elec calc™: Limitation and cascading (back-up)

By limiting the peak value of short-circuit current passing through it, a current-limiting circuit-breaker or a limiting fuse permits the use, in circuits downstream of its location, of switchgear and circuit components having much lower short-circuit breaking capacities, and thermal and electromechanical withstand capabilities than would otherwise be necessary.

Reduced physical size and lower performance requirements lead to substantial economy and to the simplification of installation work.

Most national standards admit the cascading technique, on condition that the amount of “let through” energy by the limiting circuit-breaker is less than the energy downstream circuit-breakers and components are able to withstand without damage.

In practice this can only be verified for circuit-breakers by tests performed in a laboratory in accordance with standard IEC 60947-2.

Such tests are carried out by manufacturers who provide the information in the form of tables showing the increased breaking capacity of a circuit-breaker placed downstream a limiting circuit-breaker, so that users can confidently design a cascading scheme based on the combination of recommended circuit-breaker types. Manufacturers also provide curves showing the limited “let through” energy and the limited peak current according to the prospective short-circuit current.

Concerning the fuses, the limitation capabilities are defined by the set of standards IEC 60269.

elec calc™ takes into consideration the data provided by the manufacturers to calculate the electrical installation. The main advantages are:

• Simplification, i.e. a wider choice of downstream switchgear, cables and equipment
• The use of lighter-duty switchgear, cables and equipment, with consequently lower cost
• Economy of space requirements, since light-duty equipment have generally a smaller volume