GE Vernova MMLG/MMLB Test Blocks and Test Plugs

GE Vernova MMLG/MMLB Test Blocks and Test Plugs

In the power system, the reliable operation of protective devices (such as relays) is the cornerstone of ensuring the safety and stability of the power grid. Regular secondary injection testing is an essential maintenance step to verify the performance of these protective devices. However, traditional testing methods often require temporary disassembly of wiring, which is not only time-consuming and laborious, but also poses safety hazards such as accidental contact with live equipment and high voltage caused by open circuits in the secondary circuit of current transformers (CTs). The MMLG series test blocks and MMLB multi finger test plugs launched by GE Vernova, as important members of the Midos product family, are specialized testing accessories designed to address these pain points. They provide standardized monitoring and testing interfaces for various power system protection schemes, greatly simplifying the operation process and significantly improving the safety of the testing process.

Product Overview: Born to protect testing

The MMLG test block is a fixed interface device installed on a protective cabinet, which integrates up to 14 circuit circuits internally. During normal operation, a pair of contacts inside each circuit are kept closed by spring pressure, allowing the current, voltage, and DC control circuits to continuously pass through the test block and maintain the normal operation of the protection device. When testing is required, maintenance personnel do not need to remove any terminal wiring. They only need to insert the matching MMLB multi finger test plug into the test block, and the extended contact fingers on the plug will push open the corresponding contacts inside the test block, thereby safely isolating the tested circuit from the operating primary equipment (such as CT, PT). At the same time, the testing instrument is connected to the relay side through the socket on the plug for simulating fault injection.

The MMLB 01 test plug is designed with 28 4mm safety sockets and uses a retractable shielded plug to provide enhanced "finger safe" function, effectively preventing accidental contact with live parts. This design makes testing wiring safer and more convenient, and supports 4mm stackable plugs, making it easy to parallel multiple testing circuits.

Model selection and differentiated application

MMLG test blocks provide multiple models according to different application scenarios to meet the special requirements of various protection schemes:

MMLG 01 (Standard Test Block): This is the most commonly used model, suitable for most protection devices such as lines, transformers, generators, etc. Its characteristic is that there is a pair of special normally closed contacts between terminals 13 and 14, which are usually connected in series to the DC auxiliary power supply circuit of the protection device. When the front cover of the test block is opened, a metal probe fixed on the inside of the cover will automatically disconnect the pair of contacts, thereby cutting off the DC power supply of the protective device and effectively preventing tripping due to misoperation during subsequent testing. This is an important safety interlock function. Attention: Terminals 13 and 14 are absolutely prohibited from being used to connect the CT secondary circuit, as the corresponding contact fingers on the MMLB plug are shorter than the other contact fingers. If used in the CT circuit, inserting the plug will cause the circuit to open and generate dangerous high voltage.

MMLG 02 (Busbar Protection Special Type): Designed specifically for complex applications such as busbar protection. Unlike MMLG 01, its terminals 13 and 14 are normally closed internally and do not have a front cover interlock disconnect function (as busbar protection may have special operational requirements). Therefore, it is also prohibited for CT connection to avoid the risk of open circuit.

MMLG 03 (Horizontal Installation Type 01): The horizontal installation version of MMLG 01, suitable for cabinets that require horizontal wiring for spatial layout.

MMLG 04 (Horizontal Installation Type 02): The horizontal installation version of MMLG 02.

MMLB 01 (mostly referring to test plug): a test plug used in conjunction with all MMLG test blocks. It contains 28 4mm safety sockets, each with a number corresponding to the terminal number at the back of the test block, ensuring the accuracy of the test wiring.

Key advantage: Why choose MMLG/MMLB system?

No need to disturb the original wiring: this is the core advantage. By using the fixed interface of the test block, we have completely bid farewell to the tedious process of "disconnection wiring restoration" in traditional testing, avoiding terminal looseness and wiring errors caused by frequent plugging and unplugging.

Simplified testing process: After inserting the MMLB test plug, all test points (relay side) and potentially live circuits (CT, PT side) that need to be connected are clearly divided into different areas of the plug (white area corresponds to isolated relay circuits, orange area corresponds to potentially live circuits), and testers can easily connect them at a glance.