The history of medium voltage switchgear in North America spans over 100 years, featuring a wide variety of breaker types and structures. Unlike China, where middle-mounted cabinets dominate, North American breakers retain distinctive characteristics, with major brands like GE, Powell, Eaton, and Square D having their own traditional mechanisms. European companies like ABB and Siemens later adapted their IEC version breakers to the U.S. market by modifying certain aspects to meet ANSI standards, such as VD4 breakers for global compatibility. Schneider has also developed an ANSI version of its EvoPact breaker.

Operating Mechanism Differences

Traditional American standard circuit breakers generally differ in their operating mechanisms. U.S. manufacturers such as GE, Powell, Eaton, and Square D maintain their unique traditional systems. Meanwhile, European companies like ABB and Siemens have slightly modified their IEC-based breakers to align with American standards. For instance, the VD4 breaker retains the same operating mechanism as its IEC counterparts.

Advancement in Breaker Carriage Design

In terms of carriage design, traditional U.S. breakers are pushed into position using a base or side shafts, without employing a chassis. When the breaker reaches the test position, it connects to the internal racking mechanism of the switchgear, allowing the breaker to move back and forth.

In contrast, European companies like ABB and Hyundai introduced a simpler chassis-based design, similar to IEC breakers, which adopts the same operating mechanism and pole structure but adjusts dimensions to fit the traditional 36-inch width of U.S. switchgear.

Basic Functions and Safety Features

Though the basic functions of American standard and IEC circuit breakers are the same, the key difference lies in a few additional safety requirements for American standards. These include features such as:

1. Releasing the stored energy spring when pulling out the breaker to prevent injury when performing maintenance. Since the breaker automatically stores energy after closing, releasing the stored energy spring during breaker removal ensures the system is in a safe state.

2. MOC (Mechanism Operated Cell Switch) and TOC (Truck Operated Cell Switch) are auxiliary switches for breaker position detection in American standards. MOC is mounted in the switchgear compartment, driven by the breaker's main shaft to ensure proper switching of control circuits. TOC detects when the breaker truck reaches the operational position.

IEC breakers typically mount these auxiliary switches on the breaker carriage, such as S8/S9 for TOC and S4/S5 for MOC, connecting to the switchgear via aviation plugs. ABB’s American standard breakers, as well as newer products from other manufacturers, also omit the external MOC installation, instead relying on auxiliary switches on the breaker truck.

This design is widely accepted by customers without compromising functionality.