4 Points to Check for Wiring and Connection Troubles When Power Generation Is Low

When low power generation persists, attention tends to focus on solar irradiance, weather, panel soiling, and equipment aging. However, one cause that is often overlooked on site is faults in wiring and connections. In solar power generation systems, electricity flows through multiple connection points—from DC-side string wiring, junction boxes and combiner boxes, around the power conditioner, to AC-side wiring. Therefore, if resistance increases at any single point or contact becomes poor, it can lead to reduced or halted power generation and abnormal alarms.

In this article, aimed at field personnel who search "low power generation" to isolate causes, we outline four points for checking wiring and connection troubles. Because on-site inspections carry the risk of electric shock and arcing, unqualified persons should avoid disassembly or contact with live parts, and, where necessary, confirmation should be requested from specialists such as the electrical chief engineer, the installation contractor, or maintenance and inspection companies.

Table of Contents

• How to suspect wiring problems when power generation is low

• Point 1 Check for differences in output between strings, and for open circuits and poor connections

• Point 2: Check connectors, terminal blocks, and junction boxes for looseness and overheating

• Point 3 Check for signs of cable damage, water ingress, and insulation degradation

• Point 4 Check the connection status around the power conditioner and on the AC side

• Information to record during wiring inspection

• Summary: When power generation is low, determine the wiring condition by recording and comparing.

How to Suspect Wiring Problems When Power Output Is Low

When considering the causes of low power generation, the first thing to check is whether the decline is occurring across the entire facility or is concentrated in certain circuits or during specific time periods. If generation is down overall, factors such as weather, solar irradiance conditions, snow accumulation, widespread soiling, power conditioner control, or grid-side constraints may be involved. On the other hand, if only specific strings, certain connection boxes, or particular power conditioners are showing low output, abnormalities in wiring or connections are more likely.

Wiring problems do not necessarily manifest as a complete cessation of power generation. In some cases the output clearly disappears, such as with a broken wire or a blown fuse, but increases in contact resistance, loose terminals, incomplete connector mating, damage to cable sheathing, or insulation degradation caused by water ingress can cause generation to gradually decline, abnormalities to appear only on sunny days, or alarms to increase after rain. Because these symptoms are difficult to assess from a single day's generation alone, comparison with past days under the same conditions and with adjacent circuits is important.

What you should be especially careful about is brushing off low power output as simply "because of the weather." It is true that output falls on cloudy or rainy days, but when you compare circuits with the same orientation and tilt within the same installation, it becomes difficult to explain cases where only some are clearly lower by weather alone. Also, during periods with sufficient sunlight, if only one circuit’s current does not rise, its voltage is unnaturally low, or there is an abnormal event history recorded, these are clues to check the wiring and connection conditions.

However, wiring checks require careful attention to safety. The DC side of a photovoltaic system can be subject to high voltages during the daytime. Even if you operate a breaker, the panel side may continue generating power, so different precautions are necessary compared with typical electrical installations. While visual inspections and record checks are relatively easy for on-site personnel to perform, tasks such as retightening terminals, insulation testing, disassembling connectors, or taking measurements on live circuits require specialist knowledge, appropriate personal protective equipment, and procedures. This article focuses on how to approach and narrow down causes on site.

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Point 1: Check output differences, open circuits, and faulty connections for each string

The first thing to check is the output differences at the string level. In solar power systems, multiple panels are connected in series to form strings, which are consolidated at combiner boxes and power conditioners. When generation is low, looking only at the total for the entire system makes it hard to identify where the drop is occurring. Therefore, it is important to compare the voltage and current of each string, the input status of each combiner box, and the power generation of each power conditioner.

If a string is composed of panels with the same orientation, the same tilt, and the same number of panels, it is generally the case that their output trends under sunny conditions are roughly similar. Of course, they will not be exactly identical due to shading, soiling, panel-to-panel variations, temperature differences, and so on, but if only one string shows a markedly low current, a voltage lower than expected, or if the output does not follow increases in irradiance, you should suspect a broken conductor, a disconnected connector, a poor connection, a blown fuse, a module fault, or similar issues.

If a string is close to an open-circuit condition, the current in the affected string may be almost nonexistent. On the other hand, with a poor connection or increased resistance, the output may not stop completely but remain low. To distinguish between these, it helps to look not only at instantaneous values but also at changes over time. Checking whether the output is low all day from morning to evening, whether the discrepancy appears only during periods of high solar irradiance, or whether it occurs only after rain can make it easier to narrow down the likely cause.

When checking strings with low power output, you must take into account differences in the number of panels and the connection configuration. Even areas that look identical can actually differ in the number of panels per string, orientation, tilt, and how they are affected by shading. It is important to cross-check design drawings, single-line diagrams, string tables, as-built drawings, and past inspection records to confirm whether the items are suitable for comparison. If the comparison conditions are misaligned, there is a risk of misidentifying normal differences as abnormalities.

In the field, comparing a low-output circuit side-by-side with a nearly normal circuit makes it easier to identify anomalies. For example, if one string among several housed in the same combiner box shows a low current, the cause may be on that string. If the entire combiner box is low, you should also check the wiring downstream of the combiner box, the input to the power conditioner, circuit breakers, fuses, and the status of monitoring data acquisition.

When looking at output differences between strings, also record the measurement time. Solar power generation is sensitive to changes in solar irradiance, and the readings can change even if a cloud simply passes by. If one string was measured when it was sunny and another when it was cloudy, the reliability of the comparison is reduced. It is preferable to use monitoring data obtained at the same time as much as possible, or to align conditions and verify within a short period.

To determine whether low power generation is caused by the wiring, it is important not to simply view the values as "low" but to organize the assessment as "lower compared to circuits under the same conditions," "lower compared to the same season in the past or the same solar irradiance conditions," and "the decline is concentrated in specific circuits." If these comparisons can be made, it will be easier to narrow the scope of inspection when requesting an investigation from a specialist.

Point 2: Check for looseness and overheating of connectors, terminal blocks, and junction boxes

In wiring and connection troubles, particular attention should be paid to connection points such as connectors, terminal blocks, the inside of junction boxes, and areas around circuit breakers. Because electricity flows through these connection points, looseness, corrosion, dirt, poor crimping, or deterioration of contact surfaces can increase resistance and cause heating and voltage drop. Not only can this reduce power generation, but it can also lead to unusual odors, discoloration, melt marks, burn marks, alarms, and burn damage, so prompt inspection is necessary.

Connector defects can be difficult to detect from appearance alone. Even when a connection appears to be properly made, it may not be fully seated, the locking may be insufficient, incompatible components may be combined, or moisture and debris may have entered the interior. Because solar photovoltaic (PV) systems are exposed outdoors for long periods, they are affected by ultraviolet (UV) radiation, temperature changes, wind and rain, and vibration. Even if there were no problems at the time of installation, the connection condition can change over the years or due to external forces.

In terminal blocks and junction boxes, check the tightening condition of terminals, the fastening condition of cables, insulation being pinched, exposed conductors, discoloration, traces of condensation, and signs of insect or small-animal intrusion, etc. If terminals become loose, contact resistance increases, making them more likely to heat up under load. Continued heating can deform surrounding resin parts and insulation, which may further worsen the contact condition and lead to a vicious cycle.

However, it is dangerous to casually open the inside of a junction box or touch terminals while it is energized. In particular, on the DC side an arc, once generated, may be difficult to extinguish, posing a risk of electric shock or burns. Even if an external abnormality is found, the basic rule is that on-site personnel should not tighten or disassemble components on the spot; instead, verify safety procedures and request a specialist. Opening inspection ports or performing measurement work involves the condition of the equipment, isolation procedures, personal protective equipment, and the suitability of the measuring instruments.

In a thermal inspection, it is helpful to identify which connection points are hotter than their surroundings during normal operation. Heating tends to appear during periods of high load, so checking near the power generation peak on sunny days can make it easier to detect signs. However, temperature is also affected by solar irradiation, ambient temperature, enclosure material, and the presence or absence of wind, so do not immediately conclude that a temperature difference indicates an anomaly; evaluate by comparing with adjacent components under the same conditions, with past records, and by checking for any visible abnormalities.

Around junction boxes, attention must be paid to rainwater ingress and condensation. If the junction box seal has deteriorated, the wiring penetration is poorly treated, the door does not close properly, or the cable entry lacks adequate drainage, moisture can enter the interior. Moisture can cause corrosion and reduced insulation, which may lead to decreased power generation and earth-leakage alarms. If abnormalities appear after rain, alarms increase during periods of high humidity, or droplets or rust are visible inside the junction box, both the wiring connections and the waterproofing condition should be inspected.

The purpose of checking the connections when power generation is low is not simply to search for faulty points. It is also important to clarify the areas where no abnormalities were found. For example, if there are output differences between power conditioners but records show no heating or discoloration inside the junction box, no visible abnormalities on the terminals, and no signs of rainwater ingress, that can serve as a basis for narrowing down the next investigation targets. Conversely, even slight-looking discoloration or looseness is worth prioritizing for detailed inspection if it is concentrated in the same circuit as the power generation decrease.

Point 3 Check for signs of cable damage, water ingress, and insulation deterioration

Damage to the cables themselves should not be overlooked as a cause of low power generation. Cables in solar power generation systems run under outdoor mounting racks, in above-ground wiring, inside conduits, in buried sections, and around junction boxes. Ultraviolet radiation, wind-induced swaying, abrasion at fastening points, grass cutting, snow removal, animal gnawing, falling objects, and ground subsidence can damage the outer sheath. As sheath damage progresses, it can lead to reduced insulation, leakage currents, short circuits, and poor electrical contacts.

Cable damage may appear immediately as a reduction in power generation, or it may manifest as anomalies only during rain or when humidity is high. When conditions are dry the problem may not be noticeable; if symptoms such as earth-leakage alarms after rain, shutdowns for a certain period, or unstable power output after recovery occur, suspect water ingress or insulation degradation. Pay particular attention to wiring close to the ground, areas with poor drainage, places where vegetation is dense, and sections where cables sag close to puddles.

During visual inspection, check for cracks, cuts, deformation, bulging, scorching, abrasion of cable jackets, cutting-in by cable ties, cracks in conduits, detachment of support fittings, and contact with the ground. If cables are in contact with the edges of mounting racks or metal components, their jackets can be gradually worn away each time wind causes them to sway. Even if there is no problem immediately after installation, long-term vibration and temperature changes can shift their position and lead to damage.

In conduits or buried sections, it may not be possible to directly view the condition of the cables from the outside. In such cases, we infer the cause by combining the timing of when abnormalities occur, the location of the circuit, terrain prone to flooding, and past construction history. For example, if reduced power generation or leakage is occurring in areas where the ground has settled after land development, near drainage channels, on routes where vehicles pass, or at sites where excavation work was carried out, it is necessary to check for impacts on the cable route.

Confirming insulation degradation requires dedicated measurements. This is a task that demands safety considerations and correct procedures, and it should not be carried out at the sole discretion of on-site personnel. In insulation testing, errors in isolating the measurement target, deciding whether equipment must be removed, selecting the test voltage, or verifying restoration after the test can damage equipment or produce incorrect results. For this reason, after organizing suspicions based on visual inspection and operating data, it is realistic to request the measurement from a specialized contractor.

When suspecting cable damage, it is important to link the reduced power output to the site location. Instead of simply recording "low power output," organize which inverter, which junction box, which string, and which cable route are involved. When taking site photos, in addition to close-ups, keep images that show the surrounding positional relationships so the cause can be traced more easily later.

Also, if power output drops immediately after nearby work such as mowing, snow removal, land development, fence installation, or drainage work, consider the possibility of cable damage caused by the work. Workers may unintentionally come into contact with cables, or heavy machinery and tools may affect wiring routes. Cross-referencing the work dates, the scope of work, and the date the power drop began makes it easier to pinpoint the cause.

Point 4: Check the connection status around the power conditioner and on the AC side

When power output is low, check not only the DC-side strings and junction boxes but also the connections around the power conditioner and on the AC side. Even if the panels are generating normally, a problem anywhere—from the power conditioner's input or output, through breakers, distribution boards, and collection equipment, to the grid interconnection point—can reduce or halt power output.

At the power conditioner, check alarms for input voltage, input current, output power, fault history, shutdown history, and temperature. In systems with low generation output, what appears to be a fault in the power conditioner itself may actually be caused by poor input-side connections or AC-side voltage conditions. Conversely, what looks like a wiring problem may in fact be due to the equipment’s control or protective actions suppressing the output.

On the AC side, things to watch for are loose terminals, the condition of circuit breakers, cable overheating, discoloration inside the panel, unusual odors, condensation, and poor ventilation. If resistance increases at AC-side connections, voltage drop and heating can occur, which may affect equipment operation. Also, if the grid-side voltage is high or under reverse-flow conditions, the power conditioner may reduce its output. In such cases, even if the panels and DC wiring are normal, the generated power may appear lower as a result.

However, it is necessary to distinguish the effects of output curtailment and grid voltage from generation declines caused by wiring troubles. If it is output curtailment, multiple units may show similar behavior during certain periods on sunny days. On the other hand, if it is wiring or connection failures, the tendency is for the issue to be concentrated in specific circuits or specific pieces of equipment. Check the monitoring data to see which power conditioners are declining, when, and by how much, and determine whether it is a system-wide trend or a localized anomaly.

When inspecting the interior of the panel, check for visible discoloration, unusual odors, burn marks, signs of insect or small animal intrusion, condensation, and rust. If the panel door or gaskets are degraded, or if cable penetration seals are poorly executed, moisture and water can enter and lead to deterioration of terminals and metal parts. For outdoor panels, check the surrounding environment as well because temperature rises from solar radiation and insufficient ventilation can affect the equipment.

Troubles around the power conditioner can manifest not only as reduced power generation but also as an increased number of shutdowns. If short daytime shutdowns with automatic recovery are repeating, daily or monthly reports may merely appear to show low power generation, while in reality shutdowns and recoveries may be accumulating. It is important to check fault and event logs and correlate them with the times when power generation decreased.

When checking the connection status on the AC side, contact with live parts or work inside the panel should be left to specialist contractors. On-site personnel should record whether any abnormalities are present, note the time of occurrence and the equipment involved, and use that information to arrange necessary inspections. In particular, if there is a burning smell, the panel is unusually hot, alarms are sounding frequently, or circuit breakers are operating, do not repeatedly attempt to restore service; prioritize safety in your response.

Information to record when checking wiring

To accurately isolate the cause of low power generation, it is essential to record what was observed on site. Wiring and connection problems can appear differently depending on the weather, solar irradiance, temperature, whether it is raining, and the operating condition of the equipment at the time they occur. Because verbal recollection alone makes it difficult to recreate the situation later, it is important to keep together the date and time of inspection, the equipment in question, measured values, photographs of the exterior, and a history of abnormalities.

First, what you should record is the basis for determining that power generation is low. Make the comparison targets clear: lower than the previous day, lower than the monthly average, lower than another circuit within the same installation, lower than the expected generation, etc. Simply writing "low" does not provide enough information to determine whether the cause is weather or an equipment fault. If solar irradiation conditions are known, placing the irradiation and output side by side for the same time period and checking them makes it easier to assess the reasonableness of the drop in generation.

Next, record the extent of the degradation. Clarify whether it is the entire facility, a specific power conditioner, a particular junction box, or a specific string. In addition to saving the numbers from the monitoring screen and measurements, marking the affected location on a single-line wiring diagram or layout drawing will make later investigations smoother. Site photos are also easier to understand when you combine close-up shots with overall photos, so it is clear where the anomaly is.

In a visual inspection, record items such as disconnected or loose connectors, discoloration of terminal parts, damage to cable sheathing, cracks in piping, loosened ties, water droplets or rust inside junction boxes, and unusual odors or scorch marks inside panels. Even when no abnormalities are found, it is meaningful to leave a note stating "inspected and no abnormalities were found." When specialist contractors investigate later, knowing what has already been checked reduces duplicated work and makes it easier to proceed with the root-cause investigation.

When power generation falls after rain or after days with strong winds, record the weather conditions as well. The before-and-after relationships around a drop in generation output—such as leakage alarms that only occur after rain, some strings showing reduced output after strong winds, or anomalies appearing after grass cutting—are important for determining the cause. Especially for outdoor equipment, wiring is easily affected by the environment, so check not only the equipment data but also changes in the site environment.

When keeping inspection records, it is also important to separate judgments from facts. Rather than writing "I think the terminal is bad," it is easier to verify later if you record things like "discoloration near the terminal in question," "current is lower than other strings in the same junction box," and "an insulation-related alarm occurred the day after rain." Leave speculation as speculation, and record facts with numbers and photos to reduce discrepancies in understanding among stakeholders.

Also, for systems with low power output, comparing current results with past inspections can be helpful. Changes such as discoloration appearing now in areas that showed no abnormalities at the previous inspection, a gradual decline over time in the current from the same string, or an increase in alarm history for a particular junction box are the kinds of changes that are hard to notice from a single check. By continuously accumulating inspection records, it becomes easier to detect early signs of abnormalities.

Summary When power generation is low, assess the wiring condition through recording and comparison

When power output is low, the cause is not necessarily a single one. Various factors—weather, dirt, shading, equipment control, panel degradation, and grid conditions—can overlap. Among these, wiring and connection problems can not only cause reduced power output but, if left unaddressed, may lead to overheating, shutdowns, or equipment damage, so they should be checked as soon as possible.

When checking for wiring troubles, first examine the output differences for each string and identify where the reductions are concentrated. Next, inspect connectors, terminal blocks, junction boxes, and other connection points for looseness, discoloration, overheating, or signs of water ingress. Also look for signs of cable damage or insulation degradation and verify any relationship with abnormalities observed after rain or following work. Finally, review the connection conditions around the power conditioner and on the AC side, the history of abnormalities, and the possibility of output curtailment.

What’s important is not to judge solely by the result of low power generation, but to compare with circuits under the same conditions, compare with past data, and cross-check with on-site conditions. If you record the figures, photos, date and time, weather, and the circuit(s) in question, it will be easier to explain when requesting an investigation from a specialist and will make it easier to narrow down the cause.

On the other hand, inspecting wiring and connections carries risks. The DC side continues to generate power during the day, and the AC side also poses a risk of electric shock or an electrical arc depending on whether it is energized. If you find a possible abnormality during visual inspection or while organizing records, do not force disassembly or restoration; it is safer to ask a specialist with appropriate qualifications and experience to check it.

To quickly detect low power generation and avoid overlooking abnormalities in wiring and connection points, daily record management and organization of on-site information are important. Keeping generation data, site photos, equipment locations, and inspection histories together for review makes the flow from anomaly detection to response decision-making smoother. By continuously recording changes in power output and establishing a system that allows verification together with on-site conditions, it becomes easier to achieve early detection of generation declines and improved accuracy in maintenance management.