Megatiker M4 Circuit Breaker Maintenance

Selection and Maintenance of Megatiker M4 Circuit Breaker

In low-voltage distribution systems, molded case circuit breakers play a critical role in overload, short-circuit protection, and line continuity. The Legrand Megatiker M4 series thermal magnetic circuit breakers (including MS4 isolating switches) are widely used in industrial distribution, building electrical, and new energy fields due to their wide current range (250A to 630A), multiple breaking capabilities (36kA to 100kA), and flexible accessory configurations. However, when faced with practical issues such as on-site selection, installation and wiring, accessory matching, parameter tuning, and equipment replacement, engineers often require a systematic technical guide. This article is based on the Megatiker M4 technical data sheet, providing a complete set of on-site technical references from six dimensions: model identification, installation points, electrical characteristics, accessory functions, interpretation of trip curves, and maintenance and replacement.

Model identification and selection

The Megatiker M4 series includes two main categories: circuit breakers and switch disconnectors (MS4). The model coding rules are as follows:

Model prefix, number of poles, breaking capacity, rated current

T743 3P F=36kA, N=50kA, H=70kA, L=100kA 250,320,400,500,630

T744 4P Same as above

T742 3P+N/2 Same as above (only 320-630A) Same as above

Example: T743F250=3 poles, 36kA breaking capacity, 250A rated current. T744H630=4-pole, 70kA breaking capacity, 630A.

Isolation switch models: T743S400, T744S630, etc. (S represents Switch disconnector), only providing isolation function, without thermal magnetic protection, suitable for circuits that require manual opening and closing but do not require overload protection.

Installation and wiring specifications

2.1 Fixed installation

Installation surface: It can be installed vertically or horizontally, or upside down (the direction of the power supply input can be reversed, but attention should be paid to the markings).

Fixed screws: The product comes with 4 (3P/4P) mounting screws for fixing on the mounting plate.

Wiring terminal: Comes with 6 (3P) or 8 (4P) connecting screws. Cage terminals, Lugs, External Lugs, Spreaders, or Rear terminals can be selected.

Specification for the use of cage terminals (taking M7400 kit as an example):

Applicable cables: copper or aluminum, with a maximum cross-sectional area of 300mm ² (hard wire) or 240mm ² (flexible wire).

The high-capacity cage terminal (M7400/2) supports dual cables: 2 × 240mm ² hard wire or 2 × 185mm ² soft wire.

Suggested cross-section (according to IEC 60947-1/2):

250A: Copper 120mm ², Aluminum 185mm ²

400A: Copper 240mm ², Aluminum 300mm ²

630A: Copper 2 × 150mm ², Aluminum 2 × 240mm ²

2.2 Insertion and extraction

Insertion type: Special terminals (M7B11/M7B12) and bases (M7B13~M7B16) need to be installed for the circuit breaker. The plug-in version has increased in weight by approximately 3.35-4.79kg (compared to the fixed version).

Pull out type: Adding a Debro lift mechanism (M7B22~M7B24) on top of the plug-in type allows the circuit breaker to be easily removed from the base for maintenance. It is necessary to extract the handle (M7T412) and position signal contact (M7T910N) together.

2.3 Electrical clearances and resistance to electric forces

When a short circuit occurs, a huge electric force is generated between the conductors. According to the breaking capacity, the minimum distance between conductors should be maintained:

Icc (kA) minimum spacing (mm)

36 350

50 300

70 250

100 200

If using busbars other than Legrand busbar kits, the installer needs to verify the spacing themselves. At the same time, it should be ensured that the weight of the wire does not cause additional stress on the terminal block.

Electrical characteristics and derating

3.1 Power loss

The power loss per pole (tested according to IEC 60947-2 Annex G at rated current) varies depending on the terminal type. Taking 400A as an example:

Terminal type Phase line loss (W) Neutral line loss (W)

Cage terminal 25.6 18.9

Terminal block 25.6 18.9

External terminal block 27.5 19.7

Expansion terminal 29.3 20.4

Rear terminal 28.7 20.2

Insertion 44.9 26.5

Circuit breaker+RCD 33.6 22.1

Attention: When multiple poles work simultaneously, the total loss is the sum of the losses of each pole. Cabinets with poor heat dissipation conditions should have their capacity appropriately reduced.

3.2 Temperature Capacity Reduction

The rated current varies significantly with the ambient temperature. The following table shows the maximum long-time delay protection setting current at different temperatures (example):

In (A) 10°C 20°C 30°C 40°C 50°C 60°C 70°C

250 336 307 279 250 220 193 183

400 475 460 425 400 360 320 280

630 700 683 650 630 580 530 480

Note: For every 10 ° C increase above 40 ° C, it is recommended to reduce the capacity by approximately 10-15%. Please refer to Table A in the manual for details.

3.3 Altitude Capacity Reduction

Capacity reduction is required for altitudes above 2000m:

Altitude (m) Rated insulation voltage Ue (V) Rated current multiple

2000 690 1.00×In