Component Snapshot At-a-Glance

• Model: MCTI40N1AB0751G
• Alt. P/N: MCTI40F1AB0751F (medium-burden coil variant, higher DC draw); MCTI39 (2-phase only, missing C-phase monitoring); MCTI19 (single-phase compact unit)
• Product Series: MCTI Midos static instantaneous current detection relay family, unified rack base with MCGG/MVAJ/MVAX/MVTT protection relays
• Hardware Type: Full withdrawable plug-in static 3-phase instantaneous overcurrent relay, blue painted aluminum industrial chassis, transparent front cover with three independent A/B/C phase adjustment knobs
• Key Feature: N-series low-burden 110/125VDC auxiliary coil, independent per-phase adjustable instantaneous pickup, ultra-fast reset characteristic dedicated for breaker fail backup logic
• Primary Field Use: Detect phase fault current to trigger breaker fail backup tripping, provide zero-delay high-speed short-circuit detection for MV feeders, busbars and main transformers

Hard-Numbers: Technical Specifications

• Protocol Support: Static analog sampling hardware, no digital bus communication; dry volt-free trip/alarm changeover contacts
• Port Count: Separate A/B/C 1A CT secondary input terminals, DC auxiliary coil supply terminals, collective trip + individual phase fault contact terminal blocks
• 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 N coil, minimal DC battery load on densely packed multi-relay panels
• CT Secondary Rating: 1A nominal CT input per phase
• Auxiliary DC Supply: 48–125 VDC station control battery wide range support
• Instantaneous Pickup Adjustment: 0.5×In to 20×In independent dial per A/B/C phase
• Operate Time: ≤8ms full trip contact closure at set fault current
• Reset Characteristic: Fast high-reset ratio, clears fault state immediately after CT current drops below pickup threshold
• 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 IDMT MCGG relays carry intentional time delay curves, incompatible with breaker fail protection which requires zero delay backup tripping for failed circuit breakers. Single-phase MCTI19 units only monitor one leg, cannot fully protect three-pole MV breakers.Stacking three single-phase MCTI19 relays wastes triple rack slot space and adds three sets of CT wiring splices that vibrate open after years of plant thermal cycling.Medium-burden MCTI40F coils draw heavy DC current during simultaneous multi-feeder faults; station battery voltage sags severely, instantaneous breaker fail trip logic fails to actuate breakers during critical bus faults.This unit is dedicated to breaker fail schemes, independent per-phase settings allow matching unbalanced CT ratios on each feeder leg without external shunt resistors.Where you’ll typically find it:

• Fossil power plant generator, main transformer and busbar breaker fail protection panels
• Refinery MV motor feeder backup instantaneous short-circuit detection schemes
• Urban distribution primary substation ring bus multi-pole breaker fail logic cabinetsThis single draw-out 3-phase relay delivers independent per-phase fast instantaneous detection and fast reset behavior to eliminate stacked single-phase relays and stabilize DC panel loading during mass fault events.

Hardware Architecture & Under-the-Hood Logic

This unit uses three fully isolated phase analog sampling front-end circuits paired with fixed-function static logic, no user programmable microprocessor code. It shares Midos rack base mechanical design with all ALSTHOM Midos protection hardware for standardized cabinet layout.

1. Each A/B/C phase 1A CT secondary current feeds independent filtered isolated sampling circuitry to block substation VFD and breaker switching EMI noise.
2. Dedicated per-phase RMS comparators continuously sample current magnitude, compare measured value against front dial instantaneous pickup setpoint for each phase individually.
3. Any single phase exceeding pickup threshold triggers immediate contact state change without time delay; fast internal reset logic releases contacts the instant fault current drops below threshold, critical for breaker fail coordination timing.
4. Low-wattage N-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, individual phase fault active status, and collective trip output latched state 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 kilovolt surge hazards.

Field Service Pitfalls: What Rookies Get Wrong

Swapping MCTI40F Medium-Burden Coil Variant For MCTI40N Low-Burden Unit

New technicians pull MCTI40F spare stock without matching model suffix. During multi-feeder simultaneous short-circuit faults, total DC load spikes and bus voltage collapses; medium-burden coils dropout prematurely, breaker fail backup tripping fails to operate during critical bus faults.Field Rule: MCTI40N low-burden variant mandatory for dense multi-relay breaker fail protection panels; segregate N and F coil MCTI40 spares in clearly labeled locked 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 electric shock hazard.Quick Fix: Always install factory CT short-circuit jumpers on the Midos base terminal strip before removing any draw-out protection relay unit.

Misadjusting Per-Phase Pickup Threshold Without Cross-Checking Coordination Drawings

Maintenance crews randomly tweak front phase dials during spare replacement without referencing site breaker fail coordination logic sheets. Mismatched phase pickup values break selectivity; either nuisance instantaneous trips occur on normal load swings or severe bolted faults fail to trigger backup breaker fail logic.Field Rule: Record A/B/C phase instantaneous pickup values on the relay door label after every setting change; cross-verify against substation breaker fail protection scheme drawing before returning the relay online.

Deploying MCTI40 On General Overload Coordination Without Breaker Fail Logic

Techs use MCTI40 as standard feeder overcurrent relay without dedicated breaker fail timer coordination. Fast reset characteristic resets trip signals too quickly, upstream backup protection cannot coordinate time grading.Field Rule: MCTI40 series is purpose-built for breaker fail detection; use MCGG IDMT relays for standard coordinated overload/short-circuit feeder protection.

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.