Re-ignition, Re-strike, Recovery Voltage & RC Snubbers

When a circuit breaker opens, the current does not disappear immediately. The process is normally as follows:

1. The breaker receives an opening command.
2. The contacts start to separate.
3. An arc is formed between the contacts.
4. The AC current reaches a natural current zero.
5. The arc is extinguished.
6. A voltage appears across the open breaker contacts.
7. The breaker contact gap must withstand this voltage.

The most important point is this: after the arc is extinguished, the breaker must recover its dielectric strength quickly enough to withstand the voltage appearing across its open contacts.

If the voltage across the contacts becomes higher than the dielectric withstand of the gap, the arc can return. This return of the arc is called either:

• Re-ignition, if it happens very soon after interruption.
• Re-strike, if it happens after a longer delay.

Arc extinction means that the arc between the breaker contacts has been extinguished after the current has been interrupted. In an AC circuit, interruption normally occurs at a current zero.

At current zero, the arc energy becomes very small, so the breaker has the best chance to extinguish the arc.

However, arc extinction alone is not enough. After extinction, the breaker must also withstand the recovery voltage.

Recovery voltage is the voltage that appears across the open terminals of the circuit breaker after current interruption and arc extinction.

In simple words: recovery voltage is the voltage that the open breaker contacts must withstand after the current has been interrupted.

The voltage across the open breaker contacts is the instantaneous difference between the source-side voltage and the load-side voltage:

Before the breaker opens, \(V_{CB}\) is almost zero because the contacts are closed. After the breaker opens, the source side and load side are no longer directly connected. Therefore, a voltage difference appears across the breaker.

The recovery voltage has two main parts:

• Transient Recovery Voltage (TRV).
• Power-frequency recovery voltage.

TRV is the first fast transient part of the recovery voltage immediately after arc extinction. It is usually steep and short-duration. The power-frequency recovery voltage is the slower voltage that remains after the transient part has settled.

The severity of TRV depends not only on its peak value, but also on how quickly it rises. In practice, both the TRV peak and its rate of rise (the rate of rise of recovery voltage, RRRV) are important for circuit breaker performance.

After current interruption, two curves are important:

• The voltage across the breaker contacts.
• The dielectric strength of the contact gap.

Successful interruption requires the dielectric withstand of the gap to stay above the recovery voltage:

If this condition is satisfied, the breaker remains open successfully. If the opposite happens:

then the gap can break down again and the arc can return.

The key idea can be summarised simply:

• If the dielectric recovery curve stays above the voltage curve, no restrike occurs.
• If the voltage curve crosses above the dielectric withstand curve, breakdown can occur.

Re-ignition is the re-establishment of the arc very soon after current interruption.

In many practical references, re-ignition is commonly associated with arc re-establishment within approximately one-quarter cycle after current zero, while re-strike is associated with a later breakdown. These definitions can vary slightly depending on the standard, manufacturer, or application.

For a 50 Hz system, the period is:

One-quarter cycle is:

So, in a 50 Hz system, re-ignition normally means arc return within about 5 ms after current zero.

Why re-ignition happens

Re-ignition happens because the breaker contact gap has not yet recovered enough dielectric strength. The contacts are still close together, and the gas or medium between them may still be hot and ionised. If the early TRV is too steep, the contact gap may break down again.

Re-strike is the re-establishment of the arc after a longer delay following interruption. It is commonly associated with breakdown occurring later than approximately one-quarter cycle after current zero.

In this case, the breaker appears to interrupt successfully at first. However, after some time, the voltage across the contacts becomes high enough to break down the gap again.

Why re-strike is more severe

Re-strike is usually more dangerous because the voltage across the open contacts has more time to build up before breakdown occurs. This can produce severe overvoltages and high-frequency oscillations.

Re-strikes are especially important in capacitive current switching, such as:

• Capacitor banks
• Unloaded cables
• Unloaded overhead lines
• Harmonic filter banks

A shunt reactor is mainly an inductive device. For an inductor, the voltage–current relationship is:

This equation explains why inductive switching can create overvoltage. If the current is forced to change very quickly, \(di/dt\) becomes large. Therefore, the induced voltage can also become large.

The magnetic energy stored in the reactor is:

When the breaker opens, the energy stored in the magnetic field must be redistributed. This can result in a voltage overshoot across the breaker or the reactor terminals.

In shunt reactor switching, overvoltage is often associated with current chopping. This means the breaker interrupts the small inductive current before its natural current zero. The sudden reduction of current causes a high \(di/dt\), which produces an overvoltage across the reactor and breaker contacts.

The induced voltage can also be explained using Faraday’s law:

If the magnetic flux changes very rapidly, the induced voltage becomes high. This is why sudden interruption of inductive current may create voltage overshoot.

In simple words: a reactor resists sudden current change. If the breaker tries to interrupt the current quickly, the reactor can generate a high voltage.

Voltage overshoot is a temporary voltage rise above the expected recovery voltage level. In shunt reactor switching, voltage overshoot may occur because:

• The reactor stores magnetic energy.
• The current is interrupted suddenly, sometimes before its natural current zero (current chopping).
• The magnetic flux changes rapidly.
• The breaker gap is exposed to a steep recovery voltage.
• The gap may not have recovered enough dielectric strength.

If the overshoot is high enough, it can cause re-ignition or re-strike.

An RC snubber is a damping circuit made of a resistor and capacitor. It is used to reduce switching transients.

RC snubbers are most commonly used in power electronics and control circuits. They can be connected across:

• Semiconductor switches
• Thyristors
• IGBTs
• Relay contacts
• Other low-voltage switching devices

An RC snubber is used to:

• Limit the rate of rise of voltage.
• Reduce voltage spikes.
• Damp high-frequency oscillations.
• Protect the switching device.
• Reduce the risk of unwanted breakdown or false triggering.

The capacitor initially absorbs part of the transient energy. The resistor dissipates the energy as heat and damps the oscillation.

Without the resistor, the capacitor and circuit inductance may create oscillation. With the resistor, the oscillation is damped.

For a simple RC circuit, the time constant is:

A larger RC time constant generally slows the voltage change. This means the snubber can reduce \(dV/dt\) across the switching device.

The rate of voltage rise is commonly written as \(dV/dt\). This is very important because many switching devices and breaker gaps have a maximum voltage rise rate that they can withstand.

Many switching transients are caused by inductance and capacitance in the circuit. The natural angular frequency of an ideal LC circuit is:

The corresponding frequency is:

The RC snubber adds damping to this oscillatory behaviour.

Controlled switching is used to reduce switching transients by opening or closing the breaker at a favourable point on the voltage or current waveform.

• For shunt reactor de-energisation, the aim is to avoid severe TRV, re-ignition, and re-strike.
• For capacitor bank energisation, the aim is to avoid high inrush current and overvoltage.

Controlled switching does not change the basic physics, but it improves the timing of the operation.

The breaker interruption process is controlled by the relationship between recovery voltage and dielectric recovery. The most important condition is:

If this condition is maintained, the interruption is successful. If the recovery voltage exceeds the dielectric withstand of the contact gap, the arc can return.

If the arc returns very quickly, it is called re-ignition. If the arc returns after a longer delay, it is called re-strike. Re-strike is usually more dangerous because it can happen after the voltage has built up to a higher value.

Shunt reactor switching is difficult because the reactor stores magnetic energy and resists sudden current interruption, and because current chopping can occur. The key reactor equations are:

An RC snubber helps by reducing voltage spikes, limiting \(dV/dt\), and damping oscillations. The key snubber relationship is: