Holger Urban, Product Marketing Manager at Schaffner, explains how to select a fuse or circuit breaker that will function as required in conjunction with an EMC filter, and not cause the filter to work as a fuse.
In electronic equipment the main focus often lies on the operational current demand (amperage) of a system. This means that modern systems are generally partitioned into different circuit branches which can feature over current protection devices. If a problem occurs on the feeder or at branch level a fault needs to be identified and the system needs be protected and fail safely. Failing safely not destructively means that all devices not considered as protection devices, including an EMC Filter, need to survive by virtue of the overcurrent protection.
Readily available MCBs (miniature circuit breakers) manufactured in accordance to the standard IEC 60947-2 feature current ranges from 1A to 63A. These are designed to trip over specific current ranges. EMC filters are designed to match overcurrent protection devices (thermally) and are designed to handle over current situations for a limited amount of time. Clearly, there are huge functional differences between a fuse and an MCB and by consulting the following diagram of tripping (time to current), a suitable filter can easily be determined.
Tripping characteristics of MCBs based on application:
Overall it needs to be understood that the circuit protection device protects the filter against overcurrent. So, let the system MCB trip not the filter (note that the filter needs to be placed downstream of the protection device).
Schaffner considers a calculated approach based on experience to evaluate the overload capability of any filter. Schaffner filters can be used with MCBs of class B, furthermore the filter can be verified with additional tests and certifications for special requirements. Normalised to multiples of rated currents the filter overload capability compared with the double logarithmic scale of tripping MCB is displayed below. In addition, the general overload capability of Schaffner filters is displayed.
NOTE: Characteristics may vary in a real application. Local installation guidelines may apply.
The first graph below shows how a filter compares against a Type B MCB while the second graph shows overload of a 40degC rated filter.
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