A break in the neutral line can cause overvoltage damage to electrical equipment. In Figure 1, the neutral (N) line connects the power supply neutral point (O1) and the user neutral point (O2). Before the break, as shown in Figure 1a, the potential difference between the power supply neutral point and the user neutral point is essentially equal. Once the neutral line breaks, as shown in Figure 1b, O1 and O2 no longer overlap, and the user-side neutral point UO2 will experience electrical displacement. This causes changes in the phase voltages on the user side, resulting in one phase being high and two being low, or one being low and two being high. Overvoltage will occur on the phase with the higher voltage, which can severely damage the user's electrical equipment. It may also lead to complaints from users demanding compensation. With the increasing number of electrical devices in use today, this has attracted the attention of power companies. This paper analyzes the phenomenon and identifies methods for overvoltage protection. [align=center]Figure 1 Neutral Line (a Neutral Line Not Broken b Neutral Line Broken)[/align] 1. Installing Repeated Grounding on the Neutral Line for Overvoltage Protection In the past, repeated grounding was installed on the neutral line as overvoltage protection. This way, after a break in the line, O1 and O2 could be reconnected through repeated grounding. 1.1 Technical Analysis: Assuming the voltage displacement at point O2 after a break is 60V, the repeated grounding resistance plus the transformer neutral point main grounding resistance totals 20Ω (this is a relatively ideal value). The calculated voltage displacement under different loads is shown in Figure 2. [align=center]Figure 2 Repeated Grounding Effect Curve[/align] As can be seen from Figure 2, only when the load is below 10kW does it have a reduction effect on voltage displacement; the effect is not obvious when the load is heavy. For example, with an actual load of 30kW, the voltage displacement only decreases from 60V to 53V, a drop of only 7V, which is very small. Repeated grounding is a technology from half a century ago. It is suitable for lines with very light loads below 10kW; its effect is minimal when the load is slightly heavier. 1.2 From an economic perspective, this method involves large investments and poor results. Each repeated grounding requires driving a sizable steel pipe into the ground, incurring significant costs. A low-voltage power grid would require multiple repeated groundings; a county-level enterprise with thousands of low-voltage distribution networks would need thousands or even tens of thousands of repeated groundings, resulting in enormous costs (hundreds to tens of millions of yuan). Even if this were achievable, the future maintenance workload would be substantial. Economically, the return on investment is not worthwhile. In my country, the actual installation rate of repeated groundings on low-voltage power grids has historically been low. This technology dates back to the early 20th century and should be phased out. 1.3 New Technology Challenges and Opportunities Currently, residual current operated protective devices (RCDs) are widely promoted in my country. These devices do not allow repeated groundings on the neutral line within their protection range. Otherwise, the protector may malfunction or fail to operate. Therefore, even if repeated groundings are already installed on a line, they should be removed when the main protector is put into operation. This technology faces new technological challenges. Promoting residual current operated protective devices also presents an opportunity to prevent neutral line breaks. China widely promotes residual current devices (RCDs), requiring every electricity user to install one. This adds overvoltage protection to household RCDs, achieving overvoltage protection. In the event of an overvoltage, the circuit breaker can be disconnected, ensuring the safety of other users' equipment. This only involves adding some protective components to existing RCDs, with minimal investment. Therefore, the return on investment for this method is far superior to installing repeated grounding. However, this method suffered from frequent false trips during its implementation, and its popularity waned after a period of initial enthusiasm. This was because the overvoltage protection value was selected too low, resulting in too fast an operating speed. 2. Overvoltage caused by neutral wire breakage A neutral wire breakage causes displacement at point O2 at the user end, resulting in overvoltage. This overvoltage is related to the balance of the electrical load. Assuming the electrical load is balanced (connecting the same number of incandescent lamps), no overvoltage will occur. However, the electrical load is dynamic and difficult to maintain balance throughout the day. If some lamps on phase A are turned off during a certain period, reducing the load, an overvoltage will occur on phase A, initially with a small amplitude. Over time (e.g., tens of minutes), the first bulb in phase A burns out. This reduces the load on phase A, exacerbating the overvoltage. This, in turn, accelerates the burnout of the second bulb, further reducing the load on phase A and further exacerbating the overvoltage. ... This cycle continues to accelerate until all the bulbs in phase A burn out, the O2 point shifts to the midpoint between phases B and C, and the voltage in phase A increases from 220V to 330V. The next overvoltage cycle occurs between phases B and C. If the load on phase B is light, phase B will experience overvoltage, potentially burning out the bulbs. Eventually, only phase C remains unburned. This cycle may be interrupted by a sudden change in the electrical load, resulting in only partial damage to electrical equipment. This phenomenon is similar to an avalanche, where a small instability at the mountaintop causes a small snowball to slide down, growing larger and faster until it eventually collapses. 3. Several Scenarios of Overvoltage in Low-Voltage Systems Atmospheric overvoltages have amplitudes of tens of thousands of volts and form over a period of several to tens of microseconds. It easily causes insulation breakdown. Operational overvoltage amplitudes range from several hundred to thousands of volts, lasting from a few to tens of milliseconds, and generally do not cause insulation damage. However, now that low-voltage components are widely used in low-voltage systems, these components will be damaged under such overvoltages, and such accidents are on the rise. Overvoltages caused by neutral line breaks range from tens to hundreds of volts, forming within tens to thousands of seconds. For example, if a 220V incandescent lamp experiences a 380V overvoltage, the bulb will turn white, the filament will overheat, and the filament will burn out after tens of seconds. Similarly, if a 220V power transformer experiences a 380V overvoltage, the high saturation of the iron core will increase the excitation current, which can last for hundreds to thousands of seconds, eventually causing the transformer to overheat and burn out. Therefore, overvoltages caused by neutral line breaks have a latency period, a rapid development period, and a final termination period. Their speed ranges from tens to hundreds or thousands of seconds. Therefore, the operating speed of overvoltage protection can have a large delay. 4. Conclusion Besides a broken neutral conductor, another common overvoltage is incorrect wiring. Supplying 380V to a 220V user is dangerous. Based on the above analysis, the development and damage from a neutral conductor breakage overvoltage is a relatively long process. Therefore, an improvement is to ensure that overvoltage protection has sufficient delay and that the operating voltage is increased. This can avoid overvoltages in the power grid and reduce false trips. Specifically, it is recommended that the overvoltage operating voltage be in two stages: a 1-3 second delay for 280-300V, and instantaneous operation (no delay) for 360-380V. Increasing the operating voltage may initially burn out a small number of appliances (especially a few incandescent bulbs) when an overvoltage occurs due to a neutral conductor breakage, but once the initial bulbs burn out and the overvoltage exceeds 280-300V, the overvoltage protection will activate, protecting the remaining appliances. Therefore, adding overvoltage protection to household residual current devices is a cost-effective solution.