ALSTOM MMLG Series Test Blocks

Technical Dossier

Product Details And Specifications

ALSTOM MMLG Series: Comprehensive Module Range and Technical Overview

The ALSTOM MMLG series of test blocks and terminal modules occupies a well-established position in global heavy industry infrastructure. Deployed extensively in chemical processing plants, nuclear power stations, oil refineries, and grid-scale electrical substations, the MMLG range serves as the primary interface layer between protection relays and field wiring circuits. Its modular plug-in architecture allows maintenance personnel to isolate, test, and commission protection relay circuits without interrupting primary wiring — a critical operational requirement in facilities where unplanned downtime carries significant safety and financial consequences. The MMLG series is most commonly associated with ALSTOM's MCGG, MFAC, MVTT, and KCGG relay families, and remains a standard component in legacy protection panels across Europe, the Middle East, Asia-Pacific, and Latin America.

The Evolution of MMLG Architecture

The MMLG test block system was developed as part of ALSTOM's (formerly GEC Alsthom, formerly English Electric) standardized protection relay interface philosophy. Early generations of the MMLG range were designed to interface with electromechanical and early static relay technologies, providing a consistent terminal block format that could be retained across relay technology upgrades. The core mechanical design — a flush-mounted, plug-in block with shorting and isolating facilities — remained largely unchanged across decades of production, which is a deliberate design decision to ensure backward compatibility in long-lifecycle substation environments.

As digital numerical relays (e.g., ALSTOM MiCOM P-series) became the industry standard through the 1990s and 2000s, the MMLG interface blocks were retained in many panel designs due to their proven reliability and the prohibitive cost of full panel replacement. This has created a sustained demand for MMLG components in the maintenance and retrofit market. The series entered its mature/end-of-life phase under ALSTOM Grid (post-2011 restructuring) and subsequently under GE Grid Solutions (post-2015 acquisition). New production of certain MMLG variants has been discontinued, making reliable aftermarket sourcing a primary concern for asset owners operating legacy protection systems.

MMLG Full Catalog & Functionalities (SKU List)

The following SKUs represent the documented MMLG series range. Each unit is a plug-in test block or associated terminal module designed for use with ALSTOM protection relay systems.

MMLG01: Standard plug-in test block, 6-way, for current and voltage circuit isolation and testing.
MMLG02: 6-way test block with integral shorting links for CT secondary circuit protection during relay withdrawal.
MMLG03: 10-way test block, extended terminal count for complex multi-function relay interfaces.
MMLG04: 10-way test block with shorting bar assembly, suitable for differential protection relay circuits.
MMLG05: Flush-mounted test block, 6-way, panel-mount variant for retrofit applications.
MMLG06: 6-way test block with additional auxiliary contact terminals for trip circuit supervision.
MMLG07: 14-way test block for distance protection relay interfaces requiring multiple CT/VT inputs.
MMLG08: 14-way test block with integral CT shorting facility and isolated voltage terminals.
MMLG11: Plug-in test block, 6-way, low-profile design for space-constrained panel installations.
MMLG12: 6-way test block with labeling strip and terminal identification system.
MMLG13: 10-way test block, DIN rail compatible mounting variant.
MMLG14: 10-way test block with extended current rating for high-burden CT circuits.
MMLG21: Replacement plug assembly for MMLG01/MMLG02 base units, field-replaceable contact set.
MMLG22: Replacement plug assembly for MMLG03/MMLG04 base units, 10-way contact configuration.
MMLG31: Shorting plug accessory, used to maintain CT secondary continuity during MMLG block withdrawal.
MMLG32: Test plug adapter, allows injection of test currents and voltages via standard 4mm banana terminals.
MMLG41: Blanking plug for unused MMLG base positions, prevents accidental contact exposure.
MMLG51: Panel-mount base unit for MMLG01/MMLG02 plug assemblies, surface-mount variant.
MMLG52: Panel-mount base unit for MMLG03/MMLG04 plug assemblies, 10-way surface-mount variant.

Sourcing Hard-to-Find & Obsolete MMLG Parts

Quality Control for the MMLG Range

MMLG test blocks are electromechanical interface components whose failure modes are primarily contact-related: oxidation of silver-plated contacts, mechanical wear of the plug-in retention mechanism, and degradation of the shorting bar spring force. DriveKNMS applies the following inspection protocol to all MMLG units processed through its facility:

Contact resistance measurement across all terminal pairs using a micro-ohmmeter (target: <1mΩ per contact pair). Mechanical insertion and withdrawal cycle test to verify plug retention force within OEM specification. Shorting bar continuity verification under simulated CT secondary load conditions. Visual inspection of terminal labeling, housing integrity, and mounting hardware. Dielectric withstand test at 2kV AC for 1 minute across isolated terminal groups. Units failing any parameter are quarantined and not offered for resale. Test records are retained and available upon request for regulated industry customers.