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.

Significantly improved security:

Preventing CT secondary open circuit: By mandating the pre installation of CT short-circuit links on MMLB before inserting the plug, the risk of CT open circuit is fundamentally eliminated.

Anti misoperation tripping: The front cover interlock function of MMLG 01 can automatically cut off the DC power supply of the protection device to prevent accidental tripping during testing.

Personal safety: The test plug adopts a 4mm retractable shielded socket, providing finger protection level. All test lines and short-circuit connections use shielded plugs to minimize the risk of electric shock.

Status indication: After the front cover of MMLG 01 is opened, a prominent orange stripe will appear on the panel as a clear visual prompt for the protective device to "exit operation" and avoid communication errors.

Compatibility and reliability: As a member of the Midos system, the size (Size 2) of the MMLG test block is compatible with relays and systems with a height of 4U, making it easy to install. With over 30 years of application history (launched in 1983) and over 500000 deliveries, it proves the maturity and reliability of its design.

Technical details and installation guidance

MMLG test block

Mechanical structure: Adopting the Midos system standard Size 2 chassis, the panel installation hole size can refer to Figure 4 in the manual. The rear terminal block provides two connection methods:

4mm screw crimping: suitable for pre insulated "L" type cold pressed terminals.

4.8mm x 0.8mm plug: Acceptable quick plug connectors that comply with BS5057 standard.

Grounding: There is a dedicated protective grounding terminal at the rear, which must be reliably grounded using a wire of not less than 2.5mm ².

Insulation: If using ring terminals, insulation type must be selected to maintain the design insulation level.

MMLB 01 test plug

Socket: 28 4mm safety sockets, capable of accepting 4mm test plugs with retractable shielding.

Fixed: After inserting the MMLG, it can be locked with the rolling screws on both sides to ensure a stable connection.

Attachment: Each set of MMLB 01 plugs comes standard with 6 short-circuit wires (consisting of two 4mm stackable shielded plugs and 150mm flexible wires), used to pre short-circuit the CT circuit before insertion. Longer 2-meter test leads can be ordered separately.

Safe operation guide: The golden rule that must be followed

Given that the testing work involves high-voltage primary equipment and secondary circuits, the following safety recommendations must be strictly followed. Any negligence may result in serious personal injury or equipment damage:

Personnel qualifications: Only professionals with corresponding qualifications and a full understanding of equipment risks can carry out relevant operations.

Circuit verification: Before operation, all necessary measures must be taken to verify whether the circuit connected to MMLG is correct. We cannot rely solely on the drawings. If conditions permit and it is safe, physical inspection should be conducted.

CT secondary circuit absolutely prohibits open circuit: this is the safety iron law of the power industry. Before inserting the MMLB into the MMLG, an external short circuit must be established between the sockets corresponding to the CT secondary winding on the MMLB plug using the accompanying short-circuit wire. Manual Figure 1 clearly illustrates this critical step. The MMLG system itself does not have an internal automatic short-circuit function, and this responsibility lies entirely with the operator.

Test power failure during plugging and unplugging: When inserting or unplugging the MMLB into or from the MMLG, all test power sources connected to the MMLB must be in a disconnected state.

Check the integrity of the leads: Before each use, visually inspect the shielding layer and insulation of all test leads and short-circuit wires for any damage.

Consider a power outage of the equipment: If conditions permit, it is best to temporarily put all secondary circuits (such as CT and PT circuits) connected to MMLG in a passive state before inserting or unplugging MMLB. This may require a brief power outage of the system to ensure that no current can excite the CT secondary circuit.

Stop if in doubt: If there are any doubts in the entire safety operation process, work should be stopped immediately and guidance from superiors or experts should be sought.

Application Cases and Historical Inheritance

Since their launch in 1983, the MMLG and MMLB series products have been widely used in the global power industry, from power plants, substations to industrial distribution systems, everywhere. The delivery of over 500000 MMLG test blocks and the usage of over 70000 MMLB 01 test plugs are the best proof of its outstanding performance and reliability. They have become indispensable partners for countless relay protection engineers to conduct regular calibration and troubleshooting.

In order to meet a wider range of application needs, GE Vernova further launched the MiCOM P990 series test blocks in 2001, providing users with more possibilities in test block selection. The P990 series integrates internal automatic CT short-circuit function in some models, providing users with additional convenience options. However, the MMLG series is still the preferred solution for many standard applications due to its simple structure, durability, and high cost-effectiveness.