Component Snapshot At-a-Glance
- Model: MCHN02D1AB0005A
- Alt. P/N: MCHN02H1AB0004A (high-burden coil variant, incompatible DC winding draw); MCGG series (overcurrent only, no low-load detection logic)
- Product Series: MCHN Midos static numerical protection relay family, shared rack base footprint with MCGG/MVAJ relay lineup
- Hardware Type: Full withdrawable plug-in static undercurrent protection relay, yellow aluminum industrial chassis, transparent front adjustment cover
- Key Feature: D-series low-burden 110/125VDC control coil, adjustable undercurrent pickup + independent definite time trip delay
- Primary Field Use: Detect sustained low motor/generator load to trigger dry-run pump trip, generator slip protection, and light-load abnormal equipment alarms.
Hard-Numbers: Technical Specifications
- Protocol Support: Static analog sampling hardware, no digital bus communication; dry volt-free alarm/trip changeover contacts
- Port Count: Single CT 1A secondary input terminals, separate DC coil supply terminals, dual alarm/trip contact terminal sets
- Baud/Data Rate: No serial data transmission
- Operating Temperature: -10°C to +55°C cabinet operational; -40°C to +85°C storage
- Isolation Rating: 2000Vrms dielectric withstand between CT measurement circuits and trip contact wiring
- Power Draw: Low-burden D coil, minimal DC battery draw under multi-relay panel loading
- CT Secondary Rating: 1A nominal CT input
- Auxiliary DC Supply: 110/125 VDC station control battery
- Undercurrent Pickup Range: 0.05×In to 0.8×In adjustable via front DIP switches
- Definite Trip Delay Window: 0.1s to 10s fixed time delay for low-load condition confirmation
- Contact Continuous Rating: 5A @300VDC resistive load; 40W DC inductive breaking capacity
- Physical Weight: 4.3kg fully assembled draw-out unit
The Real-World Problem It Solves
Standard MCGG overcurrent relays only flag high-magnitude overload and short-circuit faults. They cannot catch low-load undercurrent states that burn pump impellers or push generators into unstable slip operation.Stacking discrete timing relays alongside CT monitoring hardware wastes rack space and adds extra calibration points that drift with cabinet vibration and temperature swings.High-burden MCHN02H coils pull heavy DC current during simultaneous multi-feeder faults; station battery voltage collapses, and protection logic fails to pick up for critical low-load trip commands.Where you’ll typically find it:
- Fossil power plant generator auxiliary anti-slip low active power protection panels
- Refinery MV pump/compressor MCC dry-run prevention low-load trip schemes
- Offshore FPSO process pump critical low-flow undercurrent monitoring cabinetsThis single draw-out unit combines adjustable pickup and time delay to eliminate external discrete timing hardware and avoid DC bus sag failure during mass fault events.
Hardware Architecture & Under-the-Hood Logic
This unit uses a dedicated isolated analog front-end paired with fixed-function static measurement logic, no user programmable microprocessor code. It shares Midos rack base mechanical design with all ALSTHOM Midos protection relays for unified cabinet layout.
- Single-phase 1A CT secondary current feeds filtered isolated sampling circuitry to block substation VFD and breaker switching EMI noise.
- Onboard ADC continuously samples RMS current values, compares measured magnitude against DIP-switch set undercurrent pickup threshold.
- Internal fixed-time timer activates once current drops below the pickup setting; timer elapse triggers contact state change to avoid nuisance transient low-load alarms.
- Low-wattage D-series coil winding limits total DC load draw on station battery, preventing voltage collapse during concurrent multi-circuit fault events.
- Front panel LED indicators show healthy auxiliary supply, low-load active condition, and time-delay trip latched status for quick on-site visual diagnostics.
- Rear Midos rack terminal base integrates factory CT short-circuit jumpers; full chassis withdrawal possible without disconnecting CT secondary wiring to eliminate lethal open CT surge hazards.
Field Service Pitfalls: What Rookies Get Wrong
Swapping MCHN02H High-Burden Coil Variant For MCHN02D Low-Burden Unit
New technicians pull MCHN02H spare stock without matching model suffix. During multi-feeder simultaneous faults, station DC battery voltage sags severely; high-burden coil fails to energize, pumps run dry unprotected and generators drift into unstable slip.Field Rule: MCHN02D low-burden variant mandatory for dense multi-relay protection panels; segregate D and H coil MCHN spares in clearly labeled storage bins.
Skipping CT Short Jumper Installation Before Withdrawing Draw-out Relay
Apprentices pull the relay chassis straight out without shorting CT terminals on the rack base. Open CT secondary windings generate kilovolt surge that damages upstream MCGG overcurrent relay sampling boards and creates cabinet shock risk.Quick Fix: Always install factory CT short-circuit jumpers on the Midos base terminal strip before removing any draw-out protection relay.
Misadjusting Pickup & Time Delay Without Cross-Checking Protection Drawings
Maintenance crews randomly tweak DIP switch settings during spare replacement without referencing site equipment protection logic sheets. Misconfigured thresholds either trip pumps unnecessarily during normal flow fluctuations or fail to catch dangerous sustained dry-run low-load conditions.Field Rule: Record undercurrent pickup value and trip time delay on the relay door label after every setting change; cross-verify against equipment protection scheme drawing before returning the relay online.
Commercial Availability & Pricing Note
Please note: The listed price is for reference only and is not binding. Final pricing and terms are subject to negotiation based on current market conditions and availability.
