idle current at the maximum normal operating voltage in the system. Excessive idle current during normal, steady-state operation will cause heating in the varistor. Because the varistor has a negative temperature coefficient, the current will increase as the varistor becomes hotter. Thermal runaway may occur, with consequent failure of the varistor.

Littelfuse publishes varistor pulse rating curves that are shown in figure 3. The pulse rating curves plot the maximum surge current versus the impulse duration in seconds. It is noted that stresses above the conditions may cause permanent damage to the device.

Power Dissipation Ratings
If transients occur in rapid succession, the average power dissipation is the energy (watt-seconds) per pulse times the number of pulses per second. The power generated must be within the specifications shown in the chart above. Operating values must be derated at high temperatures as shown in figure 2. Note the rapid drop in rated value at temperature greater that 85C.

Varistors can dissipate a relative small amount of average power compared to surge power and are not suitable for repetitive applications that involve substantial amounts of power dissipation.

In the ANSI/IEEE C62.33 (1982) Standard for Surge Protective Devices the following is stated: "Single and lifetime pulse current ratings are appropriate tests of varistor surge withstand capability. In the absence of special requirements, energy ratings are recommended for use only as supplements to the predominant current ratings, and for application problems, which are more conveniently treated in terms of energy."7

Mean Time Before Failure (MTBF)
MTBF is a measure of the typical number of hours that a varistor will continuously operate, at a given temperature, before a failure will occur. Accelerated aging test techniques are used to understand and minimize the MOV degradation process.

To obtain MTBF value, accelerated aging testing techniques are used to acquire the necessary data accurately and reliably in a short period of time. The following is a brief explanation of how an accelerated aging test is perfomed:

1. Obtain 60-90 MOVs of the same production run.

2. Initially test the varistor voltage @ 1 mA, and the leakage current at the rated dc working voltage.

3. Place an equal count of 20-30 varistors in three separate temperature chambers that have the temperature set at 85°, 105°C, and 125°C.

4. Apply rated volts ac to the devices.

5. Every 100 hours remove varistors from testing chambers and measure the varistor voltage @ 1 mA, and the leakage current at the rated dc working voltage.

6. If the leakage current is greater than 100 uA (an arbitrary failure point) then remove the device from test and record the number of hours before failure.

7. Continue test until all devices have failed, or enough data has been collected to allow an accurate curve fit of the data.

8. Input data into a data analysis program and extrapolate the time before failure at other temperatures.

The amount of time required to perform this test can be long. Typically Maida tests its MOVs for 10,000 – 15,000 hours (416 – 625 days) before the test completes. The criteria used to signify a failure or the time between testing is arbitrary. The values shown in the procedure are what Maida uses to run its test. Other values can be used for these parameters if required.

Using the Arrhenius model, the data collected is imported into a spreadsheet and then exported into a curve-fitting program. Using the equations of the Arrhenius model, the MTBF for a given temperature is plotted and printed.

Accelerated testing has been used in reliability prediction models. Accelerated testing allows accurate reliability and failure rate estimation in a relatively short period of time. Failure rates obtained by subjecting electronic components to highly accelerated testing conditions are used to estimate failure rates under normal operating conditions.

Studies have shown that failure of many electronic components, and varistors in particular, are due to chemical degradation processes, which are accelerated by elevated temperature. The Arrhenius model has found wide application in accelerated testing technology. The Arrhenius model is applicable if:

1. The most significant stresses are thermal.

2. The expected mean life is logarithmically related to the inverse of temperature.

The model is generally described by the following equation:

ML = e A+B/T


ML: Mean life

A,B: Empirically derived constants from the life test data. The constant’s values depend on the characteristics of the material tested and the method.

T: Absolute temperature in Kelvin

The expected mean life (ML) of a varistor under normal operating temperatures is calculated using the above equation. The constants A and B are calculated from the (ML vs. temp.) graph developed during the accelerated testing experiment. The following two equations make calculating A and B easier:

B = (ln ML1 / ML2 ) ( 1 / T1 – 1 / T2 )-1


A = ln (MLI) – B / TI

T1 and T2 are high temperatures used during the accelerated testing, and ML1 and ML2 are the corresponding mean lives obtained from the accelerated test.

A varistor normally operates under 40°C, a standby current value less than 50 uA and a voltage (10-15%) less than the MCOV.

Mean life of an electronic component is the expected mean or average life of the component. Mean life is estimated by testing a sample of components for a period of time, then:

The number of "varistor hours" on test at any time can be computed by adding the lives, in hours, of the varistors that have failed up to the moment of estimation, to the lives, in hours, of the varistors observed that have not failed. The greater the number of item hours (testing time), the more confidence in the resulting estimates of mean life.

Figure 3 is an example of the MTBF Analysis completed recently for varistor Style D69ZOV251RA72.

The vertical axis (ML) is a label that signifies the mean life (or the average time before failure) of an MOV expressed in hours. The horizontal axis (1/TEMP IN K) is a label of the temperature expressed in the reciprocal of the temperature in Kelvin. As the reader can see from the example the ML, at 0.00299-1 (61.5°C or 334.5°K), equals 1e+06 or 1 million hours. The ML, at 0.0023-1 (161.8°C or 434.8°K), equals 100 hours.


[H] Admin
Staff member
Aug 29, 2004
In our critical UPS's we replace batteries every two years wether they need it or not.

Belwo are some articles on Sealed Lead Acid Batteries (SLA):

Below are a list of pointers put together from the general guidelines we have for prolonging life according to our maintence practices from my former employer and from the battery university articles:

Charge at proper voltage to prevent sulfation.
Operate batteries at or below 30c.
Always store lead acid batteries charged.
Never let the open cell voltage drop below recommended voltage.
Top and recharge batteries every six months if in storage.
Do not repeatedly deep discharge batteries.


[H] Admin
Staff member
Aug 29, 2004

The answer in most cases in microcomputer applications is no. Sine wave output UPS systems are really only necessary for continuous on-line UPS systems and certain directly supplied AC motor driven disk drives. Since most, if not all microcomputer systems are operating the CPU and disk drives off an internal DC power supply, sine waves are not necessary for emergency backup UPS systems. Let me explain why!

There are basically three waveform types used with UPS systems for use with microcomputers. They are square wave, sine wave, and quasi-sine wave pulse width modulated (PWM) stepped rectangular wave. All three types of waveforms must be tightly controlled as to frequency and should also have some form of maximum voltage "governor" limiting device to limit the maximum output average or RMS voltage to safe levels. This is necessary to prevent overheating of the computer power supply, especially for continuous on-line UPS units. While this is not as critical for emergency standby units it is a very desirable feature.

The cheapest waveform to provide is the square wave. Next in price range come your quasi-sine wave pulse width modulated stepped rectangular waves. And finally, you get to the higher priced sine wave units. Sine wave units use the same principles as square and stepped waveform units but they add an additional filtering device or transformer on the output to convert the waveform to an approximate sine wave.

Some people (those who sell only sine wave units) disparage all other type waveforms with scare tactics similar to some people I have been exposed to who sell fire alarm units door-to-door. This type waveform selected really depends on factors such as what type load will it be used with, is it a continuous on-line unit or an off-line emergency standby unit, and how much am I willing to pay to protect my system from crashing.

The backup requirements for modern microcomputer power supplies, which in turn supply DC voltage to power the CPU and the floppy or hard disk drives, is a lot different than the backup requirements for a main-frame computer or a disk drive running off a synchronous AC motor. Most, if not all, micro computer disk drive motors are DC driven and use phase locked loop (PLL) technology to maintain frequency and speed control and therefore do not need sine waves. Also the requirements are a lot different for waveform shape and tolerances if you are going to run your system off the UPS continuously on- line for 8-24 hours per day as opposed 2-10 minutes in an emergency condition to prevent a system crash due to momentary or temporary power failure.

As everyone knows the power generated and supplied by your local utility is a sine wave. This is because it is generated by rotating AC machinery and sine waves are a natural product of rotating AC machinery. Just because sine wave AC is provided by your utility at your outlet does not make it the only nor the best waveform to use to backup your computer. There are other factors to consider as outlined previously. In fact, for computer power supplies most engineers would tell you it would be better if smooth DC came out of the wall outlet instead of AC sine waves. Sine waves are great for power companies to make and transmit power over great distances but DC runs modern microcomputers. Interestingly enough it turns out that square waves, and quasi-sine wave pulse width modulated stepped rectangular waveforms, make better sources for rectification into smoother more ripple free DC voltage than do sine waves. The reason is that these "flat-topped" waveforms as I call them have a higher average output voltage value and the output voltage is at peak value longer than for "round-topped" sine waves. All engineers know that the charging of a DC power supply occurs at the peak of the waveform. Thus, since flat-topped waveforms are at the peak longer they keep the DC supply input fully charged longer and thus the DC output is smoother. This reduces ripple and improves the system power factor. This can be easily demonstrated by attaching an oscilloscope on the output of a DC power supply and observing the ripple with a sine wave input and then a square wave or stepped waveform input, all of equal RMS value. The DC is smoother with the flat-topped waveforms than for round-topped sine waves.
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