ALSTOM MMLG01 Test block
Introduction to ALSTOM MMLG01
ALSTOM MMLG01 is a high-precision, multi-finger test block designed primarily for the testing and isolation of protection relays and secondary circuits.
It serves as a vital interface between the main power system and sensitive protection equipment, allowing technicians to perform maintenance and testing without disconnecting the wiring from the panel.
Part of the renowned MMLG series, this component is recognized for its robust build and the “make-before-break” safety mechanism, ensuring that current transformer (CT) circuits are never left open-circuited during testing.
Technical Parameter Table
| Parameter | Specification |
| Model | MMLG01 |
| Manufacturer | ALSTOM (now under GE Renewable Energy / Grid Solutions) |
| Rated Voltage | 300V AC/DC |
| Rated Current | 20A (continuous) |
| Contact Resistance | < 10 mΩ |
| Insulation Resistance | > 100 MΩ at 500V DC |
| Number of Contacts | 14 Pairs (standard configuration) |
| Operating Temperature | -25°C to +55°C |
| Mounting Type | Flush panel mounting |
| Standards Compliance | IEC 60255, CE marked |
Related Models & Same Series
The MMLG series includes several variations designed for specific terminal configurations or matching test plugs.
MMLG02: Variant with specific contact arrangements for specialized relay types.
MMLB01: The companion multi-finger test plug used to insert into the MMLG01.
MMLG03 / MMLG04: Custom configurations for complex differential protection schemes.
MMLZ Series: Associated accessories and mounting hardware.
Product Advantages and Features
CT Shorting Safety: Features internal shorting links that automatically short-circuit the CT secondary before the test plug is fully inserted, preventing dangerous overvoltages.
Modular Design: Compact footprint that fits standard relay panel cutouts.
Reliability: Silver-plated contacts ensure low resistance and long-term durability in industrial environments.
User Safety: Eliminates the need for manual wire disconnection, reducing the risk of human error and accidental trips.
Versatility: Compatible with a wide range of secondary injection test sets.
MMLG01
Application Cases
Substation Automation: Used in high-voltage and medium-voltage substations to facilitate routine relay testing.
Power Plants: Installed in generator protection panels for seamless monitoring and maintenance.
Industrial Plants: Found in large-scale manufacturing facilities (steel mills, refineries) to protect critical motor and feeder circuits.
Retrofitting: Frequently used when upgrading legacy electromechanical relays to modern digital IEDs (Intelligent Electronic Devices).
Installation and Maintenance
Installation
Orientation: Mount the block vertically on the front of the relay panel.
Wiring: Connect the rear terminals using ring-type or spade-type crimps. Ensure that CT and VT connections follow the provided wiring diagram.
Clearance: Maintain adequate spacing for the insertion of the MMLB01 test plug.
Maintenance
Visual Inspection: Periodically check for signs of overheating or dust accumulation on the contacts.
Cleaning: Use compressed air to remove dust. Do not use abrasive cleaners on the contact surfaces.
Operational Check: Test the “make-before-break” sequence during scheduled outages to ensure the shorting springs are functioning correctly.
Unique Product Description
The ALSTOM MMLG01 is the “gold standard” for relay isolation.
While modern digital relays often have internal testing software, the MMLG01 provides a physical air-gap isolation that engineers trust for safety.
It acts as a bridge; by inserting a test plug, you essentially “hijack” the signal path, redirecting the system inputs to your test set while keeping the rest of the grid safe. Its mechanical simplicity is its greatest strength, offering a fail-safe point of entry for over 30 years of power system history.