Understanding the Rated Characteristics of Medium Voltage and High Voltage Circuit Breakers

Circuit Breakers are extensively used in power systems to make or break electrical circuit during normal operation as well as during fault conditions. Circuit Breaker must be able to withstand the thermal and electrodynamic stresses arise due to interruption of operating currents and short circuit currents during fault conditions. There are various types of circuit breakers (Air Blast CB, OCB, SF6 Breakers, VCBs), however the typical ratings that characterize the operation of circuit breaker in a power system remains the same.

Rated Voltage

It is the maximum RMS Voltage at which the breaker is designed to operate safely and satisfactorily. Rated Voltage is always greater than the operating voltage. For Example a breaker operating in a 132 kV Network may be rated at 145 kV or a medium voltage breaker in 11kV Network at 15kV or 17.5kV. Common Values of rated voltages are 15kV, 17.5 kV, 24kV, 36kV, 145kV, 245kV and so on.

Rated Frequency

50 Hz and 60 Hz frequency systems are the most commonly used systems around the world. The breaker is designed to operate on the system frequency.

Rated Normal Current

It is the RMS Value of the current, Circuit Breaker is designed to carry continuously in closed position, operating within permissible temperature rise limits as defined by IEC 62271-1 at an ambient temperature of 40^oC.

Rated Insulation Level

The ability a breaker to withstand switching and lightning surges is defined by the following:

Lightning Impulse Withstand Voltage

It defines the ability of the breaker to withstand the overvoltage arising due to lightning strikes. Since lightning is a natural phenomenon and its shape and size cannot be predicted, therefore basic impulse loading is defined for the impulse wave shape of 1.2/50 µsec. The most commonly used values for BIL is presented in the table.

1.2/50 µsec is the most commonly used wave shape however different standards have been in practice as well.

Power frequency withstand Voltage

It is the ability of the system to withstand the voltage at system frequency for 1 min.

Rated Voltage	Impulse Withstand Voltage	Power Frequency Withstand Voltage
KV	kV	kV
17.5	95	38
145	650/750	275/315
245	900/1050	460/530

Table 1 Insulation Levels for Different Systems

Rated Short Time Withstand Current

It is the RMS value of the current circuit breaker is expected to carry for a specified short time. The short time is usually 1 sec. or 3 sec. The value of rated short time current is normally equal to short circuit current.

Rated Short Circuit Current

Also called rated short circuit breaking current, it is the max. value of short circuit current that the circuit breaker is expected to break at its rated voltage.

Rated Peak Withstand Current or Rated Making Current

The circuit breaker is expected to withstand the peak associated with the first major loop of short circuit current. This is particularly important in case of circuit breaker (accidental) closing when the fault is already present. The rated making current for the circuit breaker is equal to 2.5 times of the short circuit current for 50Hz system, and 2.6 times for 60Hz system.

Operating Sequence

It designates the switching sequence of the circuit breaker. The operating sequence as per IEC is O - t -CO - t’ – CO. where t and t’ are 0.3 sec or 3 min depending upon the requirement of switching operations. O represents opening operation and CO represent opening operation immediately after closing.

Rated Transient Recovery Voltage

It is the voltage that appears across the terminal of the circuit breaker after the interruption of current. This voltage appearing across the terminal may be many times greater than the nominal voltage depending upon the configuration & system earthing, the breaker must be able to break all short circuit currents at all transient recovery voltages below rated TRV.

First Pole to Clear Factor

It is the ratio of recovery voltage across the 1st pole of the circuit breaker to normal phase to ground voltage across the terminal of the circuit breaker. The circuit breaker breaks the current at zero instant, since zero is reached at different instant for each phase, the high recovery voltage appears across the pole, opened first. The Value of FPC factor depends upon the system grounding and is usually taken as 1.5 or 1.3 depending upon the neutral earthing.

Rated Out of Phase Breaking Current

It is the maximum value of current, circuit breaker is expected to break when the system phases are not synchronized. The voltage appearing across terminal of the circuit breaker increases many times in such a scenario. This is an important parameter for generator circuit breakers. It’s value is usually 25% of the short circuit breaking current.

Rated Cable Charging Breaking Current

It is the RMS value of the current circuit breaker is designed to break during switching of unloaded cables.

Rated Line Charging Breaking Current

It is the RMS value of the current circuit breaker is designed to break during switching of unloaded transmission lines.

Switching of Small Inductive Current

Not only the fault interruption current capability is important, When the circuit breaker breaks or switches a smaller inductive current, significant overvoltage may occur which may damage the insulation of inductive load such as unloaded transformer.

Electrical Endurance                                      

IEC 62271-100 defines two electrical Endurance Classes for circuit breaker, Class E1 designates basic electrical endurance and Class E2 designates extended electrical endurance, Class E2 circuit breaker do not require maintenance of the interrupting part during their expected life.

Mechanical Endurance

As per IEC 62271-100, two mechanical endurance classes, Class M1 (2000 Operations) and Class M2 (10,000 Operations) are defined.


About the Author

Muhammad Adeel Khan has over 7 years experience in the field of power systems working with leading companies in the electrical industry. . He received his Bachelors degree in Electrical Engineering from NED University in 2011. He also holds Masters degree in Power Systems from NED University.