KYN28A MV Switchgear Selection Guide: Indoor Metal-Clad Switchgear for Industrial Power Distribution
Introduction
The KYN28A-12 is the workhorse of medium-voltage power distribution in industrial plants. It is the switchgear that sits between the utility transformer (or site main transformer) and your factory's production equipment — protecting expensive assets and, more importantly, the people who operate them.
This selection guide covers everything you need to specify a KYN28A switchgear panel correctly: from busbar ratings and circuit breaker selection to protection schemes, enclosure configurations, and factory testing. It is based on the latest IEC 62271-200 standard and validated by actual type test data.
1. What Is KYN28A Switchgear?
Definition and Architecture
KYN28A-12 is an indoor, metal-clad, withdrawable-type AC metal-enclosed switchgear for rated voltages up to 12kV. The name breaks down as:
| Letter | Meaning |
|---|---|
| K | Metal-clad enclosed (铠装移开式) |
| Y | Withdrawable (移开式) |
| N | Indoor installation (户内) |
| 28 | Design series number |
| A | Improved version |
| 12 | Rated voltage 12kV |
The defining characteristic of metal-clad switchgear is that all high-voltage components are enclosed in earthed metal compartments with defined partition walls. This provides the highest level of personnel safety among MV switchgear types — if one compartment experiences an internal arc fault, it is contained within that compartment, protecting operators and adjacent equipment.
Key Components
┌──────────────────────────────────────────────┐
│ KYN28A Panel (Front View) │
│ │
│ ┌────────────┐ ┌─────────────────────────┐ │
│ │ │ │ │ │
│ │ Busbar │ │ Low-Voltage │ │
│ │ Compart- │ │ Compartment │ │
│ │ ment │ │ (Relays, Meters, │ │
│ │ │ │ Terminal Blocks) │ │
│ │ │ │ │ │
│ ├────────────┤ │ │ │
│ │ │ │ │ │
│ │ Circuit │ ├───────────────────────── │ │
│ │ Breaker │ │ │ │
│ │ Compart- │ │ Cable Compartment │ │
│ │ ment │ │ (Bottom Entry) │ │
│ │ (VCB on │ │ │ │
│ │ Truck) │ │ │ │
│ └────────────┘ └─────────────────────────┘ │
└──────────────────────────────────────────────┘Four independent compartments (meeting LSC-2B classification per IEC 62271-200):
- Busbar compartment: Main horizontal busbars, branch connections, busbar PT if fitted
- Circuit breaker compartment: VCB on withdrawable truck, shutter mechanism, earthing switch
- Low-voltage compartment: Protection relays, control wiring, terminal blocks, auxiliary power
- Cable compartment: Power cable terminations, current transformers, voltage transformers, earthing switch
2. Rated Parameters — The Selection Checklist
Every KYN28A specification must define these parameters. Use this as your checklist.
Core Ratings
| Parameter | Standard Range | How to Select |
|---|---|---|
| Rated voltage (Ur) | 7.2kV / 12kV | Match system nominal voltage. For 11kV systems, select 12kV rated. |
| Rated frequency (fr) | 50Hz / 60Hz | Match country standard. 50Hz for IEC countries, 60Hz for North America and parts of Asia. |
| Rated current — busbar (Ir) | 630A / 1250A / 2000A / 2500A / 3150A / 4000A | Select ≥ transformer secondary rated current × 1.2 at the switchgear voltage level. |
| Rated current — feeder | 630A / 1250A / 2000A / 3150A | Select ≥ the load current of the largest feeder. |
| Rated short-circuit breaking current (Isc) | 25kA / 31.5kA / 40kA | Must be ≥ the calculated maximum three-phase fault current at the switchgear busbar. |
| Rated short-time withstand current (Ik) | 25kA/3s / 31.5kA/3s / 31.5kA/4s / 40kA/3s | Equal to the breaking current, with specified duration. |
| Rated peak withstand current (Ip) | 63kA / 80kA / 100kA | = 2.5 × Ik for 50Hz, 2.6 × Ik for 60Hz. |
| Rated insulation level | 42kV/75kV (impulse/power-frequency for 12kV) | Per IEC 62271-1. Not negotiable — this is type-tested. |
Example Selection
For a factory with the following data:
- 2500kVA transformer, 11kV/400V, 6% impedance
- Short-circuit current at MV busbar: 11.7kA (system contribution + motor contribution)
- 4 outgoing feeders (2 × 400A to sub-transformers, 1 × 200A to capacitor bank, 1 × 630A to large motor)
Selected KYN28A specification:
- Rated voltage: 12kV
- Busbar rated current: 1250A (transformer secondary = 131A at 11kV → 1250A provides generous headroom)
- Breaking capacity: 25kA minimum (31.5kA recommended for future-proofing)
- Incomer: 1250A VCB
- Feeders: 1× 1250A (motor), 2× 630A (sub-transformers), 1× 630A (capacitor)
- Protection: Microprocessor-based relays (50/51, 50N/51N, 27/59)
3. Vacuum Circuit Breaker (VCB) Selection
The heart of every KYN28A panel is the withdrawable vacuum circuit breaker. ZY POWER typically equips KYN28A with VS1 or ZN85 series VCBs.
VCB Comparison
| Parameter | VS1-12 | ZN85-40.5 |
|---|---|---|
| Rated voltage | 12kV | 40.5kV |
| Rated current | 630–3150A | 630–2500A |
| Breaking capacity | 25–40kA | 25–31.5kA |
| Mechanical endurance | 30,000 ops | 10,000 ops |
| Electrical endurance (at rated Isc) | 50 ops | 30 ops |
| Operating mechanism | Spring-stored energy (motor-charged) | Spring-stored energy (motor-charged) |
| Standard | IEC 62271-100 | IEC 62271-100 |
VCB Sizing Rule
The VCB rated current must be ≥ the feeder's design load current. The VCB breaking capacity must be ≥ the fault current at the VCB's location.
Important: The busbar fault level is NOT the same as the feeder fault level. A feeder 200m away from the busbar will have lower fault current due to cable impedance. Always calculate at the equipment terminals.
# Simplified fault level at feeder end
Isc_feeder = Isc_busbar / (1 + (Z_cable / Z_source))Operating Mechanism: Motor-Charged Spring
Standard KYN28A VCBs use a motor-charged spring-stored energy mechanism:
- Charging motor (110V DC or 220V AC/DC) tensions the closing spring (~15 seconds)
- Close coil releases the spring to close the breaker
- Trip coil opens the breaker on protection relay command
- Spring automatically recharges after closing (ready for O-C-O duty cycle)
Standard auxiliary voltages: 110V DC, 220V DC, 220V AC. DC systems require a station battery with charger. AC systems can operate from a UPS-backed supply.
4. Busbar System Configuration
Busbar Ratings and Material
KYN28A busbars are typically electrolytic copper (ETP grade) with tin or silver plating.
| Rated Current | Minimum Copper Size | Busbars per Phase |
|---|---|---|
| 630A | 40×5 mm | 1 |
| 1250A | 60×6 mm | 1 |
| 2000A | 80×8 mm | 1 |
| 2500A | 80×10 mm | 2 |
| 3150A | 100×10 mm | 2 |
| 4000A | 120×10 mm | 2 (forced cooling optional) |
Single Busbar vs Segmented Busbar
| Configuration | Description | When to Use |
|---|---|---|
| Single busbar | All incoming and outgoing circuits connected to one continuous busbar | Small substations, ≤ 5 panels, non-critical loads |
| Single busbar with bus-section | Busbar split into two sections with a bus-section circuit breaker (bus coupler) | Standard for industrial substations — allows maintenance on one section while the other remains energized |
| Double busbar | Two independent busbar systems with bus coupler | Critical applications where supply changeover must be seamless (rare in general industrial use) |
Industrial standard: Single busbar with bus-section. Two incomers (from two transformers or two utility feeds) feed the two bus sections. The bus-section breaker is normally open — if one incomer fails, the bus-section closes and the remaining incomer carries both sections.
5. Protection Schemes
Standard Protection Package
Every KYN28A feeder should have, at minimum:
| Function | ANSI Code | Purpose |
|---|---|---|
| Phase overcurrent | 50/51 | Protects against overloads and phase-to-phase faults |
| Earth fault | 50N/51N | Detects ground faults — essential for safety |
| Circuit breaker failure | 50BF | Initiates backup tripping if primary breaker fails |
Incomer Panel
Additional functions for the main incoming supply:
- Under/over voltage (27/59): Trip on supply loss or abnormal voltage
- Directional overcurrent (67): For parallel transformer operation
- Synchro-check (25): If manual or automatic paralleling is required
Transformer Feeder
- Transformer differential (87T): For transformers ≥ 2MVA — detects internal winding faults
- Restricted earth fault (64REF): Complements differential for winding-to-ground faults
- Overload (49): Thermal image protection for the transformer winding
Motor Feeder
- Thermal overload (49): Matched to motor thermal time constant
- Locked rotor / stall protection (51LR): Detects motor stall during starting
- Number of starts limiter (66): Prevents excessive thermal stress from repeated starts
- Unbalance protection (46): Detects single-phasing and voltage unbalance
6. Physical Dimensions and Installation
| Panel Rating | Width (mm) | Depth (mm) | Height (mm) | Weight (kg) |
|---|---|---|---|---|
| 630A–1250A | 800 | 1500 | 2300 | ~800 |
| 1250A–2000A | 800 | 1500 | 2300 | ~900 |
| 2500A–3150A | 1000 | 1650 | 2300 | ~1100 |
| Bus riser / PT | 800 | 1500 | 2300 | ~600 |
Installation requirements:
- Indoor installation only (IP4X enclosure standard)
- Concrete plinth or steel channel base frame
- Minimum clearance: 800mm front, 800mm rear, 600mm side (per IEC 61936-1)
- Operating aisle: ≥ 1500mm for VCB truck withdrawal
- Ambient temperature: -5°C to +40°C (derating above 40°C)
- Relative humidity: ≤ 95% (non-condensing)
- Altitude: ≤ 1000m (derating 1% per 100m above 1000m)
7. Factory Testing and Type Test Certificates
ZY POWER's KYN28A-12 switchgear has been type-tested by an independent IEC 17025 accredited laboratory:
| Test | Standard | Result |
|---|---|---|
| Dielectric test (power-frequency) | IEC 62271-200 §6.2 | ✅ 42kV/1min |
| Dielectric test (lightning impulse) | IEC 62271-200 §6.2 | ✅ 75kV (1.2/50μs) |
| Temperature rise test | IEC 62271-200 §6.5 | ✅ ≤ 65K rise at 1250A |
| Short-circuit test (31.5kA/3s) | IEC 62271-200 §6.6 | ✅ No damage |
| Internal arc test (31.5kA/1s) | IEC 62271-200 Annex A | ✅ IAC-A-FL (front/lateral) |
| Mechanical endurance | IEC 62271-200 §6.9 | ✅ 2000 operations |
| IP degree verification | IEC 60529 | ✅ IP4X |
Type Test Certificate: 2025XHT04078 Test Laboratory: Zhejiang High-Voltage Electrical Apparatus Testing Institute (国家电器安全质量监督检验中心) Date: 2025
Download the full type test report →
8. Common Configurations for Industrial Applications
Configuration A: Single Incomer + 4 Feeders (Small Factory)
[Incomer 1250A] — [Busbar 1250A] — [F1 630A] [F2 630A] [F3 200A] [F4 630A]
|
[Bus PT]Panels required: Incomer (1) + Feeder (4) + Bus PT (1) = 6 panels Footprint: ~5m wall length
Configuration B: Dual Incomer + Bus-Section + 8 Feeders (Medium Factory)
[Inc-1 1250A] — [Bus-A 1250A] — [F1][F2][F3][F4] — [Bus-Sec 2000A] — [Bus-B 1250A] — [F5][F6][F7][F8] — [Inc-2 1250A]Panels required: Incomers (2) + Bus-Section (1) + Feeders (8) + Bus PT×2 (2) = 13 panels Footprint: ~11m wall length
Configuration C: Dual Incomer + 2 Transformer Feeders + 4 Distribution Feeders
[Inc-1] — [Bus-A 1250A] — [TF-1] [F1] [F2] — [BS] — [Bus-B 1250A] — [TF-2] [F3] [F4] — [Inc-2]
↓ ↓
[T1 2500kVA] [T2 2500kVA]
↓ ↓
[LV Switchboard] — [Bus Coupler] — [LV Switchboard]This configuration provides N+1 redundancy at both MV and LV levels — the gold standard for continuous-process factories. The cost premium over Config A is approximately 80% (additional transformer, switchgear panels, cables, and controls), but for facilities where unplanned downtime costs exceed $50,000 per day, the payback on redundancy can be measured in weeks rather than years.
9. KYN28A vs Other MV Switchgear Types
When specifying MV switchgear for a factory, you will encounter three common types from Chinese manufacturers. Here is how they compare:
| Feature | KYN28A-12 | XGN-12 | KYN61-40.5 |
|---|---|---|---|
| Rated voltage | 12kV | 12kV | 40.5kV |
| Enclosure type | Metal-clad (LSC-2B) | Metal-enclosed (LSC-2A) | Metal-clad (LSC-2B) |
| Breaker mounting | Withdrawable (truck) | Fixed (bolted) | Withdrawable (truck) |
| Maintenance isolation | Automatic shutters | Manual disconnector | Automatic shutters |
| Panel width (1250A) | 800mm | 800mm | 1000mm |
| Panel weight (1250A) | ~800kg | ~600kg | ~1200kg |
| Relative cost | 1.0× (baseline) | 0.7–0.8× | 1.8–2.2× |
| Best use case | Standard industrial distribution | Budget-constrained, non-critical | High-voltage utility connection |
Recommendation: For 11kV industrial distribution, KYN28A is the correct choice. Its withdrawable VCB design cuts maintenance downtime from hours (XGN: disconnect, unbolt, remove, reinstall, retest) to minutes (KYN28A: rack out, rack in, done). The automatic shutter mechanism — a metal plate that closes over the busbar contacts when the VCB is withdrawn — provides a level of operator safety that fixed switchgear cannot match.
FAQ
Q: What is the difference between KYN28A and KYN61?
KYN28A is rated for 12kV maximum, while KYN61 extends to 40.5kV. KYN28A uses a more compact busbar arrangement and is the standard choice for 11kV industrial distribution. KYN61 is used where the utility supply is 33kV or 35kV and step-down is required at a remote substation. For most factory applications with 11kV intake, KYN28A is the correct choice — it is more cost-effective and has a larger installed base for spare parts availability.
Q: Can KYN28A operate at 60Hz?
Yes. The standard KYN28A is designed for 50Hz, but can be supplied for 60Hz operation. The main change is in the protection relay settings and the VCB closing/tripping coil ratings. All other components (busbars, insulators, CTs, VTs) are rated for 50/60Hz operation. Always specify the frequency at the quotation stage.
Q: How do I calculate the required busbar current rating?
Take the transformer secondary rated current at the MV voltage level. For example, a 2500kVA transformer at 11kV has a secondary current of: I = 2500 / (√3 × 11) = 131A. The busbar should be rated at ≥ 1250A to accommodate future expansion and provide thermal margin. The standard KYN28A busbar rating of 1250A covers the vast majority of industrial applications up to approximately 20MVA transformer capacity.
Q: What maintenance does KYN28A require?
KYN28A is designed for minimal maintenance. Recommended schedule: (a) Annual visual inspection — check for dust accumulation, condensation, and rodent ingress. (b) Every 3 years or 2000 operations — lubricate the VCB truck mechanism and shutter mechanism. (c) Every 5 years — test the protection relays with secondary injection. (d) Every 10 years or 10,000 operations — replace the VCB vacuum interrupters (they have a finite contact wear life). The VCB itself requires no oil changes or gas top-ups — vacuum is maintenance-free by design.
Q: Can I add new feeder panels to an existing KYN28A lineup?
Yes — one of KYN28A's advantages is modularity. New panels can be added to either end of an existing lineup. The busbar is designed with bolted joints between panels for extension. Procedure: (1) de-energize the affected busbar section, (2) remove the end cover plate, (3) bolt the new panel to the existing lineup, (4) connect busbar links, (5) test, (6) recommission. Always verify that the existing busbar short-circuit rating is adequate for the expanded configuration.
Q: Is KYN28A suitable for outdoor installation?
Not in standard configuration. KYN28A is rated IP4X — protected against solid objects >1mm but not weatherproof. For outdoor installation, the switchgear must be housed in a weatherproof enclosure (containerized substation or E-house) with climate control. ZY POWER offers pre-fabricated containerized substations that house KYN28A switchgear together with transformers and LV panels in a single weatherproof package.
Engineering Evidence
Applicable Standards
| Standard | Clause / Scope | Relevance to This Guide |
|---|---|---|
| IEC 62271-1 | Common specifications for HV switchgear | Rated insulation levels in §2 |
| IEC 62271-100 | HV switchgear — AC circuit-breakers | VCB specifications in §3 |
| IEC 62271-200 | AC metal-enclosed switchgear (>1kV) | KYN28A enclosure classification (LSC-2B) in §1 |
| IEC 62271-200 Annex A | Internal arc classification | IAC-A-FL certification in §7 |
| IEC 60529 | Degrees of protection (IP Code) | IP4X enclosure rating in §6 |
| IEC 61936-1 | Common rules for HV installations >1kV | Installation clearances in §6 |
| IEEE C37.20.2 | Metal-clad switchgear | Equivalent US metal-clad standard |
| IEC 60076-1 | Power transformers — General | Transformer protection settings in §5 |
| IEC 60255 | Measuring relays and protection equipment | Protection relay functions in §5 |
Type Test Certificates
| Certificate No. | Equipment | Rating | Laboratory | Date | Key Tests Passed |
|---|---|---|---|---|---|
| 2025XHT04078 | KYN28A-12 | 1250A / 31.5kA | Zhejiang HV Electrical Apparatus Testing Inst. (国家电器安全质量监督检验中心) | 2025 | Dielectric 42kV/1min ✅ · Lightning impulse 75kV ✅ · Temperature rise 65K ✅ · Short-circuit 31.5kA/3s ✅ · Internal arc IAC-A-FL 31.5kA/1s ✅ · Mechanical 2000 ops ✅ · IP4X ✅ |
| 2025XHT04079 | SRM-12(C) RMU | 630A / 25kA | Same laboratory | 2025 | Full type test passed ✅ |
Download the complete KYN28A Type Test Report (PDF) →
Factory Engineering Data
- VCB mechanical endurance testing: ZY POWER conducts 500-operation sequences on every VS1 VCB before shipment, verifying closing time (≤60ms), opening time (≤45ms), and contact resistance (≤45μΩ).
- Busbar joint resistance: Every bolted busbar connection measured at ≤ 1.2× the resistance of an equivalent length of continuous busbar per IEC 62271-200 §6.4.
- Protection relay calibration: Secondary injection testing on every protection relay with 3-point verification (10%, 100%, 200% of setting) before panel shipment.
- Dielectric routine test: 42kV/1min power-frequency withstand on every completed panel (routine test, not just type test).
Application Example
Automotive Assembly Plant, Middle East (2025): 13-panel KYN28A lineup supplying two 3150kVA transformers (N+1 configuration) for a robotic assembly line. Critical requirement: zero-tolerance for nuisance tripping during robot welding sequences that draw 300% inrush current for 2-3 cycles. Solution: KYN28A incomers with 1250A VS1 VCBs and microprocessor relays with configurable inrush restraint (2nd harmonic blocking). Bus-section VCB rated 2000A for full-load transfer. All 13 panels factory-tested as a complete lineup before containerized shipment. Site commissioning: 5 days from container arrival to energization. First 18 months: zero nuisance trips, zero protection failures.
Reviewed by: ZY POWER Engineering Team
Last Updated: 2026-06-25
Q: How does KYN28A compare with XGN and KYN61 series?
These three are the most common MV switchgear types from Chinese manufacturers. KYN28A (12kV, metal-clad withdrawable) is the workhorse for indoor factory substations — best value, largest installed base, easiest maintenance due to truck-mounted VCB withdrawal. XGN (12kV, metal-enclosed fixed) uses fixed-mounted breakers with disconnectors — lower cost but requires more room for maintenance access and cannot achieve the same LSC classification as KYN28A. KYN61 (40.5kV) is for higher voltage applications (33kV/35kV utility connections) — larger, heavier, and more expensive. For 11kV industrial distribution, KYN28A is the standard choice unless budget is severely constrained (in which case XGN may be considered for non-critical feeders only).
Q: What is the expected service life of KYN28A switchgear?
With proper installation in a clean, climate-controlled indoor environment and regular maintenance, KYN28A switchgear has a design life of 30-40 years. The vacuum circuit breakers have a mechanical endurance of 30,000 operations (VS1 series) and electrical endurance of 50 full-rated short-circuit interruptions. In typical industrial service with 10-20 switching operations per year, the mechanical life far exceeds the equipment's economic life. The limiting factors are: (a) protection relay obsolescence (microprocessor relays typically have 15-20 year support life), (b) busbar insulation aging in hot/humid environments, and (c) vacuum interrupter contact wear (monitored by contact wear indicators on modern VCBs).
Q: What auxiliary power supply is needed for KYN28A?
The KYN28A requires auxiliary power for: VCB spring-charging motor (typically 200W), protection relays (30-50VA each), indicating lamps, space heaters (anti-condensation, 50-100W per panel), and trip/close coils. Standard auxiliary voltages are 110V DC (battery-backed), 220V DC, or 220V AC (UPS-backed). A typical 10-panel KYN28A lineup draws approximately 800-1200W of auxiliary power. A station battery system (110V DC, 40-65Ah) with charger is recommended — it ensures protection relays and trip coils remain operational even during a complete AC power loss, which is precisely when they are needed most.
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