In addition to the normal current rating of a circuit breaker (which is the current it can handle continuously during regular operation), there are two important categories of short-circuit current ratings that define how the circuit breaker performs during fault conditions:
Rated Short Circuit Breaking Current
The short-circuit breaking current of a circuit breaker consists of two components: the AC component and the DC component. The AC component is expressed in RMS value. A standard circuit breaker is guaranteed for breaking or interrupting a high current up to its short-time withstand current. The AC component of the short-circuit breaking current is equal to the RMS value of the short-circuit withstand current, with a percentage of the DC component.
The RMS value of the AC component of the short-circuit breaking current may be selected from a few pre-specified standard values. These values are 25 kA, 31.5 kA, 40 kA, and 50 kA. There are many other standard values available, such as 6.3 kA, 8 kA, 10 kA, 12.5 kA, 16 kA, 20 kA, 63 kA, 80 kA, and 100 kA. However, 25 kA, 31.5 kA, 40 kA, and 50 kA are mostly used for this purpose.
The DC component in the short-circuit breaking current of a circuit breaker arises due to the sudden asymmetrical nature of the fault current during a short circuit. Here’s an explanation of its origin and cause.
When a fault, such as a short circuit fault, occurs in an electrical system, the current changes abruptly. In an AC system, the normal current is naturally sinusoidal. However, if the fault occurs at a point in the waveform that is not at zero, the current does not have time to settle into its sinusoidal shape immediately. This is the main reason of DC component in a short circuit current. If the fault occurs at a zero-crossing point of the AC waveform, there is no DC component, and the fault current is symmetrical. However, in the most of the cases the fault occurs somewhere else other than zero crossing. The sudden onset of the fault disrupts the balance of the magnetic fields in the system. This causes a transient offset in the current, which is the DC component. Most power systems are inductive in nature. When a short circuit occurs, the inductance resists the sudden change in current, leading to an exponential decay of the DC component over time. This is governed by the system’s L/R time constant.
In short, we can say, the DC component in the short-circuit breaking current of a circuit breaker is caused by the asymmetrical nature of the fault current, originating from the timing of the fault and the inductive properties of the system. It is a transient phenomenon that decays over time but must be accounted for in circuit breaker design to ensure safe and reliable operation during fault conditions.
Rated Short Circuit Making Current
In a power system, there is often a chance of closing a circuit breaker within a fault condition. Think of the scheme of an auto-recloser, where the circuit breaker is automatically closed after a specific time following its first faulty trip. If the fault is transient in nature and has already been cleared before the circuit breaker is closed by the auto-recloser, then it is okay. But there may be a permanent type of fault, and in that situation, the circuit breaker bridges the faulty part of the network with the healthy portion of the network. Sometimes, human operators may do the same thing manually, meaning they close a faulty network with the healthy part by switching on the associated circuit breaker. Although, in both cases, the circuit breaker will eventually trip permanently to clear the fault from the healthy part of the network. Here, the making capacity or the making current rating of a circuit breaker comes into the picture.
Making Current and the Doubling Effect
The making current of a circuit breaker refers to the maximum peak current that the circuit breaker can safely close onto during a fault condition without sustaining damage. The making current is directly influenced by the doubling effect, as explained below:
When a circuit breaker closes, particularly under fault conditions, it connects the source to the faulty circuit. That can be considered as the recreation of fault in the network. Again, when a fault occurs in a network, a sudden change in current happens. Inductive components in the network (e.g., transformers, transmission lines) resist this rapid change in current, creating a transient DC offset in addition to the sinusoidal AC fault current. If the fault occurs meaning breaker closes the fault at the peak of the voltage waveform, the current reaches its maximum value during the first half-cycle due to the superposition of the DC offset and the AC fault current. This can cause the peak value of the short-circuit current to be nearly twice the peak value of the steady-state current. This event is refered as doubling effect. As per Indian Standard this value is taken as 1.8 instead of 2. This value is refered as peak factor due to DC offset.
Making Current = Peak Current × Peak Factor
⇒ Making Current = √2 × Symmetrical RMS Current × Peak Factor
⇒ Making Current = √2 × Symmetrical RMS Current × 1.8
⇒ Making Current = 2.5 × Symmetrical RMS Current.
Rated Short Time Withstand Current
The short-time withstand current of a circuit breaker is the maximum current it can carry for a short duration (typically 1 second, 3 seconds, or 4 seconds) without sustaining damage or excessive temperature rise. This rating ensures that the circuit breaker can handle short-circuit conditions temporarily before it trips or another protective device operates. Readers may get confused, but this is not the breaking capacity; rather, it is the maximum RMS current that the circuit breaker can withstand for a specific duration while in the closed position.
| System Voltage | Short-Time Withstand Current (kA for 1s or 3s) |
|---|---|
| 33 kV | 16 kA – 31.5 kA |
| 132 kV | 25 kA – 40 kA |
| 220 kV | 40 kA – 50 kA |
| 400 kV | 40 kA – 63 kA |
The value of short-time withstand current is normally taken as equal to the breaking capacity of the high voltage circuit breaker.
Rated Line Charging Breaking Current
This rating is applicable to all types of circuit breakers with a rated voltage of 72.5 kV and above. In an extra high voltage transmission line, even when no active load is connected, the line still carries a small charging current due to the capacitance between the conductors and the ground. This is called line charging current, which is obviously a capacitive current. The value of this current is not very high. But interruption of this small capacitive current may cause very high restriking voltage across the opening contacts of a circuit breaker. Therefore, a circuit breaker must capable of interrupting safely a maximum value of this capacitive line charging current. This maximum current limit is called Rated Line Charging Breaking Current. As per standards, the rated line charging breaking currents are furnished below.
| Rated Voltage (kV) | Rated Line Charging Breaking Current (A) |
|---|---|
| 420 | 400 |
| 245 | 125 |
| 145 | 50 |
| 72.5 | 10 |
| 36 | – |
Theory of Line Charging Breaking Current
Normally, an unloaded transmission line has a small capacitive charging current. We know that a capacitive current leads the voltage by 90°. So, the voltage across the line lags behind the current by 90°, meaning that when the current is zero, the voltage is at its peak.
Let us consider a scenario where the circuit breaker opens exactly when the capacitive current is zero. At this moment, the voltage on the line is at its maximum, and the transmission line is holding the full charge, like a capacitor. The supply end of the line continues to oscillate at its normal frequency, following its normal sinusoidal function. So, after half a cycle, the voltage across the circuit breaker reaches twice the peak value.
Let me explain it a little bit more clearly. When the current crosses zero and the voltage reaches its positive maximum (assumed), the contacts get separated. As the line behaves like a capacitor, it tries to hold the voltage at its positive maximum after being disconnected from the source. However, the voltage at the supply end (i.e., the other contact of the CB) reaches its negative maximum just after one half-cycle of the sinusoidal wave.
Therefore, the voltage difference between the contacts of the CB becomes:
Positive Maximum + Negative Maximum = 2 × the peak value of the supply voltage.
If the contact gap of the circuit breaker cannot withstand this voltage stress, it breaks down, leading to a restrike arc. This arc allows the charge in the transmission line to discharge rapidly, resulting in a high-frequency oscillation. The peak of this transient oscillation can increase further, reaching up to 4 times the normal voltage peak. The arc then quenches (dies out), but if the breaker still cannot withstand the voltage, another restrike occurs. With each restrike, the voltage increases in steps. Therefore, the circuit breaker must stop arcing by increasing its dielectric strength. Otherwise, the high-frequency oscillations can cause overvoltage, which may lead to flashovers and system failures.
Rated Cable Charging Breaking Current
The Rated Cable Charging Breaking Current refers to the maximum capacitive current that a circuit breaker can safely interrupt when disconnecting a cable from a power system. Due to insulation (e.g., XLPE, oil, or gas), cables have capacitance between conductors and the ground. Even when no active load is connected, they carry a small charging current due to this capacitance. Capacitance per unit length is 10–20× higher than overhead lines (e.g., 0.2–0.5 μF/km for HV cables vs. 0.01–0.05 μF/km for lines). When a circuit breaker opens, it must safely handle this capacitive current without causing excessive overvoltage or restrikes. Thus, the Rated Cable Charging Breaking Current is the maximum capacitive current that the breaker can safely interrupt when disconnecting an underground cable, ensuring safe operation without excessive voltage spikes.
| Rated Voltage (kV) | Rated Cable Charging Breaking Current (A) |
|---|---|
| 420 | 400 |
| 245 | 250 |
| 145 | 160 |
| 72.5 | 125 |
| 36 | 50 |
Theory of Cable Charging Breaking Current
Theoretically, cable charging breaking current is the same as overhead line charging breaking current, but the values and effects of cable charging breaking current are greater than those of line charging breaking current. The cable was charged before disconnection. Once disconnected, the cable retains its charge (like a capacitor holding a voltage). However, the system voltage on the supply side continues its normal sinusoidal variation. After half a cycle, the potential difference between the breaker contacts can become twice the peak system voltage. This high voltage can cause dielectric breakdown across the CB contacts, leading to restrikes (unwanted arcing across the contacts). If restrikes occur, the charge in the cable discharges rapidly, creating high-frequency transient oscillations. The peak voltage of this transient can reach four times the normal voltage peak. These transients can cause flashovers, insulation damage, and stress on other equipment.
Rated Capacitor Bank Breaking Current
The theory behind the rated capacitor bank breaking current is nearly same as that of line charging breaking current or cable charging breaking current. The rated capacitor bank breaking current refers to the maximum current a circuit breaker (CB) can safely interrupt when switching capacitor banks in a power system without restrikes, excessive overvoltage, or equipment damage. A capacitor bank stores electrical energy and retains its charge even after the associated breaker opens. When disconnected, the capacitor bank remains at a high voltage due its charge retaining property. However, the system side voltage continues its sinusoidal waveform. As a result, just after half a cycle, the voltage difference across the breaker can rise to nearly twice the peak system voltage, leading to possible dielectric breakdown and restrikes. Restriking of arc generates high-frequency oscillations (kHz range) due to the interaction between the system inductance and capacitance.
| Rated Voltage (kV) | Rated Capacitor Bank Breaking Current (A) |
|---|---|
| 420 | Not specified |
| 245 | 250 |
| 145 | 160 |
| 72.5 | 125 |
| 36 | 50 |
Rated Capacitor Inrush Making Current
When an uncharged capacitor bank is switched on in a substation, a large inrush current flows immediately. The inrush current can be extremely high, sometimes 10-100 times the normal rated current. Before switching, let us consider, the capacitor bank is completely uncharged. So, the voltage across the capacitor is zero (ideally). When the circuit breaker closes, the capacitor suddenly connects to the system voltage. Since the capacitor voltage cannot change instantly, the voltage difference between the system and the capacitor is maximum. This large potential difference causes an initial surge of charging current into the capacitor. At the instant of switching momentarily the capacitor bank behaves like a short circuit. This is the inherent property of a capacitor. The initial current is therefore limited only by the system impedance (mainly the inductance of transformers, cables, and busbars). The inductance (L) of system and capacitance (C) of the capacitor bank creates a LC oscillation circuit. The inrush current oscillates at the resonance frequency LC oscillation circuit. The frequency of the inrush current is normally in the range of 2 kHz. This high-frequency inrush current is finally damped out due to the presence of system resistance.
Rated Small Inductive Breaking Current
The interruption might be more challenging at lower currents because the arc is less stable, leading to possible current chopping, which can induce high-frequency transients. Actually, as the arc between opening CB contacts is much weak for small inductive breaking current. This is why the arc column is not stable and often gets broken anywhere away from zero crossing. Current chopping is when the arc is extinguished abruptly before natural zero, causing a rapid di/dt and resulting in voltage spikes. The circuit breaker must be capable of handle these voltage spikes. This rating is curtail because a circuit breaker may have to interrupt, small transformer magnetizing current as when required.
Theory of Small Inductive Breaking Current
When a circuit breaker interrupts an inductive circuit (such as transformer magnetizing currents, reactor currents, or motor no-load currents), the following issues arise:
- Inductive loads store energy in their magnetic fields, and when the breaker opens, the collapsing field induces a high voltage across the contacts.
- Since the current is small, the natural zero-crossing occurs with low energy, meaning the arc can easily extinguish.
- Due to arc discontinuation, the current is interrupted, and a high voltage is induced across the CB contacts.
- The induced voltage restrikes the arc, making it difficult to finally interrupt the current.
- In high-current breaking, the arc is large and dissipates energy quickly, leading to rapid de-ionization of the contact gap; hence, it gets extinguished very close to the zero crossing.
- In low-current breaking, the arc is weak and persists longer due to the repetitive restriking of the arc, making the final extinction process difficult.
- This prolonged arc can cause contact damage, excess heat, and dielectric failure.
Rated Out of Phase Breaking Current
Out-of-phase breaking current in a circuit breaker refers to the current interrupted when two parts of the power system, such as a generator and a grid, are not in synchronization (i.e., they are out of phase with each other). In normal operation, all parts of a power system operate in synchronism, meaning their voltages and currents are in phase. However, during certain conditions like the synchronization of a generator to the grid, or in the event of a disturbance, the voltage across the breaker contacts may become out of phase. The circuit breaker needs to trip instantaneously if such out of phase condition appears in the system. This tripping of a high voltage circuit breaker is called out-of-phase breaking.
When a breaker is opened under such conditions, the difference in phase angle between two sides of the CB causes a higher voltage to appear across the breaker terminals. This results in a current that is much larger than the rated current, making it challenging for the breaker to interrupt safely. Circuit breakers are designed to interrupt a specified amount of current under out-of-phase conditions. his current is typically around 25% of the rated short-circuit current as per standards like IEC 62271-100.