GE Vernova MMLG/MMLB Test Blocks and Test Plugs

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.