Secondary Wiring of North American Medium-Voltage Switchgear

In the secondary wiring of North American switch cabinets, the circuit breaker compartment serves as the primary compartment for installing the circuit breaker cart, and at the same time, it also contains the control power supply, current transformer, circuit breaker closing and opening auxiliary switch, circuit breaker cart position auxiliary switch, terminals, as well as the secondary circuits for monitoring, control and protection. 

American standard switch cabinets are generally 36 inches (914mm) in size. Regardless of whether the circuit breaker handles high current or low current, its width is the same. Generally, the circuit breaker should not be too wide. For example, if the circuit breaker is 790mm wide, the total width on both sides would be 914 - 790 = 124mm, and the width on one side would be 124 / 2 = 62mm. This can meet the requirements for secondary terminals and wiring. However, if the circuit breaker is wider or the compartment width is too small, such as a 26-inch switch cabinet, the circuit breaker has a 185mm gap and a width of 600mm, with the width on one side being only 30mm. It is impossible to install secondary terminals and secondary wiring, and the terminals and wiring would be too high and interfere with the movement of the circuit breaker handle. 

In the front part of the general circuit breaker compartment, there is a 10-inch (254mm) deep space as the secondary compartment. That is, when the circuit breaker is in the open test position and the microcomputer and other devices installed on the door panel are at the rear, they will not interfere with the circuit breaker. The upper and lower parts of the compartment can also be used for installing low-voltage components, such as power control circuits MCB and fuses, which are installed in this position. 

The auxiliary switch TOC for the circuit breaker pusher position and the auxiliary switch MOC for the circuit breaker‘s open-close position are also installed in the circuit breaker compartment. They enable system debugging even without the circuit breaker, and can also be used for large current switching. The secondary plug is only used for the storage of the circuit breaker and the connection of open-close signals. 
The current transformer is mounted on the contact box. The current values are directly connected to the current meter and microcomputer protection installed on the door panel of the circuit breaker compartment, enabling microcomputer protection for overcurrent and short circuit, etc. The bottom secondary plug ensures that the circuit breaker cart automatically connects when it reaches the working position, or the three-position switch realizes the connection of the secondary plug. 

Microcomputer protection, closing and opening indicator lights, closing and opening switches and other components are installed on the door panel. The control, protection and display of the entire circuit breaker‘s outgoing line circuit are fully realized within one compartment. Unlike IEC switch cabinets, current transformers are located behind the cabinet, and the closing and opening positions of the circuit breaker need to be connected through secondary plugs. All lines need to be connected to the low-voltage compartment, and instruments and other components are installed on the door of the instrument compartment. One switch cabinet can only achieve the function of one circuit. American standard switch cabinets have one compartment with all the functions of one circuit, and two circuits do not interfere with each other. 

For the protection against internal arc faults, the method of adding an independent instrument room on the door panel is adopted. The compartments are sealed to prevent arc combustion. Similarly, the front part of the auxiliary function switchgear compartment is also used as an instrument room. For 3000/4000A high current switch cabinets, the upper part usually only allows the installation of voltage transformer switchgear and other auxiliary compartments, or can be used as an instrument compartment to install more system protection circuits. Of course, for metal armored switch cabinets with internal arc fault levels, there is generally a separate instrument room located between the upper and lower circuit breaker compartments. However, due to the height of the compartments being generally only 500-600mm, it is impossible to install all the secondary components of the two circuits inside this compartment. 

For the terminals and control circuits of the general circuit breaker compartments, they are still arranged on both sides, top or bottom of the compartment, while the instruments, switches, etc. are installed inside the compartment. TOC, MOC, secondary plugs, current transformers are all in the circuit breaker compartment and are connected nearby to the terminals, which is convenient for inspection and maintenance.