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Overvoltages may be produced by certain types of asymmetrical fault, mainly on systems with ungrounded neutrals. The voltages set up are of normal operating

Voltage transient due to current chopping

Figure 10.6 Voltage transient due to current chopping: if = fault current; v = system voltage, i0 = current magnitude at chop, vr = restriking voltage

frequency. Consider the circuit with a three-phase earth fault as shown in Figure 10.7. If the circuit is not grounded the voltage across the first gap to open is 1.5 Vphase. With the system grounded the gap voltage is limited to the phase voltage. This has been discussed more fully in Chapter 7.


It is well known that in resonant circuits severe overvoltages occur and, depending on the resistance present; the voltage at resonance across the capacitance can be high. Although it is unlikely that resonance in a supply network can occur at normal supply frequencies, it is possible to have this condition at harmonic frequencies. Resonance is normally associated with the capacitance to earth of items of plant and is often brought about by an opened phase caused by a broken conductor or a fuse operating.

In circuits containing windings with iron cores, for example transformers, a condition due to the shape of the magnetization curve, known as ferroresonance, is possible. This can produce resonance with overvoltages and also sudden changes from one condition to another.

A summary of important switching operations is given in Table 10.1.

Three-phase system with neutral earthing (grounding)

Figure 10.7 Three-phase system with neutral earthing (grounding)

Table 10.1 Summary of the more important switching operations (Reproduced with permission from Brown Boveri Review, December 1970)

Switching Operation


Voltage Across Contacts

1. Terminal short circuit

2. Short line fault

3. Two out-of-phase systems voltage depends on grounding conditions in systems

4. Small inductive currents, current chopped (unloaded transformer)

5. Interrupting capacitive currents capacitor banks, lines, and cables on no-load

See Figure 10.5

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