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Selection and setting of overcurrent releases of Modeion circuit breakers

modeion.png Selection and specification of the setting of overcurrent releases is a frequent task especially for designers. Therefore, we have decided to deal with this topic after several years again.

Step 1: Overcurrent release selection

By the right selection of the overcurrent release you can avoid many problems which especially end users as well as switchboard manufacturers, electro-installation firms or inspection engineers often have to face. It may happen, for example, that a simpler overcurrent release is selected, which does not make it possible to set the tripping characteristic suitably for the protected device or circuit and/or it is not possible to achieve the selectivity required with it. For these reasons the circuit breaker is then undesirably switched off quite often. This causes significant damage to manufacturers in many applications, e.g. in the metallurgical or glasswork industries. The selection of an overcurrent release should be made also with regard to the economic point of view and in simple, nondemanding applications it is not suitable to use complex releases which are more expensive. Another case is when the load or even circuit parameters are not sufficiently known. Then it is a good idea to prefer a more complex overcurrent release, with more possibilities of settings, which can be set according to particular requirements during the putting into operation. The text below includes correspondence between protected devices and overcurrent releases with basic characteristics which may help during the release selection.

Step 2: Specification of settings

An unsuitable setting of the overcurrent release may lead to undesirable switching off of the circuit breaker in a common troublefree operation or, conversely, the circuit breaker may not switch off at a short-circuit. The setting of the tripping characteristic can be divided into two principle bands.

First band: protection from overload (Dependent time release)

We often call this band “thermal release”. A too high setting of the thermal release value may lead to a thermal overload of the line or motors and at the end even to a failure of the device protected. For motors it is furthermore necessary to ensure protection also in the case of a phase failure. A low setting of the thermal release value conversely leads to the already above mentioned undesirable switching off of the circuit breaker in a common operation. For example, in the case of an unsuitable setting of the thermal release delay, the circuit breaker switches off recurrently during the motor start-up. The thermal memory may be switched off in rare cases only, but never during protection of motors, lines and transformers. During protection of e.g. spot welding machines or track cranes where a regular cyclic overload is registered it is possible to switch the thermal memory off . These devices are dimensioned for such overloads.

Second band: protection from short circuit (Independent time release)

This band is also known under the slang term “short-circuit release”. The incorrect setting of the short-circuit release value may have by far more serious consequences. A value of the short-circuit release set too low will cause the undesirable switching off of the circuit breaker again. A too high setting of the short-circuit release value may cause, in turn, that the circuit breaker at the occurrence of a short-circuit will switch off later or will not switch off at all. These two cases may lead to significant material damage or even personal injuries or death.

Through a suitable setting of the short-circuit release, possibly through a delay of its switching off, it is possible to achieve a higher selectivity between individual protection elements in the circuit, whether the matter concerns circuit breakers or fuses. The full or at least partial selectivity will again prevent the circuit breaker from switching off needlessly. This means that the circuit breakers in which there is no failure will not disconnect uselessly from the power supply system.

Step 3: Recording the data in the project

This last step is as important as two previous steps. It is necessary for the overcurrent release type to be stated in the project, together with the specified values of the overcurrent release settings. According to this type the switchboard manufacturer, installation firm or the end user will set the overcurrent release.