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Each case adopts “Customer issue → Root cause analysis → Loss incurred → Correct course of action” Its structure is designed to serve as a powerful tool for the sales team.

 

 

Case 1: Factory Buys “Special-Price Box-Type Substations” to Cut Costs, Only to Become the “Tripping King” After Summer Arrives

 

The customer’s trouble

To minimize initial capital expenditure, a certain mechanical processing plant purchased the lowest-priced European-style packaged substation available on the market. The equipment operated normally for the first two months after commissioning; however, once summer’s high-temperature weather set in, the high-voltage-side circuit breaker of the transformer began tripping frequently during the daily afternoon peak-demand period, and in some cases even caused a feeder switch to burn out. As a result, the plant’s production lines were forced to halt repeatedly, leading to delivery delays and subsequent penalty charges.

 

Root Cause Analysis

To reduce costs, the transformer substation manufacturer significantly reduced the surface area of the transformer cooling fins and the dimensions of the ventilation louvers on the high- and low-voltage switchgear, while failing to install forced exhaust fans inside the enclosure. Under summer ambient conditions of 35°C, the measured internal temperature of the substation reached 72°C—well above the thermal trip threshold for the circuit breakers. Furthermore, the transformer selection did not account for the substantial motor loads present in the plant’s actual operating profile (including harmonic distortion and inrush currents), which further exacerbated overheating.

 

Cause losses

 

Direct losses: RMB 48,000 spent on replacing the switch and repairing the transformer.

Indirect losses: Cumulative production downtime of 32 hours, resulting in order-delivery compensation and electricity cost losses totaling approximately RMB 170,000.

Hidden losses: premature aging of the transformer enclosure, resulting in an estimated reduction of service life by more than five years.

 

The correct approach

 

During the procurement of packaged substations, a type-test report on temperature rise must be required for verification. Thermal design meets local extreme high-temperature conditions with a 20% margin. 。

Mandatory requirement: Pad-mounted transformers with a rating of 630 kVA or greater must be equipped with an automatic temperature-controlled ventilation system and dust-proof filters.

When signing the contract, “During summer full-load operation, the internal temperature rise of the pad-mounted transformer shall not exceed 45 K.” Draft the technical acceptance provisions.

 

 

Case Study 2: Improper Layout of the Residential Community’s Distribution Room Leads to Complete Equipment Damage in a Heavy Rainfall

 

The customer’s trouble

During the first flood season following the handover of a newly built residential community, a 12-hour moderate rainstorm caused water to accumulate to a depth of 0.8 meters in the cable trench of the underground distribution room. The next day, a routine inspection by the power supply bureau revealed that water had entered the 10 kV incoming switchgear, the transformer, and several low-voltage switchboards, resulting in zero insulation resistance. The entire community experienced a power outage lasting more than three days, prompting collective protests by residents demanding redress.

 

Root Cause Analysis

 

Improper site selection: The distribution room is located at the lowest point of the underground parking garage, and no waterstop curb or automatic sump pump system has been installed.

Design defects in the cable trench: the outdoor cable conduit was not sealed, and the trench bottom was not sloped toward the sump pit, resulting in rainwater flowing backward along the conduit into the switchgear room.

During the construction phase, waterproofing and sealing of cable trenches were omitted from inspection as “non-critical processes.”

 

Cause losses

 

Equipment write-off: Four high-voltage switchgear cabinets, two dry-type transformers, and eight low-voltage switchgear cabinets were all damaged by water ingress, with replacement costs totaling approximately RMB 960,000.

Construction Schedule and Compensation: The property management company urgently rented a generator and compensated owners for losses totaling approximately RMB 230,000.

Social Impact: The developer has been publicly criticized by the housing and urban–rural development authority, significantly impacting subsequent property sales.

 

The correct approach

 

The distribution room must not be located on the lowest level of an underground parking garage; it shall be situated on the ground floor or in a semi-underground level with dedicated drainage, and its entrance shall be equipped with a waterstop curb no less than 30 cm high.

The cable trench must be designed with a dual drainage system: the trench bottom shall have a slope of ≥0.5% leading to a collection sump, which shall be equipped with two automatic submersible sewage pumps—one in service and one on standby—complete with high-water-level alarms.

Cable entry and exit openings shall be sealed with a dual-layer system consisting of fire- and explosion-proof sealant combined with heat-shrink sealing caps, followed by individual water spray testing for each opening after installation.

 

 

Case 3: Incorrect Selection of Prefabricated Substation for a Photovoltaic Project Leads to Repeated Grid-Connection Acceptance Inspections

 

The customer’s trouble

In a 2 MW distributed photovoltaic project on the rooftops of industrial and commercial facilities, a standard 10 kV oil-immersed pad-mounted transformer was selected as the step-up equipment. During the initial grid-connection acceptance inspection, the power supply company pointed out that the neutral-grounding scheme of the transformer did not match that of the grid. After corrective measures were implemented, the second acceptance revealed that the protection settings and the control power supply failed to meet the anti-islanding requirements. A third inspection was then conducted because the factory had not performed zero-sequence protection testing on the transformer, resulting in its disqualification. It took a total of four months from start to finish before the system was finally connected to the grid, during which the enterprise missed the subsidy application window.

 

Root Cause Analysis

 

Lack of familiarity with grid requirements: In this region, the 10 kV grid-connection requirement stipulates that packaged substations must employ low-resistance grounding with direct neutral earthing; however, the procured standard packaged substation is an ungrounded system.

Missing protection settings: PV-specific packaged substations must be equipped with reverse-power protection, frequency and voltage limit protection, and active anti-islanding protection, whereas standard packaged substations are only equipped with overcurrent and instantaneous trip protection.

Pre-connection joint commissioning was not performed: It was only after on-site installation that it was discovered that the substation’s built-in operating power supply is derived from the grid side, rendering the protection devices completely inoperative during a grid outage.

 

Cause losses

 

Direct economic impact: The three rounds of rectification, including the replacement of protective devices, the addition of a grounding transformer, and commissioning costs, totaled RMB 120,000.

Power generation loss: Due to a four-month delay in grid connection, the estimated loss, calculated based on local electricity prices and subsidies, is approximately RMB 430,000.

Reputational Damage: The owner believes the general contractor lacks professionalism and will switch suppliers for the Phase II project.

 

The correct approach

 

In the preliminary stage, it is mandatory to obtain the “Grid Connection System Design Report” issued by the power grid company and to systematically translate each item therein—specifically, the “grounding method for packaged substations and protection configuration requirements”—into procurement technical specifications.

Photovoltaic packaged substations must be specifically designed as integrated, photovoltaic-dedicated units. , equipped with: dual-tap on-load tap changer, zero-sequence current transformer, and a UPS DC operating power supply (with a minimum runtime of ≥4 hours).

During factory acceptance, the grid company or a third-party testing agency shall conduct full-protection simulation tests, including islanding-effect simulation.

 

 

Case 4: The temporary power supply plan was a direct copy of the drawings, and the cable was crushed and severed twice when it crossed a road.

 

The customer’s trouble

In a certain construction project, the temporary power supply extended approximately 300 meters from the distribution box to the job site, with the cable laid directly beneath the construction access road used by heavy vehicles and without any protective measures. Within one week, the cable was crushed and severed twice by heavy muck trucks, each outage disrupting concrete placement and resulting in cold-joint quality defects.

 

Root Cause Analysis

 

The design scheme directly adopts standard drawing collections for building construction, failing to account for the operational condition of frequent passage of 60-ton heavy vehicles at the construction site.

“Cost-saving mentality”: The belief that temporary power will be dismantled after only a few months of use leads to reluctance to invest in steel conduit or cable trays.

Lack of on-site briefing: The construction team buried the pipeline at the standard depth of 0.7 meters without providing a sand bedding layer or installing brick covers as warning markers.

 

Cause losses

 

Two emergency repair costs: excavation, cable replacement, and labor totaled RMB 36,000.

Quality Incident: Two interruptions in the concrete pouring resulted in low-strain testing and grouting reinforcement for eight pile foundations, incurring costs of RMB 70,000.

Construction Schedule Delay: A total of 2.5 days of work stoppage resulted in fines and rental fees totaling approximately RMB 40,000.

 

The correct approach

 

If the temporary power routing cannot avoid heavily loaded roads, it must be protected using galvanized steel conduit or heavy-duty cable trunking, with a conduit wall thickness of ≥3.5 mm and an burial depth of ≥1 meter.

Install highly visible marker posts at both ends of the crossing and prepare a temporary cable routing diagram to be submitted to the site safety officer.

The contract explicitly stipulates that cable crossing protection is a “mandatory item” and may not be deleted or downgraded.

 

 

Case 5: An enterprise independently increased capacity without conducting thermal imaging, resulting in a fire and destruction at the busbar connection point.

 

The customer’s trouble

A textile factory, in order to accommodate newly installed equipment, independently commissioned electricians to replace the incoming main switchgear busbar from a 630 A copper bar to a 1000 A copper bar. Three months after commissioning, a nighttime fire broke out, completely destroying the low-voltage main switchgear and spreading to adjacent cabinet sections. The fire department determined that the ignition source was at the busbar connection bolts.

 

Root Cause Analysis

 

Thermal imaging acceptance not performed: After the busbar was replaced, conductive paste was not applied to the lapped surfaces, and insufficient bolt torque resulted in excessive contact resistance. During heavy-load operation, local temperatures exceeded 150°C, and long-term thermal expansion-induced loosening led to arcing.

The electrician performed the work based on experience, without using a torque wrench or conducting the contact resistance test as required by GB 50149.

Companies assume that “switching to a larger wire gauge will ensure safety,” but they overlook the fact that the splicing process is the real key to heat generation.

 

Cause losses

 

The main cabinet and the two adjacent cabinets are scrapped, with replacement costs totaling RMB 180,000.

Production was halted for five days, resulting in approximately RMB 600,000 in lost output value and the cancellation of some orders.

The power supply company imposed a fine and ordered the entire plant to rectify all safety hazards, with a total investment of RMB 80,000 for the remediation.

 

The correct approach

 

All busbar splices must be executed in three steps: grinding away the oxide layer → applying conductive paste evenly → tightening with a torque wrench to the specified values (M10 bolts: 20 N·m; M12: 35 N·m).

A DC resistance test must be performed prior to commissioning (requirement: ≤1.1 times the resistance value of a busbar of the same length).

An infrared thermographic inspection must be conducted within 72 hours of commissioning, followed by quarterly inspections. If the temperature difference at any splice point exceeds 15 K, the equipment must be immediately shut down for troubleshooting.

 

 

Below are six additional high-frequency pitfalls encountered in distribution projects, presented in the same structured format to help expand your case library.

 

 

Case 6: A pad-mounted transformer substation was struck by lightning during the thunderstorm season—simply because its grounding resistance had never been measured.

 

The customer’s trouble

During a thunderstorm, a pad-mounted transformer at a certain scenic area was struck by lightning, causing the high-voltage fuse to blow and the transformer windings to burn out, which in turn paralyzed the surrounding surveillance and lighting systems. Strangely, no other nearby pad-mounted transformers were damaged.

 

Root Cause Analysis

 

During installation, no grounding resistance test was conducted for this packaged substation; instead, the contractor relied on experience to drive in three galvanized angle steels (resulting in an actual grounding resistance as high as 18 Ω, whereas the requirement is ≤4 Ω).

Without a surge protective device (SPD), lightning-induced overvoltages directly intrude into the transformer.

The grounding flat steel was buried less than 30 cm deep, and the welds were not protected against corrosion, resulting in rust-induced breakage after only six months.

 

Cause damage Loss

 

Replacement of transformer and fuses: RMB 52,000.

Surveillance system repair: RMB 18,000.

The scenic area experienced a two-day power outage, resulting in approximately RMB 90,000 in losses to ticket sales and shop operations.

 

The correct approach

 

The grounding system for the packaged substation must form a closed loop ground grid, with no fewer than four grounding electrodes spaced at least 5 meters apart, and a measured grounding resistance of no more than 4 Ω (no more than 10 Ω in rocky areas).

Secondary surge protectors (10/350 μs waveform) shall be installed on both the high-voltage and low-voltage sides.

Grounding concealed works must be subject to on-site supervision and documented with photographic records, and a third-party grounding resistance test report must be issued prior to commissioning.

 

 

Case 7: The capacitor compensation cabinet has never been switched on, resulting in an additional monthly reactive power penalty of RMB 20,000.

 

The customer’s trouble

A certain auto parts manufacturer discovered that for six consecutive months, the “power-factor adjustment charge” on its electricity bills was consistently listed as a penalty, resulting in monthly additional expenses of RMB 18,000 to 23,000. Even manual switching of capacitors by electricians failed to produce any significant improvement.

 

Root Cause Analysis

 

Wiring error in the automatic compensation controller: the current signal is derived from the main incoming CT, while the voltage signal is taken from the capacitor side, resulting in a phase inversion in the sampled signals.

Some capacitors have already bulged and failed internally, yet the indicator light remains on.

Incorrect switching parameter settings: The target power factor is set to 0.95, but the delay time is set to 120 seconds, resulting in a response that is too slow during overload conditions.

 

Cause losses

 

The cumulative electricity penalty for the past six months is approximately RMB 120,000.

Capacitor replacement and wiring renovation: RMB 16,000.

The transformer’s actual utilization rate is relatively low, resulting in additional losses of approximately RMB 30,000 per year.

 

The correct approach

 

The voltage and current signals for the compensation controller must be sampled in phase (both taken from the incoming line side or the transformer secondary side).

Perform a capacitor capacitance test once annually (capacitors must be replaced if capacity degradation reaches or exceeds 20%), and verify that the series reactor is properly matched (a 7% reactance rating is used to suppress the 5th harmonic, while a 14% reactance rating is used to suppress the 3rd harmonic).

Set the switching delay to 20–40 seconds and the target power factor to 0.92–0.95 (to avoid overcompensation that could lead to overvoltage).

 

 

Case 8: Burnout of the neutral terminal block leads to abnormal voltage across the entire plant, causing equipment damage.

 

The customer’s trouble

During overnight overtime at an electronics factory, a large number of office computers, workshop PLCs, and variable-frequency drives were severely damaged by fire. Upon inspection, it was found that the main neutral terminal block in the distribution room had completely melted, causing the three-phase voltage to surge above 400 V.

 

Root Cause Analysis

 

The entire plant makes extensive use of single-phase 220 V loads (computers, lighting, and air conditioning), resulting in severe three-phase load imbalance and neutral current levels that far exceed the design limits.

The main neutral terminal block is designed for a current rating of only 100 A, yet the measured neutral current reached 210 A.

An electrician connected multiple temporary circuits in parallel to the neutral terminal block; loosening of the screws caused overheating, which in turn triggered positive feedback and resulted in a fuse blow.

 

Cause losses

 

Equipment destroyed: 12 PLC modules, 5 variable-frequency drives, and 23 office computers, totaling approximately RMB 190,000.

Production Halt for Emergency Repairs: 2 Days, Resulting in Approximately RMB 400,000 in Lost Output Value.

Data loss: Some process parameters that were not backed up will need to be reconfigured.

 

The correct approach

 

The current-carrying capacity of the main neutral terminal block shall be configured at 200% of the phase conductor’s current-carrying capacity (taking into account harmonics and severe unbalance).

Conduct monthly checks of three-phase load balance, with the requirement that the difference between the maximum and minimum phase currents does not exceed 30%.

It is prohibited to crimp more than two conductors onto the neutral terminal block, and the temperature of the terminal block shall be inspected using thermal imaging every six months.

 

 

Case 9: Maloperation of the automatic transfer switch, resulting in simultaneous tripping of both incoming feeders.

 

The customer’s trouble

A certain data center employs a dual 10 kV incoming feeder configuration with automatic transfer at the bus-tie. On one occasion, a voltage fluctuation occurred on one of the incoming feeders (without a permanent fault), triggering the automatic transfer scheme to immediately open the faulty feeder and close the bus-tie. However, the instant of closing the bus-tie resulted in an overcurrent trip on the other incoming feeder, causing a total power outage across the entire data center.

 

Root Cause Analysis

 

Incorrect backup auto-transfer logic configuration: The “incoming line undervoltage delay confirmation” has not been set (typically 2–3 seconds), causing the system to trip on even momentary voltage fluctuations.

The bus tie breaker is not interlocked with the incoming line current—when it is closed onto a bus that is already under load, the inrush current is excessively high.

No actual joint commissioning tests were conducted prior to commissioning (only secondary simulations were performed).

 

Cause losses

 

The data center experienced a 2.5-hour outage, resulting in direct losses of approximately RMB 800,000 (including server damage).

The client has lost its financial industry certification, and the annual contract has been terminated.

 

The correct approach

 

Backup auto-transfer time settings: incoming line undervoltage confirmation delay ≥ 2 s; closing pulse width ≤ 0.5 s.

Add current interlock function: If the standby incoming feeder carries current (indicating that the other side is energized), closing the bus-tie breaker is prohibited.

Prior to commissioning, a real disconnection test of the incoming line (with load simulation) must be conducted, and a joint commissioning report must be prepared and filed.

 

 

Case 10: Cable Intermediate Joint Explosion—The Culprit Turns Out to Be “Failure to Leave Sufficient Allowance”

 

The customer’s trouble

After two years of operation, a 10-kV cable in a certain industrial park experienced an intermediate joint explosion, creating a 30-cm-diameter crater and causing dielectric breakdown in two adjacent cable circuits, resulting in a 36-hour power outage across the area.

 

Root Cause Analysis

 

During installation, the cable length was just sufficient to reach the cabinet; however, no thermal expansion allowance was provided when fabricating the intermediate joint. In summer, cable expansion caused the joint enclosure to crack, resulting in silicone grease leakage and subsequent moisture ingress.

The joint was not sealed with a waterproof box and was directly buried in a waterlogged underground area.

The cable oscillatory wave partial discharge test was not conducted as required, resulting in undetected factory defects.

 

Cause losses

 

Replacement of cable sections and splices: RMB 38,000.

Eleven factories in the power outage area have filed claims totaling approximately RMB 470,000.

The power supply company will deduct credit points from the contractor, resulting in restrictions on future bidding.

 

The correct approach

 

During cable installation, a 3–5 meter S-shaped slack shall be留ed on each side of every intermediate joint, and a stainless steel explosion-proof box combined with waterproof gel shall be installed.

It is strictly prohibited to make more than two splices on the same cross-section, and the distance between any two splices must be ≥10 meters.

Partial discharge testing (≤10 pC) must be performed prior to commissioning, and online monitoring shall be conducted every two years.

 

 

Case 11: DC panel battery aging results in failure to perform switching operations during a fault.

 

The customer’s trouble

A short-circuit fault occurred in the distribution room of a hydropower station. Although the protective devices operated correctly, the circuit breaker failed to trip, and the sustained short circuit ultimately burned out the entire busbar section. Subsequent investigation revealed that the DC panel voltage had dropped to only 18 V (rated at 220 V), which was insufficient to energize the operating mechanism and cause the breaker to trip.

 

Root Cause Analysis

 

The DC panel battery is a maintenance-free lead-acid battery that has been in service for six years (design life: 3–5 years) and has never undergone a verification discharge.

The charging module has been in a float-charge state for an extended period, with no regular equalization charging or battery activation performed, resulting in severe sulfation inside the batteries.

The electrician mistakenly assumed that “a lit charger indicator means everything is normal” and never measured the individual cell voltages.

 

Cause losses

 

The busbar was burned out and the transformer was damaged, with replacement and repair costs totaling approximately RMB 280,000.

A three-day station-wide power outage will disrupt downstream water supply and irrigation, resulting in ecological compensation.

 

The correct approach

 

The individual cell terminal voltage of the storage battery shall be measured quarterly (with a deviation no greater than 0.2 V), and a full verification discharge test shall be conducted annually, discharging to 80% of the rated capacity.

A backup battery pack must be procured in advance after four years of operation, and mandatory replacement is required after five years.

A battery online monitoring system is installed on the DC panel to display internal resistance and state of health in real time.

 

 

Case 12: The safety tool cabinet is merely a metal cabinet, and the failure of the voltage detector creates an electric shock risk.

 

The customer’s trouble

While performing a voltage verification on a 10 kV switchgear cabinet, an electrician at a certain factory, wearing insulated gloves and using a voltage detector, found that the detector did not emit any audible or visual alarms—yet the busbar was actually energized, nearly resulting in the electrician accidentally coming into contact with the live part. Subsequent inspection revealed that the voltage detector’s battery had been depleted and that the insulated gloves had a pinhole puncture.

 

Root Cause Analysis

 

The so-called “safety tool cabinet” is merely a standard metal cabinet and lacks constant-temperature and dehumidification functions; insulated gloves stored in it will gradually become damp and aged over time.

The voltage detector lacks routine self-inspection records, and its battery has been dead for six months without being replaced.

Lack of safety management procedures: The standard practice of “self-checking the voltage detector and testing it while wearing insulated gloves” is not followed before entering the switchgear room.

 

Cause losses

 

Although no actual electric shock occurred, the incident was classified as a serious attempted accident, resulting in a public notification and a fine of RMB 60,000 imposed by the work safety supervision authority.

The entire plant will suspend operations for three days to conduct a safety retraining program.

 

The correct approach

 

The safety tool cabinet must be an intelligent, constant-temperature, dehumidifying cabinet (with relative humidity ≤60%) and equipped with a red-light live-electricity detection socket.

Develop a daily inspection checklist: Before each day’s use, the voltage detector must be self-checked on a known energized conductor.

Insulating gloves and insulating boots shall be submitted for inspection every six months; any that exceed their service life shall be immediately scrapped.

 

 

The above cases cover the most easily overlooked “critical details” in distribution projects from multiple dimensions, including grounding, compensation, neutral conductors, automatic transfer switches, cable joints, DC switchboards, and safety tools. You can select and apply these cases based on the customer’s profile (e.g., industrial plants, residential communities, photovoltaic systems, construction crews, etc.).

 

Conclusion

 

“Each of the above cases is not a fictional tale; rather, they are hard-earned lessons—lessons that cost real money and had real consequences for our peers.”

If you don’t come to me, you’ll probably—no, make that definitely—end up as the star of our next case study collection.

We provide not only equipment, but also It is a comprehensive risk-control solution that covers the entire process, from selection and verification to operations and maintenance. 。

Leave the expertise to the experts—your job is to make money, and ours is to ensure your power distribution system never runs into trouble. ”