Circuit Breakers

MasterPacT MTZ Circuit Breaker (upper photo) and MasterPacT NW (lower photo)

• Through-the-door construction with front-accessible key interlocks

• Open/close buttons

• Open/close indication

• Individually-mounted MTZ1 or NT circuit breakers can be mounted 2 per cell

• ANSI rated and certified to ANSI C37 (UL 1066) and CSA C22.2 No. 268

• Four-position (Connected, Test, Disconnected, Withdrawn)

• Low voltage power circuit breakers

• Maximum 200 kA interrupting rating without fuses (MTZ2, 3 or NW)

• Maximum of 42 kA interrupting rating at 480 Vac (MTZ1 or NT)

• 100% rated

• Two-step stored energy mechanism

• 800 A – 5000 A frame (MTZ2,3 or NW)

• 800 A frame (MTZ1 or NT)

• MicroLogic trip units with optional Ethernet communications and monitoring features

• Enhanced functionality with low maintenance requirements

The closing and opening push buttons, racking handle, and racking mechanism are accessible through the front door cutout, allowing circuit breaker operation, testing, and disconnection without opening the door. Optional shutters (see Secondary Wiring System) can be provided, covering live parts when the circuit breaker is removed.

MasterPacT circuit breakers meet ANSI interlock requirements, including closing spring interlocking.

Through-the-Door Access

The circuit breaker compartment consists of the circuit breaker cradle, a horizontal secondary terminal block on top of the cradle, and a vertical wireway on the right side of the compartment, allowing for top and bottom customer wiring. The cradle structure is a bolted-in assembly that supports the circuit breaker. Integrated into the cradle are the circuit breaker interference interlocks or cell keys.

Cell keys are steel pins on the left side of the cradle. As the circuit breaker is pushed into the compartment, the keys on the circuit breaker move past corresponding key slots on the cradle. This feature prevents circuit breakers with greater continuous current ratings from being placed in circuit breaker compartments with lesser continuous current ratings. This feature also prevents circuit breakers with lesser short circuit current ratings from being placed in circuit breaker compartments with greater short circuit current ratings.

The racking handle storage space, racking handle insertion opening, and “Stop Release” button are on the bottom of the cradle. Circuit breaker racking is achieved with the circuit breaker compartment door closed.

When required for metering, optional current transformers are positioned around the stationary main contacts or in the bus compartment behind the cell.

Circuit Breaker Compartment

Circuit Breaker Compartment with Optional Current Transformers Located Inside the Cell

This section contains basic MasterPacT circuit breaker interrupting, sensor, operating time, control power requirements, and switch ampere ratings. In addition, an overview of Key Interlock Operation is described. Most MasterPacT accessories, trip units, and trip unit options are available in Rear Accessible, Front Accessible and Arc Resistant switchgear.

All MasterPacT circuit breakers are 100% rated and are assembled and tested in an ISO 9001 facility to applicable standards including:

Power-Zone 4 switchgear with MasterPacT circuit breakers has the highest interrupting and short-time (withstand) ratings for low voltage switchgear in the smallest physical size. The small switchgear footprint and the elimination of limiters reduces cost. The circuit breaker’s improved coordination with downstream devices increases system uptime.

Power-Zone 4 switchgear using MasterPacT MTZ1/NT circuit breakers allows the use of up to eight (8) 800 A frame circuit breakers in a single, 30-inch wide section. This lets the user maintain a high concentration of power protection devices in a small physical size. MasterPacT MTZ1/NT circuit breakers are not available in Arc Resistant Switchgear.

MasterPacT MTZ2/3, NW circuit breakers are available in various levels of interrupting ratings (AIR) as listed in Ratings for ANSI C37 Certified MasterPacT MTZ2/3 or NW Circuit Breakers below.

Power-Zone 4 switchgear with MasterPacT circuit breakers has the highest interrupting (200,000 A at 480 V without using fuses) and highest short-time (withstand) ratings for low voltage switchgear, in the smallest physical size. The small switchgear footprint and the elimination of limiters reduces cost. The MasterPacT circuit breakers’ wide range of coordination with downstream devices increases system uptime.

MasterPacT circuit breakers are available in various levels of interrupting ratings (AIR).

See Ratings for ANSI C37 Certified MasterPacT Circuit Breakers.

The MasterPacT circuit breaker has fewer parts than conventional circuit beakers while performing the same functions.

Typical MasterPacT Circuit Breaker (Front and Side View of NW shown; MTZ is similar)

The main disconnecting contacts on the rear of the circuit breaker are spring loaded and self-aligning. These contacts are designed so the pressure at the point of contact, on the stationary connectors, becomes greater under short-circuit conditions.

MasterPacT Circuit Breaker (Rear View)

Power-Zone 4 switchgear with MasterPacT circuit breakers has through-the-door circuit breaker racking. The connect, test, and disconnect positions can be made with the circuit breaker compartment door closed.

Circuit Breaker Positions

Through-the-Door Circuit Breaker Position (side view)		Primary Connectors	Secondary (Control) Connectors	Circuit Breaker Functionality	Circuit Breaker Door Position
Connect		Engaged	Engaged	Can be operated mechanically and electrically. Ready for service (operation).	Closed
Test		Disengaged	Engaged	Can be operated mechanically and electrically. Can have operation and control systems tested.	Closed
Disconnect		Disengaged	Disengaged	Can not be operated remotely. Can not be removed from circuit breaker compartment.	Closed
Remove		Disengaged	Disengaged	Can not be operated remotely. Can be removed from circuit breaker compartment.	Open Circuit breaker is fully extended on rails.

4-pole MasterPacT Drawout Circuit Breaker is an optional feature for the following:

• 1600–4000 A Feeders

• 1600–5000 A Mains/Ties

In some electrical systems the user may choose to switch the neutral on and off along with the 3-phases of power to isolate the neutral in multi-source systems. In a 3-pole breaker solution the source neutrals landed at the gear may be connected to multiple sources at the same time which some users wish to avoid by switching the neutral via 4-pole circuit breaker.

The 4-Pole Breaker in LV Drawout equipment opens and closes the phase bus connections as well as the neutral bus connection.

4-pole breakers are available for 1600–5000 A Mains/Ties and 1600–4000 A Feeders (NW, MTZ2 and MTZ3) up to 100 kAIC @ 480 V and 85 kA @ 600 V. Front Accessible and Arc Resistant construction is not available.

A number of MasterPacT circuit breaker accessories are available with Power-Zone 4 switchgear. A few of the most common accessories are listed below.

Circuit Breaker Compartment	Shutters Optional shutters in the cradle automatically block access to the main disconnects when the circuit breaker is in the disconnect, test, or remove position. An optional padlock attachment to lock shutters closed is also available. Key Interlocks, Key Locks, and Padlocks Optional key interlocking can be provided with MasterPacT circuit breakers. Key interlocking capability is available for circuit breaker operation and the cradle. Key interlock operation is outlined in the Power-Zone 4 switchgear order assembly drawings. To facilitate installation procedures, a key is supplied with each lock. After installation, only the necessary keys called out on the key interlock diagrams should be present on the switchgear.
MasterPacT MTZ2 Circuit Breaker	Cell Keying Cell keying, a standard feature on Power-Zone 4 switchgear, is provided to avoid insertion of circuit breakers with similar dimensions, but insufficient interrupting ratings, or incorrect frame sizes into an inappropriate circuit breaker compartment. Auxiliary Switches A family of auxiliary switches that include: Open and close position switches (OF) Connected and closed position switches (EF), also known as cell switches Cradle position switch (CE, CD, CT)
Field-installable and Upgradeable Options A family of field-installable coils, buttons, motors, connectors, and trip units that include, but are not limited to: Communications via Ethernet (Modbus TCP/IP) or Modbus Serial capabilities Opening and closing coils (MX, XF) Additional shunt trip unit (MX) Undervoltage release (MN) Spring charging motor (MCH) Sensor plug
Ready-to-close contact (PF): The ready-to-close position switch indicates that the following conditions are met and the circuit breaker can be closed: The circuit breaker is open. The closing springs are charged. There is no standing closing or opening order.
Diagnostic and communicating Shunt Trip (MX), Shunt Close (XF), Undervoltage Trip (MN,diagnostics only) These type of coils bring added functionality with diagnostic and communication capabilities and are only available on MasterPacT MTZ breakers: Identify the type of the coil, MX diag&com, XF diag&com or MN diag. Detect if the coil is functioning properly by comparing the device order and the opening/closing status of the circuit breaker. Check for wire breakage in the coil periodically (every 30 seconds). Identify if the order comes from the BPFE option. Identify if the order is a permanent order. Count the operating cycles of the coils. When a communication system is present, the Masterpact MTZ device can be opened and closed from a remote control system such as a SCADA, through the communication network. Diagnostic results can be read through all the different ways of retrieving information from the Micrologic X control unit including the embedded HMI, wireless communication, and Ecoreach software through a USB connection. In addition to the diagnostic function, these devices feature a continuous-type release; the release can be maintained as long as the order is maintained. This feature allows the MX diag&com to lock the circuit breaker in the OFF position when the order is maintained. Digital Modules (MTZ only)

As shown in the example below, a main-tie-main manual key interlock scheme is designed to keep all three circuit breakers from being closed simultaneously. Only two of the three circuit breakers can be closed simultaneously.

Main-Tie-Main Key Interlocking

Main 1 and main 2 circuit breakers are closed with their keys placed in the lock cylinders. The tie circuit breaker is locked open; a key is required to unlock it and put the circuit breaker into service.

The main 1 circuit breaker is opened and taken out of service. It is locked open, permitting key A1 to be removed to operate the lock associated with the tie circuit breaker. Then the main 1 circuit breaker cannot be put into service and closed until either the tie circuit breaker, or the main 2 circuit breaker, is opened and locked out of service. This would free one key for use in unlocking the main 1 circuit breaker.