Utilization categories and Releases in circuit breakers

Circuit Breakers Utilization Categories, Releases, Ratings & Markings

Circuit breakers are essential protective devices in electrical systems. They not only interrupt fault currents but also ensure selective coordination with other protective devices. To understand their performance and applications, we need to study their utilization categories, types of releases, rated values, tripping characteristics, and marking requirements.

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1. Utilization Categories of Circuit Breakers

The utilization category of a circuit breaker specifies whether it is designed for selectivity under short-circuit conditions. There are two main categories:

(i) Category A

• Not designed for selectivity under short-circuit conditions.

• Do not have intentional short-time delay.

• No short-time withstand current rating.

• Commonly used for final distribution protection (e.g., MCCBs in feeders).

(ii) Category B

• Designed for selectivity under short-circuit conditions.

• Have an intentional short-time delay (may be adjustable).

• Must have a short-time withstand current rating specified by the manufacturer.

• Typically used in main breakers and upstream protection.

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2. Releases in Circuit Breakers

A release in a circuit breaker refers to the tripping mechanism that operates under fault or abnormal conditions. The main types are:

(i) Shunt Release

• Trips the breaker when an external voltage signal is applied.

• Used for remote tripping (e.g., emergency shutdown).

(ii) Overcurrent Release

Protects against overloads and short circuits.

• Instantaneous Release → trips immediately at preset fault current.

• Definite Time Delay → trips after a fixed delay, independent of fault current magnitude.

• Inverse Time Delay → tripping time decreases as fault current increases.

  • Independent of previous load

  • Dependent on previous load

Terminology:

• Overload release → for overload protection.

• Short-circuit release → for high fault current protection.

(iii) Under-voltage Release

• Trips the breaker when supply voltage drops below a threshold.

• Ensures breaker does not reclose automatically on power restoration.

(iv) Other Releases

• Special releases based on application (e.g., earth fault release, combined protections).

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3. Characteristics of Releases

(i) Shunt Release & Under-voltage Release

• Rated control circuit voltage

• Type of current (AC/DC)

• Frequency (for AC)

(ii) Overcurrent Release

• Rated current setting

• Current type (AC/DC)

• Frequency (for AC)

• Adjustable current and time settings (on front of MCCB/CB)

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4. Rated Current of Circuit Breaker

• Defined as the RMS value of current at the maximum setting, which the breaker can carry continuously without exceeding permissible temperature-rise limits.

• For adjustable releases → current setting range must be marked (in amperes or multiples of rated current).

• For non-adjustable releases → breaker is marked with rated current only.

👉 In case of indirect releases via CTs, marking can be based on CT primary current or overload release setting, with CT ratio clearly indicated.

Temperature Considerations:

• Non-thermal releases → unaffected by -5 °C to +40 °C ambient.

• Thermal releases → values given at reference temperature of 30 °C ± 2 °C.

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5. Tripping Time Settings

1) Definite Time Delay Overcurrent Release

• Time delay is independent of fault current.

• Marked as fixed or adjustable time in seconds.

2) Inverse Time Delay Overcurrent Release

• Time delay is dependent on fault current magnitude.

• Tripping characteristics given as time-current curves by manufacturers.

• Curves provided for extreme current settings and, if adjustable, for extreme time settings as well.

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6. Marking on Circuit Breakers

Circuit breakers must be durably marked for identification and safe operation.

(a) Mandatory Markings (visible when installed)

• Rated current (In)

• Suitability for isolation (symbol if applicable)

• Position indicators (O = Open, I = Closed)

(b) External Markings (may not be visible when installed)

• Manufacturer name or trademark

• Type designation/serial number

• IEC 60947-2 compliance

• Utilization category (A or B)

• Rated operational voltage & frequency

• Rated impulse withstand voltage

• Breaking capacities (Icu, Ics)

• Short-time withstand current (for Category B)

• Terminal markings (Line, Load, Neutral ‘N’, Earth ⏚)

• Reference temperature for thermal release

(c) Data on Breaker or Manufacturer’s Literature

• Short-circuit making capacity

• Rated insulation voltage (if > operational voltage)

• Pollution degree (if ≠ 3)

• Conventional enclosed thermal current

• IP Code

• Minimum enclosure/ventilation requirements

• Distance to earthed parts (for unenclosed use)

• Suitability for environment A/B

• RMS sensing (if applicable)

(d) Markings on Operating/Tripping Devices

• Control circuit voltage & frequency (closing devices)

• Shunt/Under-voltage release ratings

• Indirect overcurrent release rating

• Auxiliary contacts details & ratings

• Terminal marking

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✅ Summary:
Circuit breakers are classified into Category A and B based on selectivity requirements. They use different releases (shunt, under-voltage, overcurrent, etc.) for protection and control. Proper understanding of rated current, tripping characteristics, and marking requirements (IEC 60947-2) is essential for correct selection, application, and maintenance of circuit breakers in electrical systems.

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