Few Words About Cascading
Cascading provides circuit breakers placed downstream of a limiting circuit breaker with an enhanced breaking capacity. The limiting circuit breaker helps the circuit breaker placed downstream by limiting high short-circuit currents.
Cascading makes it possible to use a circuit-breaker with a breaking capacity lower than the short-circuit current calculated at its installation point.
Area of Cascading Application
- Concerns all devices installed downstream of this circuit-breaker,
- Can be extended to several consecutive devices, even if they are used in different switchboards.
The installation standards (IEC 60364) stipulate that the upstream device must have an ultimate breaking capacity Icu greater than or equal to the assumed short-circuit current at the installation point.
For downstream circuit-breakers, the ultimate breaking capacity Icu to be considered is the ultimate breaking capacity enhanced by coordination.
As soon as the two circuit-breakers trip (as from point IB), an arc voltage UAD1 on separation of the contacts of D1 is added to voltage UAD2 and helps, by additional limitation, circuit-breaker D2 to open.
The association D1 + D2 allows an increase in performance of D2 as shown in figure 2 below:
- Limitation curve D2,
- Enhanced limitation curve of D2 by D1,
- Icu D2 enhanced by D1.
In actual fact, in compliance with the recommendations of IEC 60947-2, manufacturers give directly and guarantee Icu enhanced by the association of D1 + D2.
In a cascade system, both the upstream and downstream devices are expected to operate simultaneously so that the fault energy is shared by the breaking devices. Unless the combination is tested for the required fault level, the performance of the combination cannot be guaranteed in the field.
After a major fault is cleared both the devices of the combination need to be thoroughly examined and replaced if necessary to ensure safe operation during any future fault in the system.
Advantages of Cascading
Cascading allows benefit to be derived from all the advantages of limitation. Thus, the effects of short-circuit currents are reduced, i.e.:
- Electromagnetic effects,
- Electrodynamic effects,
- Thermal effects.
Installation of a single limiting circuit-breaker results in considerable simplifications and savings for the entire downstream installation:
- Simplification of choice of devices by the cascading tables,
- Savings on downstream devices. Limitation enables circuit-breakers with standard performance to be used.
Thanks to cascading, circuit breakers with breaking capacities less than the prospective short-circuit current may be installed downstream from a current limiting circuit breaker. It follows that substantial savings can be made on downstream switchgear and enclosures.
Resource: Merlin Gerin/Schneider Electric Circuit breaker application guide