Interlock requirements for north American switch cabinets

Interlock requirements for north American switch cabinets

Rock and roll

North American circuit breakers typically employ a push-pull mechanism installed on switchgear such as POWER VAC switchgear. The mechanism features screw rods with nuts on both sides. When the breaker is pushed into the cabinet, it engages with the nuts. By rotating the screw rods, the nuts drive the breaker to move forward or backward. The left and right screw rods move synchronously via chains, ensuring parallel movement and uniform force distribution. This design is particularly suitable for high-current circuit breakers, where balanced contact force prevents misalignment and guarantees reliable electrical connections.

There are also operating mechanisms that use a single intermediate lead screw, which drives the nut forward and backward, as shown in the figure.

The screw rod 4 drives the nut 5 to move, and the nut 5 is engaged with the circuit breaker to realize linkage.

Operation of the secondary plug

The Meibang switch cabinet adopts a double-layer circuit breaker structure, with the upper circuit breaker positioned higher. If the IEC standard secondary plug operation mode is adopted, it is necessary to use a ladder for insertion and removal.

The bottom secondary plug is usually used in conjunction with the three-position design of the circuit breaker trolley, namely the isolation, test and working positions. When the trolley is rotated from the isolation position to the test position, the secondary plug is connected; when it continues to be rotated to the working position, both the primary and secondary plugs are connected.

It also uses a handle to operate. In the isolation position, pull down and push the handle to achieve the secondary plug connection, that is, to the test position. Then push the circuit breaker to the working position, and both the primary and secondary plugs are connected.

If the secondary plug 3 is installed on the base plate and slides front and back, the circuit breaker reaches the isolation position. Pull the handle of the secondary plug on the base plate to connect with the circuit breaker.

Shut the door

When the circuit breaker trolley enters or exits the working position, the operation should be carried out with the circuit breaker compartment door closed.

Auxiliary switch for circuit breaker opening/closing position

In Figure 9, the Mechanism Operation Unit (MOC) switch is a lever-operated component integrated with the circuit breaker mechanism. When activated by closing the circuit breaker, it extends the lever from the base of the mechanism and presses down on the MOC's operating lever. This motion then transmits actuation to the auxiliary switch, driving its position switching.

Tactile Operating compartment (TOC)

The switch features nine auxiliary contacts. When the circuit breaker is actuated to the connected position, four contacts close while five remain open. During this process, the bracket on the breaker pushes the TOC switch lever during its final inch of travel. This mechanism enables the TOC switch to provide visual indication of the breaker's connected status.

Specifications limit board

Since North American switchgear uses standardized width specifications for circuit breakers, the compartment dimensions remain identical across models with matching current ratings and breaking capacities, differing only in contact configurations. This design necessitates specification limit plates to prevent circuit breakers from other cabinets from entering. As illustrated in Project 11 above, the opening is designed to interlock with the bottom baffle of the current breaker. This mechanism ensures only breakers from the specified cabinet type can pass through, while those of other specifications are restricted from access.

Location interlock

The circuit breaker can only close when in the working or test position, a function achieved through the interlock plate on the base plate. When in the closed state, performing push-in or push-out operations will trigger the breaker to trip. This differs from conventional practices that prohibit shaking during operation, posing a risk of accidental circuit breaker disconnection.

In addition, according to the IEEE C37.20.2 standard, the spring energy storage needs to be released when the circuit breaker is pulled out of the switch cabinet, which is also achieved through the mechanism on the base plate.

Mechanical key interlock

In power systems, the mechanical key lock is typically used to secure the two incoming lines and one bus tie switch of circuit breakers, ensuring safe operation and fulfilling the "five prevention" functions. This three-lock-two-key system works by locking the screw rod, preventing the circuit breaker from accidentally reaching or exiting its working position. This mechanism effectively avoids operational errors and potential safety hazards.