Today, selectivity can be defined in two ways. On the one hand, there is the familiar selectivity defined at 230 V(AC) in the series connection of overcurrent protective devices. On the other hand, selectivity comes into play in the parallel connection of overcurrent protection devices, especially in current-limited applications - which applies to 230 V(AC) as well as 24 V(DC). Series connection still refers to conventional overcurrent protection devices, such as fuses or circuit breakers. However, parallel connection is increasingly coming into focus, as it is very important when using electronic overcurrent protection.

The B characteristic curve for miniature circuit-breakers: With this characteristic curve, 3.2 to 4.8 times the rated current ensures rapid magnetic tripping in the event of a short-circuit.

© E-T-A Electrotechnical Apparatus

Selectivity means that in the event of a fault in a circuit of series-connected overcurrent or residual current protective devices, only the device immediately upstream of the fault location will trip. Protective devices connected in series differ in current intensity by a factor of at least 1.6, but usually by 2 to 3 times the overload. This means that the circuit breaker closest to the load must switch off before the next higher-level protective device in the event of a fault.

For example, if the load is fitted with a 10 A fuse, the next higher-level fuse must be rated at 16 to 20 A. This would ensure selectivity. This would ensure selectivity. This works safely and reliably for circuits with 230 V(AC), as the short-circuit currents required for tripping can be provided by the mains or the supply. A 3.5 to 5-fold overload is required to trip a standard type L circuit breaker; this is the only way to ensure safe and instantaneous disconnection. With a nominal value of 10 A, an overload of 35 to 50 A is therefore necessary to ensure selectivity.

In the example of a B characteristic curve, the fast disconnection due to magnetic tripping caused by a short circuit is 3.2 to 4.8 times the rated current. In this range, fast, selective tripping can occur in less than one second.

Safe shutdown

The circuit breaker is designed so that the necessary overload or the flow of the necessary short-circuit current is guaranteed for safe disconnection. This is also the case with common applications in 230 V (AC) installations, as well as when installed in a switch cabinet with a corresponding power supply. However, the miniature circuit breaker is also increasingly being used in current-limited applications - examples include an uninterruptible power supply at 230 V(AC) or downstream of a switching power supply at 24 V(DC). In these cases, overload capacity is no longer readily available, and the issue of selectivity is becoming increasingly relevant in this area.

Type ESX 10' from E-T-A switches off from a value of 1.1 times the rated current. This is combined with a current limitation above the typical value of 1.8 times the rated current. The usual switch-off time in the event of an overload is between 100 ms and 3 s.

© E-T-A Electrotechnical Apparatus

With selectivity in standard applications, the series connection of overcurrent protection devices is the main focus. When considering current-limited applications - such as UPS - the focus is not on series connection; rather, the application in the parallel connection of the overcurrent protection devices is important. Applications in the 24 V(DC) range with a switched-mode power supply are limited in their overload capacity. In the past, miniature circuit-breakers were commonly used as cost-effective overcurrent protection. These are gradually being replaced by electronic overcurrent protection, as this ensures selectivity in the parallel connection of the protective devices. Why? Back to the example of the 10 A fuse: a short-circuit current of 35 to 50 A must flow here for safe disconnection in the event of a fault. This is where the limitations in the overload capacity of the switched-mode power supply units come into play: The switched-mode power supply units are designed for nominal load, but have a certain overload capacity for a short time. For example, a device with a nominal output of 24 V(DC)/20 A can certainly provide a triple overload for 10 to 15 ms. If we consider the circuit breaker described above with an overload of 35 to 50 A, the following scenario can occur: The circuit breaker does not trip fast enough to reliably disconnect the faulty circuit from the switching power supply in the event of a fault. Instead, it causes the power source to go into uncontrolled overload and thus into fault mode. The entire secondary circuit loses its power supply. For this reason, more and more electronic overcurrent protection devices are being used, as they switch off reliably in such cases.

The characteristic curves of the electronic protection devices are designed for use downstream of the switching power supply and are set accordingly. This means that an overload switch-off occurs with these devices at a value of 1.05 to 1.35 times the rated current. The switch-off takes place within a very short time - typically after 0.1 to 1 s, depending on the multiple of the rated current. The circuit breaker guarantees selectivity by switching off quickly in the event of a fault. The other circuits remain safely connected to the 24 V (DC) power supply; the disconnection only affects the faulty circuit.

Application for uninterruptible power supplies

Type EBU' combines a typical miniature circuit-breaker with electronic disconnection. The device characteristic is based, for example, on the standard characteristic of a miniature circuit-breaker with a B characteristic, which is combined with an electronic characteristic.

© E-T-A Electrotechnical Apparatus

The application is similar for uninterruptible power supplies in the 230 V(AC) range. In general, the circuits in the secondary circuit are still protected with conventional circuit breakers. The requirement to provide a correspondingly high overcurrent in the event of a fault comes into play here. In this way, the circuit breaker reliably disconnects the circuit. For a circuit breaker with a rated current of 10 A, this means an overload of 35 to 50 A. When designing the power supplies, a 3 to 5-fold overload in the event of a fault must be included in the capacity. This often results in overdimensioning in order to ensure fast and safe tripping in the event of a fault. To avoid such overdimensioning, there are also electronic solutions for use on uninterruptible power supplies on the secondary side in the 230 V (AC) range that are combined with line protection. One example of this is the 'type EBU' (electronic breaker unit) from E-T-A. The electronic part is the sensitive measuring unit with tripping in the event of a fault, while the circuit breaker is used to switch off the load circuit. Thanks to the electronics and setting options, the protection device can be adapted precisely to the rated current of the load circuit and the UPS. The setting is made in relation to the required rated current of the load, the short-circuit current and the overload behaviour in order to parameterize the appropriate reaction of the device in the event of a fault. As a positive side effect of this precise setting, the total output of the UPS can be reduced by up to 30%, as the high overload capacity of the UPS is no longer required to switch off the conventional circuit breakers.

Author:
Ralf Dietrich is Head of Product and Market Development and a member of the management team at E-T-A Elektrotechnische Apparate in Altdorf.