Feature: Components
without spacing, thermal interaction occurs between them. T e circuit breakers heat each other, which means the maximum nominal current of the circuit breakers must be reduced again. How much the current needs to be reduced due to increased temperature of the MCB in such cases is specifi ed by the manufacturer and referred to as the Rated Diversity Factor (RDF). Electronic circuit breakers, on the other hand, are temperature-
independent. T e operating temperature of the PISA-M is between -25°C and +70°C at full output power – i.e., without derating; an RDF is not necessary. T e modules can also be installed in the control cabinet without
minimum spacing to other PISA-M units. Only a minimum distance of 15mm must be maintained to heat sources, such as the power supply.
The right power supply unit Just as important as selecting the right circuit breaker is the decision for the appropriate power supply unit. T is aspect is oſt en overlooked. Practice shows that the short-circuit current of a 10A power
Figure 2: MCB tripping curves for B, C, Z and K characteristics
earlier. High ambient temperature leads to the maximum nominal current fl owing through the circuit breaker needing to be reduced to prevent unwanted tripping. Conversely, at low temperatures, a higher current must fl ow to suffi ciently heat the bimetal to trip the circuit breaker. T e installation in the control cabinet can also aff ect the thermal fuse. When multiple circuit breakers are operated side by side
supply for a total load of 7A may not be suffi cient to trip the MCB and isolate the faulty load. Because with a load of 7A, a 1A MCB is usually used. T is means that if an MCB with the B characteristic is used, the 10A power supply must provide between 30A and 50A as short-circuit current to trip the MCB. Most 10A power supplies can’t provide this much current, as they shut down for self-protection beforehand. As a result, all loads connected to the power supply are no longer supplied, even if they do not have a fault. To avoid this problem, a power supply with a fuse-breaking
Figure 3: ECB tripping delay depending on current fast tripping characteristic 32 September 2025
www.electronicsworld.co.uk
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