Miniature circuit-breaker versus electronic selectivity module - 5 Pages

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Miniature circuit-breaker versus electronic selectivity module

Catalog excerpts

Miniature circuit-breaker versus electronic selectivity module Selective monitoring of 24 V DC load circuits In modern plant and machine construction all 24 V DC loads – whether rugged electro-mechanical loads or highly sensitive electronics – are supplied by a single, common, stabilized switch mode power supply. To ensure that a fault in one load does not bring the complete plant to a halt, the 24 V DC power supply circuit is divided into separate branches and selectively protected. Miniature circuit-breakers (MCBs) are often used for this purpose. In many cases, however, they do not offer reliable protection. Conversely, the electronic selectivity module monitors the 24 V DC branches safely and ensures. Switched-mode power supplies protect themselves with electronic output current limiting To provide power to the 24 V DC level of automated plants, primary switched-mode power supplies have long ago gained widespread acceptance and replaced the unregulated transformer power supply. In most cases, however, the distribution of the 24 V feed among multiple load groups and their protection are still handled by conventional miniature circuitbreakers. The internals of switching power supplies are dimensioned for a specific nominal power output. To protect the devices against damage from overloading, the output current is electronically limited. The current limiting normally starts at 1.1 to 1.5 times the nominal value. This limited maximum current also has an effect on the tripping response of miniature circuit-breakers. The typical behavior of miniature circuitbreakers The function of miniature circuit-breakers is the protection of wires against thermal damage to the insulation from too high a current. The tripping characteristics are thus matched to the current voltage characteristics of the wires. Looking at the various current-time characteristics of MCBs, two general tripping ranges can be distinguished resulting from two different disconnection mechanisms. In the range of small overcurrents, a time-delayed thermal tripping occurs through a bimetal. The duration until the tripping depends on the level of the overcurrent and can range from minutes to hours in the case of small overcurrent. The disconnection in the case of larger overcurrents (electrical shorts), on the other hand, is tripped electromagnetically without a delay. Here, the immediate tripping takes place within milliseconds by means of an electromagnet. So that the electromagnetic tripping of the MCB does not already respond at the normal operational starting current surges caused by the connected loads, different sensitivities exist, which are reflected in the tripping characteristics “A” to “D”. In all cases, however, a multiple of the nominal current is required for the MCB to trip rapidly.

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If, for instance, a miniature circuit-breaker of the common type C is considered, the electromagnetic instant tripping occurs at 5 to 10 times the nominal current; if operated with DC voltage, the limit current values increase by a factor of 1.4. For a safe configuration, 14 times the nominal current thus has to be taken into account to achieve the instant tripping required for the selectivity. The problem with modern power supplies and conventional miniature circuit-breakers The electronic current limiting of a power supply limits the output current during overloads to, for example, 1.5...

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Terminal strip MCB = Miniature circuit breaker mmediate tripping Transfer resistance of short- circuit from dead shorts of creeping shorts of several Calculation is based on assumed The short-circuit current of 46A DC will not cause MCB C4 to trip reliably within 12ms (required tripping current: 56A DC) Application example with limited short circuit current The short circuit current is reduced by various ohmic impedances which means that the current required for fast tripping of the miniature circuit-breaker cannot be achieved regardless of the output performance of the power supply....

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Extensive functionality assures a targeted fault localization Additional benefits commissioning The tripping current of each output can be individually set with a potentiometer accessible from the front. This reduces the device variety and allows changes to be easily made even during commissioning. Two device versions with adjustment ranges from 0.5 to 3 A and 3 to 10 A are available for covering the widest possible current range. The status of the separate load circuits is indicated by one multi-colored LED per channel. In the event of a disconnection, the indicator changes from green to...

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Application range of miniature circuit breakers and selectivity module: Selectivity module and standard power supply + Safe shutdown, even for low short-circuit current. e.g. due to long cables, small core cross- sections, creepage short-circuits + Uninterruptible operation of electronic loads such as PLCs, due to immediate disconnection of faulty branches when there + Fault diagnosis via LEDs, common signaling contactor single channel signaling MCB and power supply with power boost + Rapid shutdown of faulty branches. Requires appropriate dimensioning of power supply, MCB, core...

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