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.

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.

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).

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.

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.

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: I_N = 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.

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.

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).

Examples:

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.