How to judge whether the parameters of the distribution cabinet are normal?

To udge whether the parameters of the distribution cabinet are normal or not, it is necessary to systematically check the electrical parameters, mechanical state, operating environment and other dimensions, usually by means of instrument measurement, visual observation and logical verification. The following are the specific criteria and operational points:

A, electrical parameter detection (core indicators)

1. Voltage detection

Testing tools: multimeter, voltmeter or multifunctional power meter.

Jdgment standard:

Rted voltage: the three-phase voltage should be 380 V 7% (line voltage) and the single-phase voltage should be 220 V 7% (phase voltage). If it deviates from the range, check the power supply side or load balance.

Three-phase voltage balance: the voltage deviation of each phase should be ≤10% (for example, phase A is 380V, phase B is 375V and phase C is normal). If the deviation is too large, there may be poor line contact or load imbalance.

Key operation points:

Measure the voltage at the incoming terminal (the upper opening of the circuit breaker) and the outgoing terminal (the lower opening) respectively to confirm whether the on-off of the circuit breaker affects the voltage transmission.

For the dual power switch cabinet, it is necessary to test whether the main and standby power supply voltages are consistent and whether the voltage fluctuation is within the allowable range during switching.

2. Current detection

Testing tools: clamp ammeter, current transformer (with instrument).

Judgment standard:

Load current: Each phase current shall be ≤ the rated current of the equipment (for example, for a circuit breaker with rated current of 100A, the long-term running current shall not exceed 80A), and the instantaneous surge current shall be within the breaking capacity of the circuit breaker.

Three-phase current balance: the deviation of each phase current is ≤10%. If a phase current is abnormally high, it may be a single-phase load or equipment failure (such as motor phase loss).

Key operation points:

When testing, it is necessary to distinguish between active current and reactive current (judged by power factor). If the reactive current is too high, it is necessary to check whether the capacitor compensation device is normal.

For frequently started and stopped loads (such as motors), it is necessary to observe whether the starting current is within the instantaneous multiple allowed by the circuit breaker (such as 5-10 times the rated current).

3. Insulation resistance test

Testing tool: 500V or 1000V megohmmeter (shaking table).

Judgment standard:

Phase-to-phase insulation: the insulation resistance between phase and ground (metal cabinet) shall be ≥0.5MΩ (low-voltage system), and the high-voltage cabinet shall be ≥10MΩ.

Secondary circuit insulation: the insulation resistance of control circuit and signal circuit is ≥1MΩ. If it is lower than the standard, there may be damp lines or aging components.

Key operation points:

Before testing, it is necessary to disconnect all power supplies and remove electronic components (such as PLC and relay) to avoid damaging precision devices.

For the distribution cabinets that have not been operated for a long time, it is necessary to conduct "electrifying and preheating" (no-load operation for 0.5-1 hour) before testing to eliminate the influence of condensed water.

4. Grounding resistance detection

Testing tool: grounding resistance tester.

Judgment standard:

Protective grounding: the resistance between the grounding terminal of the cabinet and the grounding grid is ≤4Ω. If it exceeds the standard, check whether the grounding wire is loose or the grounding body is corroded.

Lightning protection grounding: independent lightning protection grounding resistance ≤10Ω, common earthing (shared with protective grounding) ≤1Ω.

Key operation points:

When testing, it is necessary to disconnect the connection point between the cabinet and the grounding grid (if there are multiple grounding points, it is necessary to test them one by one) to avoid other grounding bodies interfering with the data.

In rainy season or when the soil humidity is high, the grounding resistance may be low, which needs to be compared with historical data.

Second, the mechanical and visual inspection (intuitive judgment)

1. Installation of cabinet and components

Inspection points:

Stability of cabinet: whether the cabinet is firmly installed, whether the screws are loose, and whether there is abnormal sound when shaking the cabinet (internal components may fall off).

Component status:

The surfaces of circuit breakers, contactors and other components are free of scorching and deformation, and the terminals are free of traces of oxidation and ablation (normally silvery white or light gray).

The indicator light and instrument display normally (e.g. the power indicator light is on, and the voltmeter display value is consistent with the actual measurement), and the buttons and switches are operated flexibly without sticking.

Abnormal features:

The temperature of the component shell is too high (if the contactor surface is hot, it should normally be within room temperature+30℃), which may lead to poor contact or overload.

There are traces of arc blackening at the terminal, so it is necessary to cut off the power immediately for maintenance.

2. Lines and connection parts

Inspection points:

Conductor color and identification: the three-phase line (A/B/C) should be yellow/green/red, the zero line should be light blue, the grounding line should be yellow and green, and the identification plate should be clear without falling off.

Connection tightness:

Gently shake the conductor with an insulating wrench, and make sure that the terminal block and the incoming terminal of the circuit breaker are not loose (if the shaking is obvious, it is necessary to re-crimp or replace the terminal).

There is no heating and discoloration at the connection between cable head (such as copper nose) and components (normally it should be metallic, and if it is dark red, it will have poor contact).

Special note:

For electronic equipment such as soft starter and frequency converter, it is necessary to check whether the cooling fan is running and whether the internal capacitor is bulging (bulging needs to be replaced immediately).

III. Functional and Logical Verification (Dynamic Test)

1. Protection function test

Test items:

Overload protection: observe whether the circuit breaker trips within the set time by simulating the load increase (such as gradually connecting the resistance load) (for example, when the rated current is 1.1 times, the trip is normal within 1 hour).

Short-circuit protection: use a short-circuit tester or temporarily short-circuit the outlet terminal (to ensure a safe distance) to test whether the breaking time of the circuit breaker is ≤0.1 second (to meet the technical requirements of the product).

Leakage protection: Input rated leakage current (e.g. 30mA) with a leakage tester, and test whether the leakage circuit breaker operates within 0.1 second.

Judgment standard: All protection functions shall be consistent with the design drawings, and the error of action value shall be ≤ 5%.

2. Automatic control and linkage function

Test scenario:

Dual power supply switching: disconnect the main power supply, and observe whether the standby power supply is automatically switched on within 5 seconds (it needs to be set to "automatic switching" mode in advance).

Fire fighting linkage: simulate the fire signal (such as pressing the smoke test button), check whether the power distribution cabinet cuts off the non-fire power supply as designed, and feed back the signal to the fire control room.

Intelligent monitoring: read the data of distribution cabinet remotely through the background system (such as SCADA), and confirm that the parameters such as voltage, current and power factor are consistent with the site, and the remote control opening and closing function is normal.

Exception handling: If the linkage logic fails, it is necessary to check the secondary circuit wiring (such as whether the relay contacts are on) and program settings (such as whether the PLC logic is wrong).

Four, the environment and temperature monitoring (long-term operation guarantee)

1. Temperature and humidity detection

Detection tool: Thermohygrometer (or intelligent sensor installed in the cabinet).

Standard requirements:

The temperature in the cabinet is ≤40℃ (when the ambient temperature is 25℃). If it exceeds the temperature, check whether the cooling fan is faulty or the vent is blocked.

The relative humidity is ≤85%. If the humidity is too high, start the dehumidifier (such as heating plate and dehumidifier) to avoid short circuit caused by condensation.

2. Temperature rise test (key part)

Detection tool: infrared thermometer (accuracy 2℃).

Key inspection points:

Abnormal warning value of normal temperature rise of components/parts (when the ambient temperature is 25℃)

Circuit breaker terminal ≤40K (surface temperature ≤65℃) ≥70℃

Main contact of contactor ≤50K (surface temperature ≤75℃) ≥85℃.

Cable joint ≤30K (surface temperature ≤55℃) ≥65℃

Key operation points:

The test should be carried out after the load runs stably for 30 minutes, and the temperature at each point should be recorded and compared with the initial value. If the temperature rises suddenly, the reason should be investigated immediately.