Electrical equipment requires reliable power supply, laborsaving maintenance, and internationalization in addition to upgrading and diversification, and it is current subjects to realize economically. This paper describes the recent tendency and objectives of molded case circuit breakers and earth-leakage circuit breakers, essential devices for low-voltage distribution, and also introduces the basic design philosophy, target, and features of the new series Fuji Auto-Breakers and earth-leakage breakers newly developed for 225-A and less frames.

Fuji Electric has developed new series molded-case circuit breakers and earth-leakage circuit breakers, TWIN BREAKERS, on the basis of new design philosophy toward 1990s. In the TWIN BREAKERS, the external dimensions of molded-case breakers and earth-leakage breakers fully unified first in the world. As a result, the sizes of Fuji Electric's breakers for 30-A to 225-A frames have been reduced as much as from 15 to 7 types. The TWIN BREAKERS not only flexibly suit low-voltage switchboards and controlboards becoming more complicated, but also contribute to reduction in total cost.

The TWIN BREAKER has been developed from an epochmaking idea to unify molded-case circuit breakers and earth-leakage breakers in full modular structure. The work has been accomplished by compiling all the former designs, evaluations, and manufacturing technologies into this development and refining upon it. This paper describes some new technologies supported the development such as current limiting technology, component minimization, reliable rotary sliding contacts, and high-strength molding materials.

Various accessories were provided for former molded-case circuit breakers and earth-leakage breakers for enhancement in handling and safety and for increase in connection and control variations. Many improvements have been done to the accessories together with TWIN BREAKERS development.
The main points are (1) Fully common accessories for both breakers, which simplifies production, distribution, and selection, (2) Measures for uses in combination with electronic control devices, (3) Complete safety measures with plenty of insulating components, and (4) Compact remote control devices.

For configuration of reliable protection systems, devices suitable for the systems should be selected and be put to their proper use. It is important to grasp the features of the construction and performance of a molded-case circuit breaker and to install it correctly.
This paper introduces an outline of the TWIN BREAKER application based on its data.

Inverters have rapidly spread since the second oil shock in 1980 and are now used in various fields. A new problem appeared in the application of an earth-leakage circuit breaker (ELCB) is to have to give attention to excessive operation due to a high-frequency leakage current.
Fuji Electric has investigated the problem and improved the characteristics considering safety in high-frequency ranges, and give a standard for users to select sensitivity in applications to inverter circuits on the basis of actual test data with ELCBs and inverters combined.

Stable power supply has been regarded important increasingly with recent spread of OA devices and automation equipment. Also molded-case circuit breakers require advanced functions such as a notice function to announce operation beforehand.
To meet the requirements, Fuji Electric has developed a series of Fuji Auto-Breakers with solid-state relays. The new series is composed of four series of electronic motor breakers, electronic TWIN BREAKERS, SA-E series, and Power breakers to optimize the merit of the Fuji breakers in each application field.