Switching in Electrical Transmission and Distribution Systems

4 Load Switching

4.1 Normal-Load Switching

Load-current switching is mainly characterized by currents up to the normal-load-current rating of the switching device. For this duty, the term switching is used, meaning energizing or de-energizing load circuits. This is in contrast with the terms making and breaking operations, reserved for a fault-current establishment and interruption.

Another fundamental difference is the frequency of operations: whereas a fault-current making/breaking is a relatively rare event, the switching of load currents is practised on a regular basis.

Switching of normal load has to be distinguished from switching of capacitive or inductive loads. Due to the reactive nature of these loads, they are considered “special” switching duties deserving particular attention – also in testing.

The main reason for this is that the TRV of normal-load switching is much less severe than in all other switching duties discussed so far. The circuit power factor is close to unity, the current and voltage have only a small phase difference, and current zeros occur far from the recovery voltage maximum. This is outlined in Figure 4.1.

Figure 4.1 Effect of the circuit power factor on TRV.

The interruption of normal-load current is generally easy for any type of circuit-breaker that has proven its short-circuit making and breaking capabilities. Therefore, there is no test-duty ...