Automatic switches: features and functions

Automatic switches are regulatory devices for overcurrent protection: overloads and short circuits. Their function is to act when a fault is detected, that is to say, to cut off the electricity supply of the installation and allow its restoration when the anomaly has been solved.

According to the norms, the automatic circuit breaker is defined as the “mechanical connection device capable of establishing, supporting and interrupting currents in the normal conditions of the circuit, as well as of establishing, supporting for a certain time and interrupting currents in specified abnormal conditions of the circuit such as short circuit “.

The symbol of a circuit breaker is IEC 60617-7.

Classifications of circuit breakers

It is possible to classify the switches considering multiple criteria. One of them is its use function:

Power switches are those that are installed in the connection of an electrical panel from a transformer, a generator, etc. They must be coordinated with the distribution and power switches  to guarantee the selectivity of protections.
Distribution switches. They protect the conductors that connect the distribution panel with another point of the installation.
Consumer switches. In this case, the protection is direct, since it is located as close as possible to the load.

A second criterion is the type of use:

Industrial use. It must meet the IEC 60947-1 and the 60947-2 standards, or another equivalent
Domestic use. It must comply with standard 60898 or an equivalent standard.

It is also possible to classify the circuit breakers according to their type of construction:

Modular automatic switches. Commonly called PIA (small circuit breaker), they are usually DIN rail mounted, and are typically used for final consumption; nominal current values up to 50 or 63 A.
Molded case switches. They are more robust than modular switches, they are used for currents up to around 1000 A.
Air cut switches. Its use applies to the highest currents, up to around 6300 A.

What are the basic parameters for the selection of the circuit breakers?

In order to calculate and select a circuit breaker, the following basic parameters must be taken into account.

Rated operating voltage (Ue). It is the voltage at which the switch operates in the conditions of the installation without disturbance. It is common that the switches have different nominal voltage ranges, with different behaviors between them, as well as voltage values in case of disturbances in the installation.

Rated current (In). It is the maximum current value that can indefinitely circulate by a circuit breaker, without exceeding the admissible temperature values of the current carrying components.

Adjustment of the thermal trip current (Ith). It is the current above which the circuit breaker will trip in case of overload.

Adjustment of the magnetic trip current (Im). It is the current above which the circuit breaker will trip in the event of a short circuit. Sometimes this current is defined according to the magnetic, depending on the type of switch trigger curve.

The breaking power is the value of the expected short-circuit current that a switching device is capable of interrupting under a given voltage. The correct choice of a circuit breaker has to be made with the values of power of cut that indicates the norm IEC 60947-2: Icu (power of last cut) and Ics (power of cut in service). This value must be greater than the maximum short-circuit current expected at the point of installation. It may be inferior to it if upstream there is a switch that guarantees adequate cutting power through manufacturer filiation tables.

elec calc™

elec calc™ has an extensive library of manufacturer’ s circuit breaker catalogs. The software automatically sizes the switches according to the electrical installation and its regulations. It also allows obtaining a list of material of the switches. The software for the sizing and calculation of electrical installations also allows carrying out  selectivity studies and limitation and cascading.

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