Component Snapshot At-a-Glance

• Model: MVUA11B1BD0784B
• Alt. P/N: MVUA11D1BD0784B (low-burden coil variant, different DC winding draw); MVAX series (DC trip supervision, incompatible AC measurement logic)
• Product Series: MVUA Midos draw-out AC voltage protection relay family, shared rack base footprint with MCGG/MVAJ relay lineup
• Hardware Type: Full withdrawable plug-in static single-phase AC undervoltage relay, yellow aluminum industrial chassis, transparent front adjustment cover
• Key Feature: B-series medium burden 110/125VDC auxiliary coil, adjustable AC undervoltage pickup with fixed definite time trip delay
• Primary Field Use: Detect sustained AC bus voltage drop to trigger motor stall alarms, generator undervoltage trip, and distribution bus fault alerts.

Hard-Numbers: Technical Specifications

• Protocol Support: Static analog sampling hardware, no digital bus communication; dry volt-free alarm/trip changeover contacts
• Port Count: Single AC voltage measurement input terminals, separate DC auxiliary 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 AC measurement circuits and trip contact wiring
• Power Draw: Medium burden B coil, balanced DC battery draw under multi-relay panel loading
• Measured AC Input Range: 0–130% nominal AC system voltage
• Auxiliary DC Supply: 110/125 VDC station control battery
• Undervoltage Pickup Adjustment: 40%–80% nominal AC voltage via front rotary dial
• Definite Trip Delay Window: 0.2s to 10s fixed time delay for voltage collapse confirmation
• Contact Continuous Rating: 5A @300VDC resistive load; 40W DC inductive breaking capacity
• Physical Weight: 0.43kg fully assembled draw-out unit

The Real-World Problem It Solves

Standard overcurrent relays only react to high-magnitude overload and short-circuit faults. They cannot flag sustained AC bus undervoltage, which burns motor windings from reduced torque and stalls generator excitation systems.Stacking discrete voltage transducers and timing relays wastes rack space and adds calibration points that drift with cabinet vibration and temperature swings.Low-burden MVUA11D coils create unbalanced DC load distribution on dense multi-relay panels; simultaneous fault events cause uneven DC bus sag and intermittent relay dropout.Where you’ll typically find it:

• Fossil power plant generator auxiliary AC bus undervoltage protection panels
• Refinery MV motor MCC low AC supply stall prevention trip schemes
• Urban distribution substation transformer secondary bus voltage collapse monitoring cabinetsThis single draw-out unit combines adjustable voltage pickup and time delay to eliminate external discrete timing hardware and stabilize DC panel loading during mass fault events.

Hardware Architecture & Under-the-Hood Logic

This unit uses a dedicated isolated AC 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.

1. Single-phase AC bus voltage feeds filtered isolated sampling circuitry to block substation VFD and breaker switching EMI noise.
2. Onboard voltage divider network continuously samples RMS AC voltage values, compares measured magnitude against front dial set undervoltage pickup threshold.
3. Internal fixed-time timer activates once AC voltage drops below the pickup setting; timer elapse triggers contact state change to avoid nuisance transient voltage sag alarms.
4. Medium-wattage B-series coil winding balances total DC load draw on station battery, preventing uneven voltage sag across multi-relay panels during concurrent fault events.
5. Front panel LED indicators show healthy auxiliary supply, undervoltage active condition, and time-delay trip latched status for quick on-site visual diagnostics.
6. 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 MVUA11D Low-Burden Coil Variant For MVUA11B Medium-Burden Unit

New technicians pull MVUA11D spare stock without matching model suffix. During multi-feeder simultaneous faults, uneven DC bus load distribution creates localized voltage sag; low-burden coils dropout prematurely, leaving AC undervoltage faults undetected.Field Rule: MVUA11B medium-burden variant mandatory for dense multi-relay protection panels; segregate B and D coil MVUA 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 front dial settings during spare replacement without referencing site equipment protection logic sheets. Misconfigured thresholds either trip motors unnecessarily during normal grid voltage fluctuations or fail to catch dangerous sustained AC bus collapse conditions.Field Rule: Record undervoltage 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.