Component Snapshot At-a-Glance

• Model: MFAC14K1AA0001A
• Alt. P/N: MFAC14B1AA0001A (medium-burden coil variant, higher DC draw); MCAG14 (alternate high impedance frame, different CT burden handling); MCGG series (overcurrent only, no differential zone comparison)
• Product Series: MFAC Midos static high impedance differential protection relay family, shared rack base with MCGG/MVAJ/MVTT relays
• Hardware Type: Full withdrawable plug-in static high impedance differential relay, blue aluminum industrial chassis, front stabilising voltage adjustment dial
• Key Feature: K-series low-burden 110/125VDC auxiliary coil, high impedance differential principle, adjustable stabilising voltage for zone fault discrimination
• Primary Field Use: Detect internal zone phase/earth faults for busbars, transformer windings and generator stators while remaining stable through external through-fault currents.

Hard-Numbers: Technical Specifications

• Protocol Support: Static analog sampling hardware, optional rear RS485 IEC 60870-5-103 bus; dry volt-free trip/alarm changeover contacts
• Port Count: Multi-set CT differential input terminals, DC auxiliary coil supply terminals, dedicated zone trip and fault alarm contact terminal blocks
• Baud/Data Rate: RS485 max 9600bps; core differential logic operates independent of communication link health
• 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 K coil, minimal DC battery load on densely packed multi-relay panels
• CT Secondary Rating: 1A nominal CT input per differential leg
• Auxiliary DC Supply: 48–125 VDC station control battery wide range support
• Stabilising Voltage Adjustment: 20V to 200V front dial adjustable for through-fault CT saturation stability
• Operate Time: ≤12ms full trip contact closure on internal zone fault
• Reset Characteristic: Fast auto-reset once differential voltage drops below pickup threshold
• Contact Continuous Rating: 5A @300VDC resistive load; 40W DC inductive breaking capacity for breaker trip coils
• Physical Weight: 4.5kg fully assembled draw-out unit

The Real-World Problem It Solves

Standard overcurrent MCGG relays cannot distinguish internal zone faults from external through-faults. CT saturation during heavy external short-circuits creates false differential signals that trigger unwanted busbar trips and plant-wide blackouts.Stacking discrete voltage stabilising resistors plus standalone timing relays takes two rack slots and requires annual re-calibration to match CT kneepoint voltage characteristics.High-burden MFAC14B coils draw heavy DC current during simultaneous multi-feeder faults; station battery voltage sags severely, and differential trip logic fails to actuate breakers during critical internal winding faults.Low impedance differential relays require perfectly matched CT ratios; minor manufacturing mismatch creates permanent standing differential current and constant nuisance alarms.Where you’ll typically find it:

• Fossil power plant generator stator and main transformer restricted earth fault protection panels
• Refinery MV switchgear single-zone busbar differential fault detection schemes
• Urban distribution primary substation transformer winding internal short-circuit protection cabinetsThis single draw-out high impedance differential relay eliminates external stabilising resistors, provides inherent immunity to CT saturation, and minimises DC panel loading during mass fault events.

Hardware Architecture & Under-the-Hood Logic

This unit uses isolated multi-leg CT summation front-end paired with fixed-function static high impedance logic, no user programmable microprocessor code. It shares standardized Midos rack mechanical design with all ALSTHOM Midos protection hardware for unified cabinet layout.

1. Multiple zone CT secondary currents feed summation circuitry to generate a differential voltage proportional to the mismatch between incoming and outgoing zone currents.
2. Isolated high impedance measurement circuit compares differential voltage against front dial stabilising threshold; external through-faults produce low differential voltage and do not trigger trip logic.
3. Internal fast comparator bypasses intentional delay for internal zone faults exceeding stabilising voltage setting, delivering sub-12ms trip speed to limit winding fault damage.
4. Low-wattage K-series coil winding limits total DC load draw on station battery, preventing uneven voltage sag across multi-relay panels during concurrent multi-circuit fault events.
5. Front panel LED indicators show healthy auxiliary supply, differential fault active status, and zone trip latched state for quick on-site visual diagnostics without workstation access.
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 kilovolt surge hazards.

Field Service Pitfalls: What Rookies Get Wrong

Swapping MFAC14B Medium-Burden Coil Variant For MFAC14K Low-Burden Unit

New technicians pull MFAC14B spare stock without matching model suffix. During multi-feeder simultaneous through-fault events, total DC load spikes and bus voltage collapses; medium-burden coils dropout prematurely, internal transformer/generator faults go undetected.Field Rule: MFAC14K low-burden variant mandatory for dense multi-relay bus differential protection panels; segregate K and B coil MFAC14 spares in clearly labeled locked storage bins.

Misadjusting Stabilising Voltage Without Cross-Checking CT Kneepoint Data Sheets

Apprentices randomly turn the front stabilising dial during spare replacement without referencing CT kneepoint voltage calculations. Setting too low triggers nuisance trips on external through-faults; setting too high desensitises protection and fails to clear minor internal winding faults.Field Rule: Record stabilising voltage setting on the relay door label after every adjustment; cross-verify against CT kneepoint and coordination scheme drawing before returning the relay online.

Skipping CT Short Jumper Installation Before Withdrawing Draw-out Relay

Maintenance crews pull 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 electric shock hazard.Quick Fix: Always install factory CT short-circuit jumpers on the Midos base terminal strip before removing any draw-out MFAC differential relay unit.

Deploying MFAC14 On Multi-Zone Busbar Schemes Without Interposing Blocking Logic

Techs install MFAC14 for multi-section bus differential without intertrip blocking contacts. Fault transfer between bus zones creates cross-zone differential voltage, tripping all connected breakers simultaneously.Field Rule: MFAC14 only suited for single-zone bus/transformer/generator differential schemes; add intertrip blocking auxiliary relays for multi-zone bus layouts.

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.