Circuit Protection Technical Information

  • Circuit Breaker Features and Benefits

    Circuit Breaker Features and Benefits

    • Nuisance-free operation when subject to high in-rush currents or transients, allowing ratings to be closely matched to the needs of the system and enabling the most efficient sizing of wiring and components.
    • Wide selection of time/current operating characteristics ensure tailoring to many different applications.
    • Quality design ensures resistance to premature failure through corrosion, fatigue, shock or vibration.
    • Unlike fuses, no need for spares. Risks of using temporary inappropriate substitutes are eliminated, and warranty costs are reduced.
    • Convenient resetting reduces down-time and service repair costs.
    • Many types also function as on/off switches, simplifying installation - fewer components result in higher overall reliability.
    • Internationally approved, avoiding the need for different models for different national standards.
    • Positive physical interruption of the circuit is ensured. There is no risk of equipment remaining live. Low leakage currents are eliminated.
    • Fail-safe if operated beyond specified performance limits. Unlike PTC devices, such as resettable fuses, which may arc and flame; also they require power to be removed before they can be reset.
  • Circuit Breaker Technologies

    Circuit Breaker Technologies

    A number of factors arise in choosing a circuit breaker to protect against overloads and short circuits. E-T-A specialists can advise on your requirement, according to the specific field of application. Four types of tripping operation cover most situations.

    Thermal Circuit Breakers (TO)

    The tripping mechanism comprises a thermal actuator and mechanical latch, designed to discriminate between in-rush/ temporary current surges and prolonged overloads to ensure effective overcurrent protection. Applications include motors, transformers, solenoids and low voltage wiring.

    Thermal-Magnetic Circuit Breakers (TM)

    Combining a solenoid in series with a bimetal thermal actuator, these provide time/current characteristics with two distinct steps. A high overcurrent value causes the solenoid to trigger the release mechanism rapidly, the thermal mechanism responds to prolonged low value overloads. These circuit breakers are well suited to telecommunications, process control, and similar applications requiring precision performance.

    Magnetic Circuit Breakers (MO or HM)

    A well-proven design of solenoid coil with optional hydraulic delay provides tripping that is highly tolerant of changes in ambient temperature. A wide range of performance characteristics is available in single, double and three pole configurations.

    Serie 808 is a fast acting magnetic device sensitive to small overload currents. Typical applications include printed circuit board and power semi-conductor protection.

    High Performance Circuit Breakers

    Where ultimate operation under adverse conditions is required, E-T-A high performance circuit breakers provide high interrupting capacity and excellent environmental specifications. Available in thermal and thermal-magnetic versions, they offer current ratings up to 500 A. Special models are designed for aerospace, defence and similar heavy-duty applications.

  • Trip Free and Method of Operation

    Manual Trip - ON/OFF Switches

    Many models are available with a manual trip feature, either standard or as an option.

    Others are specifically designed as combined switch/circuit breakers

    with rocker, push button, or toggle actuation, styled for front panel mounting. Rocker types are available with illumination as an option.

    According to IEC 934/EN 60934:

    • R = manual reset only
    • M = with manual release but not intended for frequent use as a switch
    • S = combined switch/CBE function
    • J = automatic reset

    Trip Free Mechanism

    E-T-A circuit breakers cannot be held closed against an overload. This is achieved through the use of positively trip free designs in accordance with IEC 934/EN 60934 (with the exception of models 1410, 1610, 1658 and 808 which are designed for specialised applications).

    Snap-Action Mechanism

    The snap-action mechanism featured in many E-T-A models ensures that the contact closing speed is independent of the speed of operation of the actuator (push button, rocker, toggle etc.). The moving contact is retained until the actuator causes a defined force to act in the closing direction of the contacts. Once this force is exceeded, the mechanical retention is overcome allowing the contacts to snap closed (tease free mechanism.) The closing speed is a function of this force alone.

    Snap-action mechanisms eliminate contact welding upon switching on to sustained short-circuits and minimise the risk of contact wear over the circuit breakers' life.

  • Terminals and Auxiliary Contacts


    E-T-A circuit breakers are available with

    • blade terminals (quick connect)
    • stud terminals
    • screw terminals
    • round connectors
    • solder pins

    Auxiliary contacts

    Electrically separate low current contacts can be included for use with alarm and control switching circuits.

    • N/C (Si1) = Normally closed contacts are open when the main contacts are closed (break or b-contact).
    • N/O (Si 2) = Normally open contacts are closed when the main contacts are closed (make or a-contact).
  • Interrupting Capacity

    Interrupting Capacity

    Overload and maximum interrupting capacities are specified for each series, defined as the maximum current levels that can be switched safely for a minimum of 40 operations, and a minimum of 3 operations respectively. For thermal circuit breakers back-up protection is advised if higher currents are possible. Please contact us for further advice on specific applications.

    IEC 60934/EN 60934 defines interrupting capacity as the rated conditional short circuit current performance. According to category PC1, this is the value of rated conditional short circuit current (interrupting capacity) for which the prescribed conditions do not include fitness of the CBE for its further use. PC2 is defined as the value of rated conditional short circuit current for which the prescribed conditions do include fitness of the CBE for its further use.

    Switching Sequence

    The switching sequence for short circuit tests is normally abbreviated as follows, according to relevant international CBE standards.

    O   Break operation (open)

    The circuit breaker in the closed position is caused to open through a short circuit current applied by means of a separate switch. Referenced as co (closed open) in earlier specifications.

    CO   Make operation with subsequent break operation (close open)

    The circuit breaker in the open condition is closed onto a sustained short circuit and must immediately re-open. This operating mode requires the circuit breaker to be fail-safe as the actuator cannot be released as quickly as the circuit breaker mechanism will open. Referenced as oco (open close open) in earlier specifications.

    t   Time period between switching operations

    Normally 3 minutes, or the period required before the circuit breaker can be re-set.

    Common switching sequences are: O-t-CO or O-t-CO-t-CO.

  • Ambient Temperature Influence

    Ambient Temperature Influence

    To ensure optimum matching of circuit breaker performance to the system requirements, E-T-A thermal and thermal-magnetic circuit breakers are not normally compensated for fluctuations in ambient temperature. The circuit breaker is usually subjected to the same heat source as the system so will automatically track its protective requirements.

    However, some applications require the circuit breaker to operate continuously in either high or low temperatures. The following table shows the correction factors that typically should be applied. The performance of magnetic circuit breakers and type 1410 is not affected significantly within this temperature range.

    Ambient temperature °C   -20     -10      0       +23       +40       +50       +60

    Ambient temperature °F    -4      +14     +32    +73.4   +104    +122    +140

    Multiplication factor           0.76   0.84    0.92    1          1.08      1.16      1.24

    (approximate values)

    Example: IN = 10 A at +50 °C. By applying the factor of 1.16 the current value obtained is 11.6 A. A 12 A CBE rating is recommended.

    Close mounting of CBEs

    When several devices are mounted together, an air gap between each is recommended. If this is not possible, each device should carry only 80 % of its rating.

    Horizontal installation is preferable.

    Plug-in Mounted E-T-A Devices:

    The continuous rating capability of E-T-A sockets for plug-in circuit breakers is a function of the total number of circuit breakers fitted and the individual ratings of each. Please enquire with details of your application.

    ON duty

    Some applications require short-time loads with high currents (e.g. remote trip coils or higher current ratings). In order to avoid overheating, the ON period must be limited (percentage referred to cycle period).


    50 % ON duty / 60 minutes:
    allows a load duration of 30 minutes,
    followed by a period of 30 minutes without load.or25 % ON duty / 20 minutes:
    allows a load duration of 5 minutes,
    followed by 15 minutes without load.

  • Additional Technical Documentation

    Additional Technical Documentation

    Download technical documentation     

    This file includes the following topics:

    • Typical Time/Current Characteristic Curve
    • Principal Types of CBE:
      • Thermal Circuit Breakers (TO)
      • Thermal-Magnetic Circuit Breakers (TM)
      • Magnetic Circuit Breakers (MO or HM)
      • High Performance Circuit Breakers
      • Electronic Overcurrent Protection
    • Manual Trip / ON-OFF Switches
    • Snap-Action Mechanism
    • Trip Free Mechanism
    • Terminals
    • Auxiliary Contacts
    • Solderability of Silver-Plated Terminals
    • Current Ratings and Time/Current Characteristic Curves
    • Ambient Temperature Influence
    • Close Mounting of CBEs
    • Typical internal resistance values
    • ON Duty
    • Interrupting Capacity Icn
    • Inductive and Resistive Load
    • Switching Sequence
    • Degrees of Protection of Electrical Equipment According to IEC 60529 / DIN EN 60529
    • Preferred Degrees of Protection
    • Tolerances in Dimensional Drawings
    • Cable Ratings:
      • to DIN EN 60934
      • for road vehicles
      • for aerospace applications
    • Representation of Operating Status
    • Definition of Make Contact and Break Contact
    • Terminal Identification
    • Graphical Symbols
      • in accordance with DIN EN 60617/IEC 60617 and ANSI Y32.20/CSA Z99
    • Contact Resistance:
      • The physical causes of contact resistance
      • Influence of outside environmental conditions on contact resistance
      • Influence of internal environmental conditions on contact resistance
      • Typical applications for different contact materials
      • Measuring contact resistance