In any electrical installation, aside from faults like short circuits or overloads, there are phenomena of overvoltage that can compromise the safety of equipment and people. An overvoltage refers to an abnormal increase in electrical voltage above the nominal levels of the network. This increase, which can last from microseconds to extended periods, is capable of causing serious damage to electrical installations and devices if not properly controlled.
Below, we explain in detail what an overvoltage is, its types, common causes, the consequences it produces, and how to prevent it effectively, emphasizing the importance of electrical safety according to current regulations (such as the Spanish REBT and international standards).
What is an overvoltage?
An overvoltage is an increase in the electrical supply voltage above the established nominal value. In low-voltage domestic systems (for example, 230 V single-phase supplies common in Europe and the UK), any significant rise beyond the typical tolerance margin (about 10% above 230 V) is considered an overvoltage. In other words, it occurs when the voltage abnormally exceeds the levels for which equipment is designed. Overvoltages can be transient (very short duration, commonly called voltage spikes or surges) or permanent (more prolonged or virtually continuous). Each type has different origins and characteristics, but both represent a risk to the integrity of an electrical installation and its connected appliances.
Transient overvoltages (brief voltage spikes)
Transient overvoltages are sudden voltage increases of very short duration, typically on the order of microseconds to milliseconds. Despite their fleeting nature, they can reach extremely high amplitudes (hundreds or thousands of volts above normal). The most destructive source is usually atmospheric discharges, i.e. lightning strikes that hit near power lines or the lines themselves. A lightning strike induces an electromagnetic impulse that travels through the grid and can arrive at our sockets in the form of a voltage spike.
Other causes of transient overvoltages are switching operations in the electrical network: for example, abrupt disconnections or reconnections of highly inductive loads (motors, transformers) or capacitive loads, as well as switching operations in transformer centers or substations. Even a short circuit or a sudden fault in a nearby circuit can generate a transient impulse that is transmitted through the electrical system.
Due to their instantaneous nature, a transient overvoltage can damage sensitive electronic components in the blink of an eye. Semiconductors, printed circuits, and internal insulation of equipment can suffer puncture or degradation from the excessive voltage. Although the spike lasts only microseconds, the energy released in that instant may be enough to burn out an integrated circuit, ruin power supplies, or disrupt the operation of control systems. Moreover, repetitive surges (even if not extremely high) gradually fatigue the insulation and reduce the useful life of devices. For this reason, it is essential to protect connected devices—especially sensitive ones like computers, audiovisual equipment, or communication systems—against this type of overvoltage.
Permanent overvoltages (prolonged overvoltage)
Permanent overvoltages (also called *temporary overvoltages* at mains frequency) are voltage elevations that persist for several cycles of the AC waveform or even continuously until the cause is removed. In contrast to transient spikes, here the voltage rises and stays at an abnormal level (for example, on the order of 20–50% above nominal) for seconds, minutes, or longer, until some protective device intervenes or the fault is corrected. The most common cause of a permanent overvoltage is a failure in the neutral conductor of the installation or the distribution network.
When the neutral breaks or is disconnected in a three-phase system with unbalanced loads (typical in low-voltage networks), the phases no longer have a stable common reference. This causes some single-phase circuits to become subjected to the phase-to-phase voltage (instead of phase-to-neutral), roughly doubling the voltage that domestic equipment receives (rising from ~230 V to ~400 V). This sustained overvoltage places enormous electrical stress on appliances for which they were not designed.
Other situations that can generate permanent overvoltages include malfunctioning regulators in transformer substations (which could send incorrect voltage for a time) or wiring errors (for example, accidentally feeding a 230 V circuit with a higher-voltage line). Whatever the source, the effect is that all connected equipment receives an abnormally high voltage continuously.
This typically causes overheating in transformers, motors, and power supplies of devices, leading to increased current draw and heat dissipation. If the overvoltage is not cut off in time, windings can burn out, electrolytic capacitors may burst, and in general, catastrophic failures can occur in appliances. Fires can even be triggered due to the heat generated in conductors or equipment overwhelmed by the voltage.
| Aspect | Transient Overvoltage | Permanent Overvoltage |
|---|---|---|
| Typical duration | Microseconds – milliseconds (brief impulse) | Seconds, minutes, or until the fault is corrected |
| Voltage magnitude | Very high spikes (can reach kV) | Moderate but sustained rise (10%–100% above nominal) |
| Common causes | Atmospheric discharges (lightning), switching operations, start/stop of inductive loads | Neutral break, wiring errors, faulty regulator, prolonged network faults |
| Main effects | Instant damage to sensitive electronics, cumulative insulation degradation, equipment resets | Overheating of devices, destruction of appliances, elevated fire risk |
| Proper protection | SPD (surge protective devices) Type 1, Type 2 and Type 3 power strips | Permanent overvoltage protectors (POP) that disconnect the installation |
| Protector location | Main panel, sub-panels, and special multi-outlet strips (e.g., ION Series) | Main incoming board before the RCDs (residual current devices) |
| Key regulation | ITC-BT-23 (Spain): mandatory SPD installation depending on risk | Art. 16.3 REBT (Spain) and related guides: obligation to protect against over- and under-voltage |
| Residual risk if unprotected | Sudden failure of electronic boards and data loss | Massive destruction of appliances and potential fire |
Frequent causes of overvoltages
Summarizing the above, overvoltages in low voltage systems usually originate from three main causes, related to atmospheric or operational factors in the electrical network:
- Lightning and atmospheric phenomena: A lightning strike near power lines, on antennas, or on lightning rods connected to the network induces a transient overvoltage wave that travels through the installation. This is one of the most dangerous causes, since lightning-induced spikes reach very high values in a very short time.
- Network switching operations: Switching operations in the electrical system (for example, the sudden connection or disconnection of powerful motors, switching of power factor correction capacitors, or changes in the distribution network topology) generate transient overvoltages through electromagnetic transient effects. Likewise, faults such as short circuits or sudden circuit breaks under load produce voltage spikes that propagate through the system.
- Neutral break or poor neutral connection: This is the typical cause of permanent overvoltages. Loss of the neutral in polyphase systems causes an imbalance of voltages, raising the phase voltage in some circuits far above normal. Also, defects in transformers or wiring errors can cause a sustained voltage increase until the problem is fixed.
Consequences of overvoltages
The consequences of an uncontrolled overvoltage can be very serious for both electrical equipment and the installation itself and even the people relying on it. Among the main harmful effects of overvoltages (whether transient or permanent) are:
- Instant damage to electrical and electronic devices: A voltage spike can *destroy* sensitive components instantly. For example, integrated circuits, power supplies, chargers, electric motors or household appliances can be rendered unusable after an intense surge.
- Reduced lifespan and latent faults: Even when an overvoltage doesn’t immediately burn out equipment, it can *degrade* its internal components. Insulation subjected to repeated electrical stress is weakened, electronic components suffer strain and age prematurely, which leads to early failures weeks or months later.
- Fires and material damage: Both strong transient surges and permanent overvoltages can cause fires. A violent spike can induce sparks or burn insulation, while a permanent overvoltage generates overheating in cables and devices. This increases the risk of fire in electrical panels, appliances or power strips, endangering the safety of people and property.
- Supply interruptions and blackouts: If a severe overvoltage occurs, various protective devices (such as fuses, RCDs, surge protectors) may trip and disconnect the installation to prevent greater harm. This results in power cuts or unexpected equipment shutdowns. In industrial environments, these stoppages can lead to significant production losses or damage to sensitive processes.
- Data loss or IT system failures: In the digital age, an overvoltage can affect servers, computers, and storage systems, causing loss of unsaved information or even data corruption. IT and communication systems are often very sensitive to abrupt voltage variations, so a spike can knock out networks, routers, alarms and other critical equipment.
In summary, an overvoltage can cause anything from the inconvenience of having to replace a damaged appliance to catastrophic situations like electrical fires. Therefore, understanding its causes and effects leads us to the most important step: prevention.
How to prevent overvoltages and protect the installation
Preventing the destructive effects of overvoltages involves two fundamental strategies: on one hand, designing the installation with appropriate protective devices, and on the other, adopting good usage and maintenance practices. Below are the most effective measures to protect an electrical installation against transient voltage spikes and permanent overvoltages:
- Installation of transient overvoltage protectors: These are devices commonly known as SPDs (Surge Protective Devices) that are connected in parallel with the supply and divert voltage surges to earth. There are different types according to their capacity: Type 1 SPD (also called lightning current arresters) are usually installed at the main service entrance when there is a high risk of lightning (for example, if the building has a lightning rod or overhead supply lines exposed to storms). Type 2 SPD are placed in the main electrical panels of homes and buildings, acting against transient overvoltages of both external origin and internal switching; they are practically mandatory in any new or refurbished installation according to current regulations. Finally, Type 3 SPD (plug-in protectors or power strips) are located near sensitive equipment, providing point-of-use protection to absorb any residual surge that might pass through after the upstream SPDs have acted. This cascaded coordination (Types 1, 2 and 3) ensures an optimal protection level, gradually reducing the surge energy and shielding even the most delicate devices.
- Use of permanent overvoltage protectors: To deal with prolonged overvoltages, devices are used that disconnect the power to the installation in the event of a sustained overvoltage. These protectors monitor the network voltage and operate a cutoff switch (for example, a relay or shunt trip that opens the circuit) when they detect the voltage exceeding a set threshold for a brief period (for instance, more than ~265 V for some tenths of a second in a 230 V system). By isolating the internal installation from the grid, they prevent permanent overvoltages from damaging equipment or causing fires. Some models of permanent overvoltage protector include an auto-reset function, so they reconnect power once normal values are restored—useful for avoiding prolonged outages (for example, for refrigerators or critical equipment) without manual intervention.
- Sockets and power strips with built-in protection: In addition to panel-level protection, it’s advisable to individually safeguard expensive or sensitive electronics. For this, there are multi-outlet power strips with surge protection, such as Solera’s ION Series, which act as extension strips for plugging in computers, TVs, audio equipment, etc. These special strips incorporate MOVs (varistors) or other components that absorb voltage spikes, and in many cases also include fuses or thermal breakers against overload, providing extra defense right at the device’s connection point. Using these is a good practice in offices and homes with plenty of delicate electronics.
- Earthing and equipotential bonding: A proper earthing system is indispensable for surge protectors to work effectively. The earth connection should have a low resistance and comply with regulations, since SPDs divert the surge energy into the protective conductor. Likewise, it’s important to interconnect all the installation’s exposed metal parts (protective conductors, lightning rods, antenna masts, metal pipes) via an equipotential bonding network, so that in the event of a surge, the entire system rises and falls in voltage uniformly, minimizing dangerous differences in potential.
- Good usage and maintenance practices: Installers and technicians should follow the guidelines of the Low Voltage Electrical Code (REBT in Spain) and its technical guides (for example, ITC-BT-23) regarding surge protection. This includes sizing and selecting the appropriate protectors for the type of installation, its geographic location and exposure level (a rural storm-prone area has different needs than an urban setting). It’s essential to use devices certified according to international standards (such as IEC/EN 61643-11 for transient SPDs and EN 50550 for permanent overvoltage protectors) and to install them following the manufacturer’s instructions, respecting coordination between them and compatibility with other protective elements (for example, coordinating with circuit breakers and RCDs to avoid nuisance tripping). After a severe storm or significant surge, it’s recommended to check the status of the protectors – many include visual indicators that show if they have operated or need replacement. Additionally, in critical installations it’s wise to schedule periodic inspections of the protection systems.
Applicable regulations and obligations
Spanish legislation underlines the importance of protecting installations against overvoltages. The current REBT (Reglamento Electrotécnico de Baja Tensión) in Article 16.3 establishes the obligation to incorporate surge protection systems in all low-voltage installations, as a safety measure for people and connected equipment. In particular, the Complementary Technical Instruction ITC-BT-23 details the need for transient surge protectors in indoor installations, especially those exposed to lightning or with overhead supply lines. Although that ITC focuses on transients, the general criterion of the regulation also encompasses permanent overvoltages.
In practice, every new build or full renovation should include both transient surge protectors (for example, a Type 2 SPD in the main panel) and permanent overvoltage protection devices, thus ensuring complete protection. Professional electricians are required to follow these guidelines when designing and executing installations—not only to comply with the law but to guarantee service continuity and preserve the user’s equipment.
In addition to national rules, there are European and international standards guiding the manufacture and application of these protectors. For example, IEC/EN 61643 defines the categories and tests for SPDs Type 1, 2, and 3, and EN 50550 defines devices for temporary overvoltage protection at power frequency (POP – Power frequency Overvoltage Protectors). Using devices that meet these standards ensures an adequate level of quality and performance. In short, the regulations demand—and good practice dictates—that one should not skimp on installing surge protection, as its cost is minimal compared to the damage it can prevent.
UK perspective: Likewise, in the United Kingdom the importance of surge protection is recognised in the wiring regulations. The 18th Edition of the IET Wiring Regulations (BS 7671:2018) introduced specific requirements for surge protective devices. Under these regulations (Section 443 and 534), SPDs must be fitted to protect against transient overvoltages in most installations unless a risk assessment deems them unnecessary or the installation owner explicitly accepts the risk of damage. In essence, for many new or modified installations, the default is to install SPDs to safeguard sensitive equipment and prevent harm. This approach, reinforced by Amendment 2 of BS 7671 (2022), aligns with the international trend of making surge protection mandatory as a standard safety measure.
Conclusion: electrical safety against overvoltages
Understanding what an overvoltage is and what causes it is the first step in recognizing the electrical risks we face in our homes and workplaces. Overvoltages—whether brief and intense or sustained—can have devastating impacts on our equipment and on the installation itself if we do not have adequate defenses. Fortunately, we have technology and regulations that, when properly applied, allow us to minimize these risks. The installation of cascaded surge protectors, proper earthing, and strict compliance with the regulations are key actions that every installer and user should consider.
In short, investing in surge protection is investing in peace of mind. From avoiding the breakdown of an expensive appliance to preventing fires, protection systems safeguard both people’s safety and their property. Therefore, it is highly recommended to consult professional electricians about the status of our installation: whether it meets current standards and if it is equipped with the proper protective devices. At the slightest hint of voltage problems (lights dimming or flickering, appliances getting damaged for no apparent reason), it’s important to act. With the correct preventive measures, we can ensure that overvoltages remain mere anecdotes rather than electrical disasters.