A Surge Protective Device (SPD), also known as a surge suppressor, is a device designed to limit transient overvoltages and divert surge currents. It contains at least one non-linear component, such as a Metal Oxide Varistor (MOV) or a Gas Discharge Tube (GDT), to achieve this protective function. The performance requirements and testing methods for low-voltage power SPDs are primarily detailed in the IEC 61643-11 standard, while their selection and application principles are covered in IEC 61643-12.
Figure 1 – Metal oxide varistor
Based on their voltage-current (U-I) response characteristics, SPDs are classified into three types: voltage limiting, voltage switching, and combination type SPDs. This article will focus on the most common type used in low-voltage applications: the voltage limiting SPD.
The Fundamental Operating Principle of an SPD
The installation of SPDs is a critical element of a comprehensive Surge Protection Measure (SPM) strategy. By protecting a system against the effects of Lightning Electro-Magnetic Pulses (LEMP), SPDs significantly reduce the probability of internal system failures, thereby improving the surge withstand capability of protected equipment and enhancing overall system reliability.
Typically, an SPD is installed in parallel with the equipment it protects. Its operation can be understood in two distinct states:
Standby Mode (High Impedance): During normal operation, the system’s power-frequency voltage is below the SPD’s Maximum Continuous Operating Voltage (Uc). In this state, the SPD exhibits a very high impedance and is effectively in a “standby mode.” It does not interfere with the normal operation of the electrical system.
Activation Mode (Low Impedance): When a surge event, such as one caused by a lightning strike, occurs, the transient voltage on the line rises sharply. Due to its non-linear characteristics, the SPD’s impedance instantly drops to a very low level. This creates a safe path to divert the surge current to the ground, clamping the voltage at the equipment’s terminals to a safe level. Once the surge has passed, the SPD automatically returns to its high-impedance standby mode.
Figure 2 – Illustration of an spd clamping transient overvoltage
Technical Characteristics of Voltage Limiting SPDs
Voltage limiting SPDs are the most widely used type in the low-voltage market, commonly found in Type 2 and Type 3 applications.
Advantages:
● Cost-Effectiveness: They offer a high performance-to-cost ratio.
● No Follow Current: They do not trigger a power-frequency follow current after the surge has been diverted.
● Fast Response Time: Their activation is extremely rapid.
● Mature Technology: The manufacturing processes and application technologies are well-established.
Disadvantages:
● Susceptibility to Aging: They can degrade when exposed to repeated surges and Temporary Overvoltages (TOVs).
● Leakage and Residual Current: They have inherent leakage currents which can increase over time.
● High Capacitance: Their internal capacitance is relatively large.
● Limited TOV Withstand: Their ability to withstand Temporary Overvoltages is relatively poor compared to other technologies.
- Protection Principle
The protective action of a voltage limiting SPD is based on its continuous, non-linear impedance characteristic. In the absence of a surge, the SPD has high impedance. As the surge voltage and current increase, the SPD’s impedance smoothly and continuously decreases, which clamps the residual voltage at a specific protective level.
- Product Design Configurations
To meet diverse electrical design requirements, the internal MOVs of an SPD can be arranged in various configurations:
- Parallel Connection: Connecting multiple MOVs in parallel increases the SPD’s overall surge current handling capacity and provides redundancy. If one MOV channel fails, the others can continue to offer protection.
- Series Connection: Connecting multiple MOVs in series increases the SPD’s Maximum Continuous Operating Voltage (Uc). This allows the device to be used in higher voltage applications or to satisfy specific circuit requirements, such as the “Y-circuit” often used in photovoltaic (PV) systems to handle both common and differential mode surges.
Figure 3 – Series connection of multiple MOVs
Key Performance Tests for Voltage Limiting SPDs
SPDs must undergo a series of performance tests as stipulated by their relevant standards. For voltage limiting, voltage switching, and combination type SPDs, there are notable differences in the testing procedures. The following are key tests for voltage limiting devices.
- Voltage Protection Level (Up) Test
The Voltage Protection Level (Up) is a critical parameter that indicates the maximum voltage that will pass through the SPD to the protected equipment. The test procedure for Type I and Type II SPDs involves applying 8/20 µs impulse currents of varying peak magnitudes (e.g., 0.1, 0.2, 0.5, and 1.0 times the rated In or Iimp) in both positive and negative polarities. The highest measured residual voltage across the SPD’s terminals during these tests determines its official Up rating. This measured value must be less than or equal to the Up declared by the manufacturer.
For Type I SPDs, the test is performed using the peak impulse current (Iimp).
For Type II SPDs, the test is performed using the peak nominal discharge current (In).
- Residual Current (Ipe) Test
Residual current (Ipe) is the current that flows through the SPD’s Protective Earth (PE) terminal when installed according to the manufacturer’s instructions and energized at the reference test voltage (Uref).
It is important to distinguish Ipe from the MOV’s leakage current. Leakage current is typically measured at a DC voltage (e.g., at a percentage of the nominal varistor voltage, Vn) and is used to assess the MOV’s quality or degradation. In contrast, Ipe is measured at the AC reference voltage and includes the effects of:
● The system’s AC voltage and frequency (capacitive currents).
● The total number of MOVs connected in series or parallel.
● The specific internal circuit structure (e.g., CT1 or CT2 wiring configurations).
● The measurement of Ipe is primarily for ensuring proper coordination with Residual Current
● Devices (RCDs) in the electrical system. If the combined residual current from the SPD and other devices on the circuit is too high, it can cause the RCD to trip unnecessarily.
- Response Time
Response time is not a formal test parameter for an SPD. This is because the activation characteristics of the components are already inherently reflected in the measured Voltage Protection Level (Up). All else being equal, a slower response time would result in a higher measured residual voltage.
The theoretical conduction principle of a voltage limiting SPD is similar to that of a semiconductor; it is extremely fast, with no significant delay (<1 ns). However, due to the parasitic inductance present in any real-world test circuit, the measured response time is typically around <25 ns. The measured value is a reflection of the test setup’s limitations, not the actual intrinsic response time of the MOV component itself.
The Core Component: The Metal Oxide Varistor (MOV)
The performance of a voltage limiting SPD is almost entirely defined by its core active component: the Metal Oxide Varistor (MOV). An MOV is a non-linear ceramic resistor whose resistance changes dramatically with the applied voltage.
- MOV Manufacturing Process
The typical MOV is produced through a ceramic engineering process:
● Composition: The primary material is Zinc Oxide (ZnO), which is mixed with 5-10% of other metal oxide additives, such as Bismuth Oxide and Cobalt Oxide.
● Formation: This powder mixture is pressed into a block or disc shape (the “green” body).
● Sintering: The body is fired at a high temperature in a kiln. During this sintering process, the ZnO grains grow, and the additives form insulating layers at the grain boundaries, creating the unique microstructure essential for the varistor’s function.
● Metallization: Metal electrodes are applied to two opposite faces of the ceramic body.
● Finishing: Leads are soldered to the electrodes, and the entire component is encapsulated in a protective material, such as epoxy or lacquer, to provide electrical insulation and mechanical stability.
- MOV Working Principle
The MOV’s electrical behavior is determined by its microstructure, which consists of conductive ZnO grains separated by thin, insulating grain boundary layers formed by the additives.
Figure 4 – Visual representation of metal oxide varistor (mov) microstructure
In the Low-Voltage State: When the MOV is subjected to a low electric field (i.e., at normal system voltages below the SPD’s Uc), the grain boundaries act as high-resistance insulating barriers. The overall resistance of the MOV is therefore extremely high (in the megaohm range), and it behaves like an open circuit.
In the High-Voltage State: When the electric field becomes sufficiently large (i.e., during a voltage surge exceeding Uc), the grain boundaries experience a rapid “tunneling effect.” They break down and become highly conductive. This connects the millions of individual, highly conductive ZnO grains, creating a massive parallel network of conductive paths. The result is that the overall resistance of the MOV drops to an extremely low level in nanoseconds, allowing it to divert large surge currents.
MOVs come in various shapes, but for power SPDs, square or rectangular blocks are most common. Their shape provides a larger cross-sectional area, resulting in higher surge current capacity and better thermal management, albeit at a higher cost. For board-level EMC (Electromagnetic Compatibility) applications where surge currents are much lower, smaller, circular disc-type MOVs are typically used.
Figure 5 – Varied shapes of MOV chip
- MOV V-I Characteristic and Key Parameters
The most important property of an MOV is its highly non-linear voltage-current (V-I) characteristic. The curve is typically divided into three regions:
- Pre-breakdown Region: A high-resistance region where a small leakage current flows.
- Non-linear Region: The active protective region where a small increase in voltage causes a massive increase in current.
- Upturn Region: At very high currents, the resistance of the ZnO grains themselves begins to dominate, causing the voltage to rise again.
The key parameter used to specify an MOV is its Varistor Voltage (Vn). This is the voltage across the MOV when a specific current (e.g., 1 mA DC) is passed through it. Vn represents the “knee point” or the start of the non-linear conduction region.
Figure 6 – Metal Oxide Varistor Voltage-Current Characteristics
A critical point for SPD selection is understanding the difference between the MOV’s Vn and the SPD’s Maximum Continuous Operating Voltage (Uc). It is a common and dangerous mistake to directly match an MOV’s Vn to the system’s operating voltage. SPD manufacturers design their products by selecting an MOV with a Vn that is significantly higher than the intended Uc. This creates a crucial safety margin that accounts for:
● The normal manufacturing tolerances of the MOV’s Vn.
● The non-linear properties and required operating space.
● A buffer to ensure a long operational life and the ability to withstand Temporary Overvoltages (TOVs).
● Therefore, users should always select an SPD based on its stated Uc value as specified by the manufacturer in accordance with application standards like IEC 61643-12, rather than attempting to determine the internal MOV’s characteristics.
- SPD and MOV Degradation Characteristics
An MOV’s life is finite. Each surge it diverts, and any exposure to TOVs, causes a small, irreversible change in its physical structure, leading to degradation.
The underlying cause of this degradation is believed to be ion migration at the grain boundaries. Over time, this process leads to:
- A decrease in the Varistor Voltage (Vn).
- A corresponding increase in the leakage current at normal operating voltage.
The increased leakage current accelerates the aging process by generating more heat. If left unchecked, this can lead to a positive feedback loop known as thermal runaway, where the MOV continues to heat up until it is destroyed, which can result in several failure modes, including:
● Thermal Failure: A localized weak point in the MOV overheats and melts, causing a short circuit.
● Electrical Breakdown: A high-energy surge causes a flashover across or through the MOV.
● Mechanical Failure: The encapsulation cracks, or in extreme cases, the MOV ruptures due to the powerful electrodynamic forces of the surge current.
To manage the end-of-life behavior safely, a well-designed SPD must include a thermal disconnector. This component is thermally coupled to the MOV and is designed to activate if the MOV overheats. When it activates, it safely disconnects the MOV from the circuit, preventing a fire hazard. This ensures that the MOV fails in a safe manner, at which point the SPD must be replaced.