Surge protection circuit is the one referred by many as protector for voltage spikes in AC grid lines; however it is not limited particularly in AC grid lines. Surge protector or surge protection device is a device that will provide surge suppression or voltage spike suppression so that sensitive devices will not get damage.

Surge protector can handle voltage spikes as high as some kilovolt range (depending on the type of surge protection device). There are also surge suppressors that are intended only to handle few hundred volts, and so on. Although surge protector is design to withstand to high voltage spikes in a short period of time, it is not rated to handle high voltages in longer duration.

What is a Surge?

Surge in general is a sudden increase in level or magnitude from a normal or standard value. In electricity, surge is often used to describe voltage transient, voltage surge or voltage spikes. Voltage surge or spike or transient is not a permanent event. It is occurring only within a short period of time but more than enough to destroy devices if there is no counter measure.

Voltage surge is not only present in power lines but also in circuits with inductive property. However, the voltage surge in the power lines is the most destructive one as it can go as high as few kilovolt range.

Below illustration shows a voltage surge on the AC power line.

AC line voltage surge

Surge protector for AC line transients is installed commonly in houses, offices and buildings to prevent appliances or devices from damage. It should be installed in the section where all devices or appliances get their sources. By doing so, all appliances will be protected by line surges and spikes. This approach is called universal surge protection. Universal surge protector may not be needed if all the appliances or devices have its local surge protection circuit.

Two Main Categories of Surge Protection Circuit used in Power Lines

1. Primary Surge Protector

Primary surge protection device is installed in the entry point of house, office or building electrical wiring. It will protect all the devices or appliances which connect the line after the entry point. In general, primary surge protector is very powerful; however, it is huge and bulky as well as expensive.

2. Secondary Surge Protector

Secondary surge protector is not as effective and powerful as the primary surge protector.

However, it is portable and convenient to use. Mostly, this type of surge protector is easily plugged in to the power outlets. It will offer protection only to the devices that derive its power from the power outlet by which the secondary surge protector is installed.

A diagram below is showing how the primary and secondary surge protectors are installed in a building.

Common types of Secondary Surge Protection Circuits

There are few known secondary surge protection circuits. One is the so called power strips. Power strips are easily plugged in to a power outlet. Aside from this, it comes with a multiple power outlets wherein multiple devices and appliances can plug into and being protected by power surge. The most important feature of a power strip is the ability to terminate power in the event of a voltage surge.

The other known type of secondary surge protector is the well-known UPS or uninterruptible power source. Some sophisticated UPS has built-in surge protector in it that provide same safety feature as the power strip can do.

How a Surge Protector Works?

There is a kind of a surge protection device

that can cut off power once there is a voltage surge. This type of surge protector is sophisticated, more complex and of course expensive. The basic components of this type are voltage sensor, controller and latch/unlatch circuit. Voltage sensor will monitor the line voltage, the controller will read the sense voltage and decide when to signal voltage termination to the latch/unlatch circuit. The latch/unlatch circuit is a controllable power contactor or power switch that can able to connect or disconnect the line voltage.

There is also a kind of surge protector that will not offer voltage shutdown but just clamping the voltage transient and absorbs the energy. This type of surge protection is commonly used as built-in surge protection such as in switching mode power supply. This type of protection is effective up to few thousand volts. This type of surge protection is best described in circuit as below illustration.

Surge protection using MOV

Surge protector 1 across AC LINE 1 and 2 is called differential mode surge suppression. While both Surge protector 2 and 3 are called common mode surge suppression. Differential mode surge suppression clamp any voltage spikes across AC LINE 1 and 2. It is called differential because it is installed across two hot wires. On the other hand, common mode is the term used for surge protector 2 and 3 since both are clamping voltage transients on individual hot wire with respect to the earth or ground. In not so tight surge requirements, surge protector 1 is already enough to pass the standard. However, for very strict requirements like a higher surge voltage, surge protector 2 and 3 are added.

Causes of Voltage Surge

There are several factors why voltage surge occurs. It can be due to a lightning, power system switching like capacitor banks, resonating circuits with switching devices, faulty wirings, and suddenly turning on and off switches, electric motors and other highly inductive appliances and devices. AC line voltage surge is present anywhere in the world. Therefore, it is recommended to protect devices and appliances from this destructive event.

Some Common Medium of Surge

These are the common path where surge or voltage spikes can enter the appliances or devices using it.

Power lines – This is the number one medium for surge as all electrical and electronic devices uses power from the AC line. AC line surge is common worldwide.

RF lines – this includes antenna. Antenna is susceptible to lightning strike. Lightning is able to put up a very high voltage spike in a short duration. When a lightning strikes an antenna, it will penetrate to the RF receiver.

Car Alternator – In automotive electronics, voltage surge is also defined. It is because the alternator is able to create a high voltage spike during load dump.

Inductive circuits/loads – any inductive circuits or loads always introduce surge voltage. Most often, this surge is called an inductive kickback.

Surge Standard Defined by IEC

IEC 61000-4-5 define the standard for AC power lines surge. Table below give specific explanations for the classes and voltage levels.

IEC 61000-4-5 surge classes

Based on this standard, the maximum voltage transient the device should withstand and pass is 4kV at class 4 (though there is class 5 but it still calls class 4).

The transient voltage defined by IEC 61000-4-5 is modelled with below illustration. It has 1.2usec rise while a pulse width of 50usec. Figure below is taken from below link from which is taken also from AN4275 of ST microelectronics. STMicroelectronics is always offering great solutions for your products. Visit their website here.

If you have trouble passing EMC on your product, is an expert on it. Visit them at

IEC 61000-4-5 surge voltage waveform

IEC 61000-4-5 also defines the short circuit current shapes as below figure. It has a 8usec rise and 20usec pulse width. Figure is taken from

IEC 61000-4-5 surge current waveform

Below table is corresponding level of surge current or short circuit current per class. The worst value is 2000A. Table is taken from

IEC 61000-4-5 surge current definition

What is this short circuit current specified by IEC 61000-4-5? To answer this question, let me start by saying that all equipment connected to power lines is required to have surge protection. Surge protection works by clamping the voltage transients to a safer level. Once the surge protection circuit clamps, there will be a short circuit path from the source to the protection device and back to the source ground.

VOltage surge in AC power line

How to Design Surge Protection Circuit

It is not difficult to design a surge protection device. Actually, built-in surge protections to some electronic equipment are can be a single device only. This can be a MOV or metal oxide varistor or transient voltage suppressors TVS. Supposing in below illustration, the surge protection 1 to 3 are can be MOV or TVS.

Surge protection circuit

Sometimes, a surge protection device in between AC lines is enough to pass the IEC standard. On few cases, surge protection circuit is needed across line and ground. This is especially at higher surge voltage requirement (4kV and up).

Using MOV as Surge Protection Device

Basic Properties

  • MOV stands for Metal oxide varistor; is commonly used surge protection in power lines
  • MOV is a voltage dependent resistor
  • MOV Operation is like a diode which is non-linear and non-ohmic current and voltage characteristics but bidirectional
  • Its operation is can also be compared to a bidirectional transient voltage suppressor TVS
  • When the clamp voltage is not reach, it is acting an open circuit

Device Selection

For universal 90-264Vac line, the usual MOV voltage rating will be 300Vrms. 300Vrms is the RMS or continuous applied voltage that the MOV can sustain. This is not the clamping voltage yet. For instance we are going to use TMOV14RP300ML2B7 from Littelfuse, its AC voltage rating is 300Vac but its clamping voltage is 775V at 50A peak current based on the datasheet. Littelfuse is offering great solutions for circuit protections. Visit them at

The next thing to verify is that the surge current rating of the MOV able to handle the level specified by Table 2 above (considering the maximum level). Based from the selected MOV datasheet below, at 2000A and 20usec pulse duration, the MOV is able to handle more than 15 strikes but less than 100 strikes. I put dashed line on the device graph estimating the 2000A.

Metal oxide varistor pulse current

Though the datasheet specified a clamping voltage, it may not anymore valid at 2000A. Below graph tells the corresponding clamping voltage at 2000A using the selected MOV. The intersection of the yellow lines is the clamping voltage. Take note that it is more than 1000V already. Ensure that all devices used in the equipment are able to stand this voltage level. Otherwise, consider another MOV with lower clamping voltage.

Metal oxide varistor clamp voltage

MOV Ideal Location for Power Line Surge Protection

MOV that acting as a surge protection device must be mounted very next to the fuse as below illustration. With this wiring, once the surge current is too much to handle by the MOV, the fuse will break and open the circuit and avoid possible catastrophic failure.

Where to install MOV

Surge Suppression in Automotive

As mentioned above, surge is not happening only to AC power lines. Voltage surge is also very common to automotive systems. Automotive system uses only Lead Acid Battery with a typical full charge voltage of around 12.9V for 6 cells in series with a 2.15V each cell. In calculation, a 14V maximum battery voltage is often used. This level is not destructive and devices with 30V rating are more than enough to survive in the long term. However, this perception is only correct at steady state but not during the so called “load dump”. Load dump is the term used to describe when the battery is being suddenly disconnected while it is being charged by the alternator. For a 12V system, load dump may result to as high as 120V spike and more than enough to destroy devices if not considered.

To counteract this load dump scenario, surge protection circuit like varistor is often used.

In automotive, load dump waveform is defined by ISO 7637 as below figure. The peak voltage is 125V maximum. The rise and pulse width duration (T1 and T) are longer compared to the standard defined by IEC 61000-4-5.

 IEC 61000-4-5 load dump waveform

Example on How to Select a Varistor for Low Voltage DC such as Automotive Systems

Design Requirements

Input: 24V DC

Current waveform for surge is 8/20μs; voltage is 1.2/50μs

Peak Surge Current: 800A

Should survive 40 surges

Sensitive devices to protect is rated 250V maximum

Define the Varistor DC Voltage

For a 24V system, do not select a 24V rated Varistor as well. Instead, include at least 20% safety margin. However, do not exaggerate the margin also since it will correspond to a physically big Varistor and a higher clamping voltage.


Varistor Voltage = 24V x 1.2 = 28.8V

Based from Littelfuse list of low voltage varistors, I would prefer to use the part with 31V DC


MOV options

Select the Part that Meet the Surge Current and Number of Pulses

Above parts with 31V DC rating is candidates. However, there are few more criteria to be satisfied. Let us consider the peak surge current and number of pulses and select the part that can satisfy it with margin.

Below is the impulse duration in microseconds versus the peak surge current capability in amps for a 14mm part listed in the table above. Based from the graph, at 800A, a 14mm part cannot meet the required number of pulses. Therefore, do not select this part.

Below is the graph for 20mm part. At 800A peak surge current, the device can guarantee more than 40 pulses requirement. Therefore, select the part with a 20mm size.

MOV pulse current

From the above table, there are two parts that has 20mm size. We’ll consider first V20E25P. As mentioned earlier, we cannot over choose the part because it will correspond to a higher clamping voltage.

Checking the Clamping Voltage

The final step is to check the clamping voltage. All we did so far will be useless if the maximum clamping voltage is beyond the requirement. Below is the maximum clamping voltage of 20mm parts. From the graph, V20E25P is the perfect device as a surge protection circuit.

MOV clamping voltage


  1. thanks very much,,, for this work.
    pls can i get a complete circuit to design a surge protector..

    1. There are many available in the net. You can simply follow them. a simple surge protection is using an MOV, VDR and TVS…For anticipating huge surge such as from the power line, it is better to buy off the shelf with safety standards rating…

  2. I hope to get any confirmation for my surge protector is ok or not with IEC 61000-4-5 regulation.
    Which things I have to check it? If I want to choice a suitable TVS diode for the class 4. (=Surge 4kV)
    When i checked datasheet for TVS diode. I can see clamping voltage(VCL), maximum current(IPPM), peak power(PPM). Except that I failed got any information yet.

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