Switching in Electrical Transmission and Distribution Systems

3 Fault-Current Breaking and Making

3.1 Introduction

This chapter deals with breaking and making of current in faulted networks. Commonly, the term fault-current breaking or fault-current interruption is used for switching operations where faults are involved. The circuit-breaker is the only switching device to perform this duty. For switching in a faulted network, the term making is in common use. The making in faulted networks occurs, for example, when, after maintenance work, earth connections are erroneously left installed.

Fault-current making by a switching device (a circuit-breaker or an earthing switch) leads to similar system transients as occur after the initiation of a fault in a healthy system: the system cannot distinguish whether the circuit-breaker closes a system already faulted or whether an external effect, such as lightning-induced flashover, caused a fault.

3.2 Fault-Current Interruption

In general, a fault-current breaking can be classified as:

• Short-circuit fault interruption (see items 1 and 2 below); or
• Out-of-phase fault interruption (see item 3).

Short-circuit faults are related to the current flowing in an erroneous path, mostly initiated by climatic, environmental, or mechanical interaction with power systems, such as lightning strokes, line galloping due to storms and wind, falling trees or branches, animals, fire, dredging accidents, and so on.

Different locations of faults call for the following fault characteristics:

1. Short-circuit current, ...