What is XL-21 Low Voltage Power Distribution Cabinet

What is XL-21 Low Voltage Power Distribution Cabinet

I. Basic Model Definition & Parameter Description

1. Definition & Model Meaning

Definition of XL-21 Low Voltage Power Cabinet: XL-21 is a national standard indoor enclosed fixed-type low-voltage power distribution cabinet. It is applicable to AC 50Hz, 400V three-phase low-voltage power distribution system. It is mainly used for power distribution, start-stop control of fan & water pump motors, line overload and short-circuit protection in industrial workshops, building machine rooms and electromechanical supporting equipment. It is the most widely-used terminal power distribution complete equipment covered by CCC certification in China, with all components fixed-mounted and non-drawable structure.

X: Enclosed cabinet type; L: Special for power distribution; 21: The second-generation optimized design serial number of national standardized indoor fixed power cabinet

Remark: XL-21 is a customized standardized cabinet in China, not drawer-type or assembled cabinet, all internal components adopt fixed installation mode.

2. Rated Electrical Parameters

II. Cabinet Structure, Plate Technology & Standard Dimension

1. Cabinet Plate & Production Process

1. Standard Plate: Cabinet main body adopts 1.5mm cold-rolled steel plate, door load-bearing frame adopts 2.0mm cold-rolled steel plate, processed by CNC integral bending and CO₂ seamless welding

2. Surface Treatment Process: Degreasing & Acid Pickling → Phosphorization Anti-corrosion → Epoxy Resin Electrostatic Powder Coating, standard film thickness 60-80μm, anti-corrosion and flame-retardant

3. Material Upgrade: 304 stainless steel cabinet adopted for basement, chemical and humid environment, high anti-corrosion performance without surface coating

4. Installation Mode: Wall-mounted floor fixed type, single front door, opening angle ≥120°, only front operation and maintenance, no rear door, double-side maintenance unavailable

2. Universal Standard Dimension

1. Universal Main Model: 800×400×1800mm

2. Small-circuit Economical Model: 600×400×1700mm

3. Multi-circuit Large-capacity Model: 1000×500×2000mm

4. Wall-mounted Mini XL-21: 500×300×800mm

3. Three Independent Compartments Inside Cabinet

1. Top Busbar Compartment: Independent closed structure, TMY copper busbar fixed by insulator, standard phase-to-phase clearance ≥20mm to prevent phase-to-phase flashover short circuit

2. Middle Component Installation Compartment: 25mm modular adjustable mounting rail, centralized installation of circuit breaker, contactor, instrument and button, high-low voltage wiring layered layout

3. Bottom Cable Inlet & Outlet Compartment: Equipped with detachable cable grommet and cable fixing crossarm, supporting top cable entry and bottom cable entry, reserved grounding bar fixing position at cabinet bottom

4. Heat Dissipation & Protection Requirements

• Standard Configuration: Top dust-proof louver for natural ventilation

• Forced Heat Dissipation Requirement: Axial flow fan + temperature control switch must be equipped when main circuit current ≥400A, starting temperature threshold 40℃

• Protection Application Classification: IP30 for indoor dry dust-free environment; IP40 for indoor dust-proof and anti-touch environment; IP54 for basement and water plant splash-proof environment; IP65 for temporary outdoor rainproof application

III. Primary Main Circuit Professional Specifications

1. Power Supply System Standard

Standard system: Three-phase five-wire system, phase color specification: L1-Yellow, L2-Green, L3-Red, N-Light Blue, PE-Yellow Green Bicolor. It can be modified to three-phase four-wire or three-phase three-wire system. N neutral wire and PE protective wire are forbidden to be connected inside cabinet, only single-point grounding allowed at transformer neutral point.

2. TMY Tinned Copper Busbar Selection Standard

Design current criterion: Under ambient temperature 40℃, allowable current density of copper busbar ≤2.5A/mm², long-term operating temperature rise ≤70K, excessive temperature rise will accelerate insulation aging.

Matching specification of tinned copper busbar:

1. ≤250A: TMY-30×4

2. ≤400A: TMY-40×5

3. ≤630A: TMY-60×6

Busbar Construction Specification: Uniformly apply electric power compound grease on contact surface, fasten bolts diagonally with standard tightening torque, full-surface tin plating required for all copper busbars for anti-oxidation.

3. Three Standard Primary Circuit Configuration Schemes

Scheme 1: Power Feeding Distribution Cabinet

Incoming molded case circuit breaker + multi-outgoing circuit breakers, with overload and short-circuit protection, delay residual current protection optional.

Scheme 2: Special Motor Control Cabinet

Motor start-stop classification standard:

• Motor ≤15kW: Direct full-voltage start

• Motor 15kW~75kW: Star-delta reduced voltage start

• Motor ≥75kW: Auto-transformer reduced voltage start / Soft start

Motor protection setting: Thermal relay setting current = 1.0~1.05 times of motor rated current, with overload and phase-loss dual protection.

Scheme 3: On-site Reactive Power Compensation Cabinet

Incoming switch + shunt capacitor + switching contactor + reactor, on-site reactive power compensation, improve plant power factor above 0.9 and reduce transformer loss.

4. Selectivity of Upper & Lower Level Protection

Upper main molded case circuit breaker: Cancel instantaneous short-circuit trip, retain long-time delay overload protection and short-time delay short-circuit protection; Lower outgoing breaker: Retain instantaneous short-circuit trip, to realize local circuit tripping only without whole cabinet power failure during fault.

IV. Secondary Control Circuit Standard Specifications

1. Four Functional Zones of Secondary Circuit

1. Measurement Circuit: Pointer voltmeter & ammeter, multi-function power meter

2. Control Circuit: Start-stop button, transfer switch, AC contactor, intermediate relay

3. Signal Circuit: Power indicator light, operation indicator light, fault indicator light

4. Interlock Protection Circuit: Thermal relay interlock, fire linkage tripping, under-voltage & loss-voltage protection

2. Secondary Wiring National Standard Specification

1. 1.5mm² multi-core flame-retardant copper flexible wire adopted for control wire, equipped with permanent cable number tube

2. Secondary weak current wire and primary power busbar wired in separate layers and grooves, mixed wiring forbidden

3. Handover test standard: Insulation resistance of secondary circuit to ground ≥2MΩ tested by 500V megohmmeter

3. Core Electrical Protection Logic

• Loss-voltage Protection: Contactor releases automatically after power cut, motor self-starting forbidden after power recovery to avoid safety accident

• Phase-loss Protection: Thermal relay cuts off control circuit immediately once any phase power loss, preventing motor single-phase burnout

• Fire Linkage Protection: Connect fire passive contact to cut off power load emergently in fire condition

V. Environmental Derating & Installation National Standard Requirements

1. Altitude Requirement: Standard applicable altitude ≤1000m; Component derating 10% for 1000m-2000m; Component derating 20% for 2000m-3000m with enhanced phase-to-phase insulation

2. Ambient Temperature: Standard applicable temperature -5℃~+40℃, 24h average temperature ≤35℃; Equipment derating 15% for long-term ambient temperature above 40℃

3. Humidity Requirement: Relative humidity ≤90% at 20℃ without condensation; Dehumidifier heater required for condensation environment

4. Forbidden Application Environment: Environment with flammable & explosive gas, strong acid-base corrosive medium, conductive dust and severe vibration

5. Installation Standard: Cabinet verticality deviation ≤5°, splicing gap of adjacent cabinets ≤2mm, foundation channel steel reliable grounding, total cabinet grounding resistance ≤0.1Ω

VI. Common On-site Fault Causes & Troubleshooting

1. Terminal & busbar heating ablation: Insufficient bolt tightening torque, unplated copper busbar, no electric power compound grease coated, long-term overload operation

2. Accidental motor tripping: Undersized thermal relay setting, motor winding aging, power phase loss, excessive cable voltage drop

3. Contactor adhesion after power-off: Contact ablation caused by motor short circuit, coil voltage deviation, damaged arc extinguishing cover

4. Whole cabinet selective tripping failure: Mismatched protection curve of upper and lower breakers, unclassified instantaneous trip parameter

5. Cabinet condensation tripping: Condensation caused by temperature difference, no dehumidifier equipped, insufficient protection grade selection

VII. Frequently Asked Questions(FAQs)

1. Q1: Can XL-21 cabinet be used outdoors directly?
A: Standard IP30 model is only for indoor dry environment. IP54/IP65 rainproof customized model is mandatory for outdoor application, standard cabinet is forbidden for open-air installation.

2. Q2: Can XL-21 carry load above 630A?
A: Negative. National standard limits XL-21 maximum rated current at 630A, GGD low voltage cabinet must be adopted for load above 630A.

3. Q3: Can neutral wire and protective ground wire be connected together inside cabinet?
A: Forbidden in standard three-phase five-wire system. Only single-point grounding at transformer neutral point is allowed, N busbar and PE busbar shall be independent inside cabinet.

4. Q4: What current value needs cabinet built-in cooling fan?
A: Temperature-controlled cooling fan is mandatory when main incoming current ≥400A to prevent busbar overheating tripping.

5. Q5: Recommended motor starting mode?
A: Direct start for motor below 15kW; Star-delta start for 15kW-75kW motor; Soft start preferred for motor above 75kW for better protection stability.

6. Q6: Core difference between XL-21 and GGD cabinet?
A: XL-21 is single-door fixed cabinet with small floor area and low cost for terminal power load; GGD is double-side maintainable cabinet with large current capacity for substation main incoming power distribution.

7. Q7: Auxiliary accessories required for humid basement application?
A: Temperature & humidity dehumidifier equipped, protection grade upgraded to IP54 to avoid condensation short circuit inside cabinet.