Electrical Faults

In power systems, an electrical fault is any abnormal condition that causes current or voltage to deviate from normal operating values. More specifically, a fault creates an unintended current path that bypasses the normal load, resulting in extremely high currents that can damage equipment if they are not cleared quickly.

Types of Electrical Fault

Faults are classified according to which conductors become connected together.

• Three-phase fault – shorts all three phases together. Although this produces the highest fault current and is therefore the most severe fault, it is also the least common.
• Phase-to-phase fault – occurs when two phases become connected together.
• Phase-to-phase-to-ground fault – involves two phases and earth.
• Single line-to-ground fault – the most common type, where one phase comes into contact with earth. These occur frequently on overhead distribution networks due to lightning, trees, wildlife, or damaged insulation.

Figure 1: The four types of electrical fault – three-phase, phase-to-phase, phase-to-phase-to-ground, and single line-to-ground.

What Happens When a Fault Occurs?

Before the fault, the system operates normally and the current follows a standard sinusoidal waveform while supplying the connected load.

When the fault occurs, the circuit impedance suddenly drops. This is because the fault effectively bypasses the load impedance, creating a much shorter path back to the source. Instead of flowing through the load, current now flows through only:

• the generator impedance,
• transformer impedance,
• transmission line impedance, and
• the small impedance at the fault itself.

Since this total impedance is much smaller than the original load impedance, the current increases dramatically — often reaching tens or even hundreds of times the normal full-load current. At the same time, the voltage at the fault location collapses to almost zero because the fault is essentially a short circuit.

Why Does the Current Become So Large?

At first this seems to contradict Ohm’s Law. If the voltage at the fault is almost zero, shouldn’t the current also be small?

The answer is that Ohm’s Law is being applied across the entire circuit, not just at the fault point. Think of the system as one complete electrical loop. The generator produces an internal voltage (its EMF), which sits behind its internal impedance. Current then flows through transformers, transmission lines, and finally to the fault.

Before the fault, the load impedance limited the current. Once the fault occurs, that large load impedance disappears from the circuit. Current only has to overcome the relatively small source and network impedances. The fault current is therefore approximately:

where:

• E – the generator’s internal EMF
• Z_source – the combined source impedance
• Z_line – the network impedance up to the fault
• Z_fault – the fault impedance

Because the denominator becomes very small, the resulting current becomes very large.

This is why determining the maximum fault level of a substation simply requires knowing which generators are connected and the network impedances. Traditional synchronous generators naturally supply large fault currents, whereas many inverter-based (static) generators are designed to supply only a limited, controlled amount of fault current regardless of system conditions.

Two Applications of Ohm’s Law

There are actually two applications of Ohm’s Law occurring simultaneously during a fault.

Across the entire network, the following determines the fault current:

At the fault itself, the following determines the voltage across the fault:

Since the fault impedance is extremely small, the voltage drop at the fault is also extremely small — even though an enormous current is flowing. Both equations are true simultaneously: one describes the behaviour of the complete circuit, while the other describes only the voltage at the fault location.

Why Doesn’t the Current Jump Instantly?

If the network impedance suddenly becomes very small, it might seem that the current should instantly jump to its new value. In reality, it cannot.

Power systems contain significant inductance in generators, transformers, and transmission lines. One of the fundamental properties of an inductor is that current cannot change instantaneously. If the fault occurred at exactly t = 0, the current must begin from whatever value it had immediately before the fault. However, the current also wants to become the new fault current, known as the symmetrical AC fault current. These two conditions cannot both be satisfied unless another component is temporarily added to the current waveform — this is called the DC offset.

The Symmetrical AC Component and DC Offset

If the fault had existed forever, the symmetrical AC fault current is the sinusoidal current that would naturally flow. However, because current continuity must be maintained, the actual fault current is made up of two components:

• the symmetrical AC component, and
• a temporary DC offset.

The DC offset is not an independent current source. It simply shifts the AC waveform vertically so that the total current starts from the exact value that existed immediately before the fault.

For example, suppose the pre-fault current is zero when the fault occurs. The symmetrical fault current at that same instant might naturally be −1.0pu. Current cannot instantly jump from 0pu to −1.0pu. Instead, a +1.0pu DC component is added so that −1.0 + 1.0 = 0, allowing the current to remain continuous.

Figure 2: Fault current waveform showing the symmetrical AC component, DC offset, and actual combined current.

Looking at the graph, the pre-fault current is at its negative peak of −0.12. The symmetrical AC component (as it would appear had fault conditions already existed) is at 1.0. Therefore the DC offset is −1.12, which decays as per the X/R ratio of the network.

Why Are the Pre-Fault and Fault Currents Out of Phase?

The normal load current and the symmetrical fault current are not aligned on the waveform graph. This is because they flow through two completely different circuits.

Before the fault, current flows through the electrical load, whose impedance contains a mixture of resistance and reactance. The current therefore typically lags the voltage by around 20–30°.

After the fault, the load is bypassed. Current now flows only through generators, transformers, and transmission lines. These elements are highly inductive, giving the fault path a much higher X/R ratio. As a result, the fault current lags the voltage by almost 90°. The change in impedance angle explains why the pre-fault and fault current waveforms appear shifted relative to one another.

Decay of the DC Offset

Immediately after the fault, the actual current equals:

The DC offset exists only because it is needed to satisfy current continuity at the instant the fault occurs. Since it is not part of the circuit’s natural steady-state response, resistance gradually dissipates its stored energy. The DC component therefore decays exponentially, eventually leaving only the symmetrical AC fault current as a pure sinusoid.

Subtransient, Transient, and Steady-State Current

The fault current does not remain constant after the fault occurs, it passes through three distinct periods:

• Subtransient period – lasting roughly the first one or two cycles, the current reaches its highest value. Both the generator’s subtransient reactance and the DC offset contribute to this peak.
• Transient period – as the DC offset decays and the machine fluxes redistribute, the fault current decreases into this intermediate phase.
• Steady state – after several cycles, the current settles at its symmetrical value, determined by the generator’s synchronous reactance and the network impedance.

Next I recommend reading the interruption theory here.