The GEC Alsthom MMLG01B1AA0001C is a specialized Test Block designed for the safe and efficient testing of protection relays within a power system. While often categorized under “protection relays,” the MMLG01 actually serves as the physical interface between the external wiring and the relay itself. It allows technicians to isolate the relay from the primary circuit, short-circuit current transformers (CTs), and inject secondary test signals without the need to disconnect any permanent terminal wiring.

Product Parameters and Technical Specifications

The MMLG series is engineered to provide high-integrity connections for utility-grade protection schemes.

• Model Number: MMLG01B1AA0001C
• Type: Multi-finger Test Block
• Number of Circuits: 14 independent circuits (standard for MMLG01)
• Current Rating: 20A continuous (for CT/VT circuits)
• Voltage Rating: 300V AC/DC
• Mounting: Standard M-series modular case (can be mounted adjacent to the relay)
• Compatibility: Used with the MMLB multi-finger test plug for simultaneous circuit isolation.

 

Product Datasheet Summary

The MMLG01B1AA0001C datasheet emphasizes its “Make-Before-Break” internal logic for current transformer circuits. When the test plug (MMLB) is inserted, the internal contacts automatically short-circuit the CT secondaries before they are disconnected from the relay. This is a critical safety feature that prevents high-voltage “open-circuit” spikes that could damage equipment or injure personnel. The “01” designation identifies the standard 14-circuit configuration, while the “AA” suffix indicates the standard hardware revision and contact material specification.

 

Country of Origin

This protection accessory was manufactured in the United Kingdom at the GEC Alsthom (later Alstom/GE) Stafford facility.

 

Application Fields

The MMLG series is a standard fixture in modern and legacy protection panels:

• Transmission & Distribution: Installed in every protection bay to facilitate annual maintenance and calibration.
• Generator Protection: Used to isolate complex differential and loss-of-excitation relays for testing.
• Industrial Substations: Providing a safe interface for commissioning new protection logic.
• Relay Retrofitting: Serving as the permanent test point for both static and numerical relay installations.

 

Product Usage Instructions

1. Normal Operation: The MMLG01 remains in the panel with its cover attached. The internal contacts are closed, allowing the CTs and VTs to feed the relay normally.
2. Testing Preparation: Remove the MMLG front cover. This may trigger a “Test Mode” alarm if the cover-removal contact is wired to the DCS/SCADA.
3. Insertion: Insert the MMLB test plug. As the plug enters, the CT circuits are automatically shorted and then isolated from the relay.
4. Secondary Injection: Connect your relay test set (e.g., Omicron or Doble) to the sockets on the MMLB plug. You can now inject currents and voltages directly into the relay.
5. Restoration: Once testing is complete, remove the plug and replace the cover. The MMLG internal contacts will automatically restore the live connections to the relay.

 

Technical Description

The MMLG01B1AA0001C consists of 14 pairs of internal spring-loaded contacts housed in a robust, insulated molding. Each pair of contacts is connected to the rear terminal block. The design ensures that the contact resistance remains extremely low (< 10 mΩ) to prevent any impact on the accuracy of the protection relay’s measurements. The housing is designed to fit perfectly within the M-series modular racking system, ensuring a uniform appearance across the protection panel.

 

Q&A – Frequently Asked Questions

Q: Does the MMLG01 require an auxiliary power supply? A: No. The MMLG01 is a passive mechanical interface. It does not require power to operate its internal shorting and isolation logic.

Q: Can I use the MMLG01 with non-GEC Alsthom relays? A: Absolutely. While it matches the GEC Alsthom “M-series” aesthetic, it is a universal test block that can be wired to any brand of protection relay (ABB, SEL, Siemens, etc.) that requires 14 or fewer test circuits.

Q: What is the purpose of the MMLG front cover? A: The cover provides dust protection and prevents accidental contact. More importantly, it often includes a small metallic bridge that completes a “Healthy” signal circuit; removing the cover alerts the control room that the relay is being tampered with or tested.

Q: Where can I source MMLG01 test blocks and MMLB test plugs? A: You can find the GEC Alsthom MMLG01B1AA0001C and compatible MMLB test plugs through specialized industrial automation and power system suppliers.

 

Recommended Similar Models

1. GEC Alsthom MMLG02 – A variant with a different number of circuits or contact arrangements.
2. GEC Alsthom MMLB01 – The essential 14-finger test plug used in conjunction with the MMLG01.
3. ABB ESSAILE – A similar European-standard test block system.
4. GEC Alsthom MCAA – Auxiliary relay often mounted near the MMLG for signal multiplication.
5. States FMS – A common North American alternative to the MMLG test block series.