An electrical switchgear is a combination of protection and switching devices assembled in one or more adjacent compartments. A switchgear is formed by compartments, which standards name “enclosure” (with support and mechanical protection functions for different components enclosed), and electrical equipment, constituted by apparatus, internal connections and incoming and outgoing terminals for connection to the installation.

This paper is intended to deal in detail with the equipment in the switchgear, providing the reader with the basic information necessary to choose the circuit breakers to be installed inside low voltage switchgears in the easiest and most correct way.

After a quick survey of main product standards concerning switchgears and circuit-breakers, IEC 61439-1 and IEC 60947-2 respectively, main problems which a manufacturer has to face when designing a switchgear are analyzed.

This paper is divided into three main parts dealing with the problems of overheating in switchgears, general prescriptions to improve the current carrying capacity of the circuit-breakers inside enclosures and derating calculations in switchgears.