Rated characteristic of circuit breakers

4. Rated characteristic of circuit breakers

        The rating of a circuit breaker denote its capabilities under specified condition of use and behavior. The following paragraphs are generally based on the recommendation of  IEC-56: "High Voltage Alternating Current Circuit-Breakers" and IS-2516: " Specifications of Alternating current circuit-breaker".

           The capabilities of a circuit breaker of a particular type are proved by conducting type tests as per the recommendations of the standards.
4.1 Rated Voltage

       The rated voltage of a circuit-breaker corresponds to the higher system voltage for which the circuit breaker is intended. The standards values of rated voltages are given in table 2. The rated voltage is expressed in KVrms and refers to phase to phase voltage for three-phase circuit. The earlier practice of specifying the rated voltage of a circuit breaker as nominal system voltage is no more followed.

Remarks	Design Feature	Voltage. Breaking capacity	Medium	Type
Used for medium and low voltage, A.C., D.C., Industrial circuit breakers. Have current limiting	Incorporates: Arc runners, arc splitters, magnetic coils	430-600 V, 5-15-35 MVA  Recently 3.6-12 KV, 500 MVA	Air at atmospheric pressure	Air- break circuit breaker
Used for low and medium voltage	Small size, current limiting feature	430-600 KV	Air at atmospheric pressure	Miniature C.B.
Getting obsolete, used up to 12 KV, 500 MVA	One tank up to 36 KV, 3 tank above 36 KV, fitted with arc control devices	12-36 KV	Dielectric oil	Tnak type oil circuit breaker
Used for metal enclosed switchgear up to 36 KV. Outdoor type between 36 and 245 KV	The circuit breaking chamber is separate from supporting chamber. Small size, arc control device used	3.6-245 KV	Dielectric oil	Minimum oil  circuit breaker
Suitable for all EHV applications, fast opening-closing. Also for arc furnace duty	Unit type construction, several units per pole, auxiliary compressed air system required	245 KV, 35.000 MVA up to 1100 KV, 50.000 MVA, also 36 KV, 500 MVA	Compressed air (20-30 kgf/cm2)	Air blast circuit breaker
Suitable for SF6 switchgear, and medium voltage switchgear. EHV circuit breaker. Maintenance free	Live tank/Dead tank design, single pressure type preferred	145 KV, 7500 MVA 245 KV, 10.000 MVA 12 KV, 500 MVA 36 KV, 2000 MVA	SF6 gas (5 kgf/cm2)	SF6 circuit breaker -single
Suitable for a variety of applications from 3.6 KV up to 36 KV	Variety of designs, long life, modest maintenance	Preferred for indoor switchgear rated up   to 36 KV, 750 MVA	Vacuum	Vacuum circuit breaker
Recently developed, used in HVDC systems. Installed in USA	Artificial current zero by switching in capacitors	500 KV DC, 15 KA/20 KA	Vacuum or SF6	H.V.D.C circuit breaker

Table  1   Comparison of circuit breakers

4.2 Rated Insulation Level

         The rated insulation level of a circuit breaker refers to the power frequency withstand voltage and impulse voltage withstand values which characterize the insulation of the circuit breaker.

Rated Voltage of Circuit Breaker ( KVrms )	Nominal System Voltage ( KVrms )
0.246 0.440 3.600 7.200 12.000 24.000 36.000 72.500 145.000 245.000 420.000 525.000 765.000	0.240 0.415 3.300 6.600 11.000 22.000 33.000 66.000 132.000 220.000 400.000 500.000 750.00

Table 2 : Rated Voltage of Circuit Breaker

 Power-frequency over voltages are due to regulation, ferranti effect, higher tap-setting, etc. The circuit breaker should be capable of withstanding the power frequency over-voltages which are likely to occur. These capabilities are verified by conducting power frequency voltage withstand tests and impulse voltage withstand tests. The circuit breaker is subjected to impulse over-voltage due causes like lighting surge and switching surge.

          During single-line to ground faults, the voltage of healthy lines to earth increases to √3 time the normal value in the system with insulated neutral. Hence higher values of insulation are recommended for circuit breaker connected in noneffectively earthed systems. The following insulations are provided in the circuit breaker :

• Insulation between live parts and earth for each pole external and internal.
• Insulation between poles.
• Insulation between terminals of the same pole-external and internal

         The design of these insulations depends upon the structural form of the circuit breaker and the rated insulation level desired.

4.3 Rated Frequency

       The standard frequency for a three pole circuit breaker is the frequency of the power system (50/60 HZ). The characteristics like normal current breaking capacity etc. are based on the rated frequency.

       The frequency of the current influences the circuit breaker behavior as follows

• The temperature rise of current carrying parts and neighboring metallic parts is influenced by eddy-current heating . The increase in frequency results in increased eddy currents. Hence, with specified limits of the temperature rise the rated current of a circuit breaker needs de-rating for application on higher frequency. 
• The frequency corresponds to the number of current-zeros per second. Since the breaking time of the circuit breaker is associated with the time for half cycles during the arc extinguishing process, the breaking time is influenced  by the frequency of current. The breaking time increases with reducing in frequency. 
• The increase in frequency influences the TRV and rate-of-rise TRV. Hence a circuit breaker designed and rated for a certain frequency cannot be recommended for other frequencies unless capabilities are proved for those frequencies. 
• The d.c. circuit breakers generally adopt a different principle of arc extinction and have different construction than a.c. circuit breaker.

Related articles :

•  HIGH-VOLTAGE A.C. CIRCUIT-BREAKER 
• Classification of Circuit Breaker
  
• Rated characteristic of circuit breakers  (part2)
• Circuit Breaker Time ( total break time) 

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