The GEC Alsthom MMLG01 (Part Number: 478069G) is a specialized test block designed to provide a safe and reliable interface for the testing and monitoring of protective relay schemes without disconnecting the primary wiring.
Product Parameters (Datasheet)
- Model Number: MMLG01
- Part Number: 478069G
- Device Type: Relay Test Block
- Number of Circuits: 14 independent circuits (Contacts)
- Current Rating: 20A Continuous (Standard for CT circuits)
- Voltage Rating: 300V AC/DC
- Insulation Class: 2kV rms for 1 minute
- Insertion Life: >500 operations with the MMLG02 test plug
- Mounting: Standard vertical flush or projection mounting (Size 4 case)
- Compatibility: Compatible with all MMLG02 multi-finger test plugs
Country of Origin
This essential protection accessory was manufactured by GEC Alsthom (now a part of GE Grid Solutions) in the United Kingdom, known for its long-standing history in power system protection engineering.
Application Fields
The MMLG01 test block is a standard requirement in high-voltage and medium-voltage substations:
- Substation Maintenance: Allowing technicians to inject current or voltage into relays for calibration.
- Fault Recording: Interfacing portable disturbance recorders with live CT and VT circuits.
- Relay Retrofitting: Facilitating the temporary bypass of trip circuits during the installation of new protective hardware.
- Secondary Injection Testing: Serving as the primary access point for secondary injection test sets like Omicron or Megger units.
Product Usage Instructions
- Installation: Mount the MMLG01 block into the relay panel. Ensure the rear terminal wiring for Current Transformers (CTs) and Voltage Transformers (VTs) is correctly mapped to the internal finger contacts.
- Normal Operation: During standard system operation, the internal contacts of the MMLG01 are closed, maintaining the integrity of the protection loops.
- Testing Preparation: To begin testing, remove the front cover. This action automatically opens the trip circuits (depending on internal wiring) to prevent accidental circuit breaker operation.
- Plug Insertion: Insert a compatible MMLG02 test plug into the MMLG01 block. The plug’s fingers will automatically short-circuit the CT secondaries before opening them to the test set, ensuring safety.
- Completion: After testing, remove the plug and replace the cover. The mechanical interlock ensures that the protection system returns to its “Armed” state.
Product Technical Description
The MMLG01 acts as a “gateway” for protective relaying. Technically, its most critical feature is the make-before-break sequence when used with a test plug. When a CT circuit is accessed, the block ensures that the CT secondary is never left open-circuited (which would cause dangerously high voltages). The 478069G variant features 14 poles, allowing it to handle complex three-phase systems plus neutral, along with multiple trip and alarm contacts simultaneously. Its robust design is engineered to withstand the harsh electromagnetic environment of a high-voltage switchyard.
Q&A (Common Questions)
Q: Do I need to manually short the CTs before using this block? A: No, the combination of the MMLG01 block and the MMLG02 plug is designed to automatically short the CT circuits upon insertion to ensure operator safety.
Q: What happens if I lose the front cover? A: The front cover often acts as a bridge for certain “Trip” circuits. Operating without the cover may leave some protection functions disabled; it is highly recommended to replace a lost cover immediately.
Q: Can this block be used with modern digital relays from other brands? A: Yes, the MMLG01 is a universal hardware interface. While it was designed by GEC Alsthom, it is commonly used to provide test access for ABB, SEL, and Siemens relay panels.
Q: Is the 478069G the same as the MMLG01? A: MMLG01 is the model series, while 478069G is the specific manufacturing part number that defines the internal contact arrangement and case style.
Similar Model Recommendations
- GEC Alsthom MMLG02 (The companion Test Plug)
- GEC Alsthom MMLG03 (Alternative contact configuration)
- GEC Alsthom METW (Legacy test block variant)
- GE P991 (Modern successor to the MMLG series)
- Areva MCAA (Auxiliary current transformer module)
For further technical data or to procure the GEC Alsthom MMLG01, please consult our protection and control equipment specialists.
