Correct Selection of the generator circuit-breaker

The general selection will follow the rated values and the obvious parameters for normal operation and the case of fault.

Selection criteria are:

  • Rated voltage

  • Rated current

  • System-side short circuit

  • Generator-side short circuit


Design for normal operation

In the vicinity of generators, special conditions arise. Thus, every application is specifically adapted to the customer’s requirements. Basis for the solutions is the application guide in IEEE C37.013.

Design for the case of fault


In general, the symmetrical system-side short-circuit current (case of fault a) is higher than the generator-side short-circuit current (case of fault b), and therefore determines the required breaking capacity of the generator circuit-breaker. The DC component of the generator-side short-circuit current is higher, which must also be taken into account for assessment of the breaking capacity.

Overvoltage protection measures

From the view of the vacuum switching principle, no overvoltage protection measures are necessary for switching generators, unless the short-circuit current of the generator is smaller than 600 A. However, surge arresters are usually equipped for this switching application in order to protect the expensive generators from all other overvoltages.

Correct Selection

You know your Application.
And we know the behaviour and features of our switching devices.
Together we work out the perfect solution for your application.

For this purpose, we kindly ask you to submit the following data:

Data sheets of:

  • Generator

  • Transformer

  • auxillary transformer and motors


if applicable

  • Single-line diagram

  • Information on equipment operation, e.g. interconnected circuits


Based on the information concerning your application, our experts select a circuit-breaker which reliably controls all service conditions, including tripping in case of fault.
Among other things, the result of the calculations contains a graphical repre­sentation of the current characteristics, as shown beside: