The GEC Alsthom MMLG01 (specifically identified by the part number 081798J) is a 14-circuit safety Test Block designed for use with electrical protection relays. While it is often listed in the “protection relay” category, the MMLG01 is actually a passive interface device. It provides a safe and convenient means for technicians to isolate a relay from its current transformer (CT) and voltage transformer (VT) circuits, allowing for secondary injection testing without disconnecting any permanent panel wiring.

 

Product Parameters and Technical Specifications

The MMLG01 is engineered to meet the high reliability standards of utility and industrial power systems.

• Model Number: MMLG01
• Part Number: 081798J
• Number of Circuits: 14 independent circuits
• Current Rating: 20A continuous
• Voltage Rating: 300V AC/DC
• Insulation Resistance: > 100M\Omega at 500V DC
• Dielectric Strength: 2kV RMS for 1 minute between all independent circuits
• Mounting: Standard M-series modular case (Flush or Projection mount)
• Compatible Plug: MMLB01 (Multi-finger test plug)

Product Datasheet Summary

The MMLG01 081798J datasheet highlights its Make-Before-Break internal contact logic. This is a critical safety feature: when the companion MMLB test plug is inserted, the internal contacts automatically short-circuit the CT secondary leads before they are disconnected from the relay. This prevents dangerous high-voltage spikes that occur when a CT is open-circuited under load. The 081798J revision features high-integrity silver-plated contacts to ensure low contact resistance and long-term stability in corrosive industrial environments.

 

Country of Origin

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

 

Application Fields

The MMLG01 is a standard component in any high-security protection scheme:

• Transmission Substations: Facilitating the annual maintenance of distance and busbar protection relays.
• Generator Control Panels: Allowing for the safe isolation of differential and reverse-power relays.
• Industrial Microgrids: Providing a standardized test point for overcurrent and earth fault protection.
• Marine Power Systems: Ensuring that maritime protection systems can be calibrated without shutting down essential shipboard power.

 

Product Usage Instructions

1. Normal State: The MMLG01 is fitted with a finger-safe cover. The internal contacts are closed, allowing the relay to monitor the system normally.
2. Accessing Circuits: Remove the front cover. (Note: In some schemes, removing the cover trips an alarm to alert the control room that the relay is in “test mode”).
3. Insertion: Insert the MMLB01 test plug. The plug’s fingers will engage the internal contacts, automatically shorting the CTs and then isolating the relay terminals.
4. Testing: Connect your secondary injection test set to the MMLB01 sockets. You can now simulate faults and verify the relay’s pickup and timing settings.
5. Restoration: Slowly remove the test plug. The internal contacts will return to their normal position. Replace the cover to restore the “Healthy” signal to the DCS.

 

Technical Description

The MMLG01 081798J consists of a robust, glass-filled polyester molding housing 14 pairs of spring-loaded contacts. The design ensures that even after hundreds of insertions, the contact pressure remains sufficient to keep contact resistance below 10 m\Omega. The modular “M-series” case is designed to match the footprint of GEC Alsthom relays (like the MCGG or METI series), allowing for a clean, professional appearance on the protection panel.

 

Q&A – Frequently Asked Questions

Q: Can I use the MMLG01 with a different brand of relay, like SEL or ABB? A: Yes. The MMLG01 is a universal test interface. While it matches the aesthetic of GEC Alsthom equipment, it can be wired to any device that requires up to 14 test circuits.

Q: What is the difference between the MMLG01 and MMLG02? A: The MMLG01 is the standard 14-circuit version. Other variants like the MMLG02 may feature different internal link configurations or a different number of active terminals.

Q: Why is my relay alarm triggered when I just take the cover off? A: Many MMLG01 installations use the cover as a “supervision” link. Removing the cover breaks a small internal circuit that signals the SCADA system that the relay is potentially being bypassed for maintenance.

Q: Where can I find the MMLG01 081798J and MMLB01 test plugs? A: You can source the GEC Alsthom MMLG01 and compatible testing accessories through specialized industrial automation and power system suppliers.

 

Recommended Similar Models

1. GEC Alsthom MMLB01 – The essential 14-finger test plug required to use the MMLG01.
2. GEC Alsthom MMLG03 – A specialized variant with different contact logic for voltage-only circuits.
3. ABB ESSAILE – A common alternative European-standard test block system.
4. GEC Alsthom MCAA – Auxiliary relay often mounted near the test block for signal multiplication.
5. GEC Alsthom MMLG04 – A vertical-mount version of the standard test block.