General Information

Core Description

The MMLB 01 is a Midos-compatible multi-finger test plug featuring 28 finger-safe shrouded 4mm sockets, working seamlessly with MMLG test blocks to deliver safe, rapid secondary injection testing and circuit isolation without disrupting permanent protection relay wiring.

Specific Application

• Secondary Current and Voltage Injection: Routing simulated power system faults from external test sets directly into protection relays without disrupting permanent panel wiring termination points.
• Substation Protective Scheme Monitoring: Accessing live operational signals and measurement loops safely through standard 4 mm test leads to analyze system performance.
• Safe Circuit Isolation: Breaking active contact pairs inside power monitoring frameworks to isolate specific differential, busbar, or feeder protection logic strings during maintenance outages.

Ordering Information

System Compatibility

The MMLB 01 multi-finger test plug is fully compatible with the MMLG series test blocks, including the MMLG 01 standard vertical block, MMLG 02 special busbar protection block, MMLG 03 horizontal standard block, and MMLG 04 horizontal busbar block. It natively accepts standard 4 mm test leads equipped with retractable shrouded safety plugs.

Alternative Model: MiCOM P990 Series

Within the GE Grid Solutions portfolio, the MiCOM P990 series test block and plug system serves as the modern alternative configuration.

Key Technical Differences

Current Transformer Shorting Architecture

The MMLB 01 relies completely on the operator to construct and verify external shorting wire links across the plug sockets prior to insertion. Conversely, the

MiCOM P990 series features an automatic internal CT shorting mechanism embedded inside the block itself to completely prevent open-circuit hazards.

Test Socket Geometry and Accessories

The MMLB 01 utilizes standard 4 mm sockets arranged in a compact 28-point cluster accompanied by shorting links. The P990 alternative utilizes a modified, key

indexed plug shape engineered to restrict incorrect connection indexing.

Historical Generation and Field Record

The MMLB 01 belongs to the legacy Midos product family with massive global field installations. The P990 represents the newer generation platform introduced to

meet altered subrack standardization parameters.

Installation & Troubleshooting FAQ

Q1: What critical safety step must be performed before inserting the MMLB 01 plug into a live CT circuit?

A: It is essential that the sockets in the MMLB 01 test plug corresponding to current transformer secondary windings are shorted using external links before insertion. Never open the secondary circuit of a live CT, as it produces lethal voltages. Pre-configuring these jumper links ensures that the CT secondary circuits are short-circuited before they disconnect from the protection relay.

Q2: Why are the test plug sockets divided into orange and white sections?

A: When external instrumentation is connected to odd-numbered terminals on the test block, insertion of the MMLB 01 ensures that the sockets in the white half represent isolated relay circuits. The sockets in the orange half connect to the potentially live substation supplies. This visual distribution helps test engineers keep test inputs separated from active facility voltages.

Q3: What causes an unexpected open-circuit condition on terminal 13 and 14 when using an MMLG 02 block?

A: Terminals 13 and 14 on MMLG 01 and MMLG 02 blocks must never be used for CT connections. The MMLB 01 test plug contact finger corresponding to these positions is deliberately shorter than the others. Inserting the plug into an MMLG 02 will result in an immediate open-circuit path on that specific channel due to the abbreviated finger length.

Q4: How should the MMLB 01 be secured to the test block during long secondary injection procedures?

A: The test plug body is equipped with two integrated knurled thumb screws. Once the plug is fully pushed into the test sockets of the MMLG block, tighten these two knurled screws into the matching threaded standoffs on the block faceplate to secure the connection against accidental removal.

Q5: What pre-test inspections should be conducted on the MMLB 01 hardware before beginning work?

A: Technicians must visually inspect all flying leads and shorting plugs for insulation cuts, exposed copper, or cracked shrouds. Additionally, the electrical continuity of the pre-wired shorting links must be checked with a multimeter before insertion into the MMLG block to ensure low-resistance bonding paths.