Can standard and QDC breakers be different?

Can circuit breakers (QDC) circuit breakers located inside homes be larger than the standard circuit breaker? Although the answer seems obvious, doubts are very common as it is. Thinking about it the World of Electrical decided to end this doubt once and for all! We will answer this question in a very simple way and in accordance with the rules.

We know that to have effective protection of the electrical installation and electrical equipment, the protective components of this installation, such as circuit breaker, DR and DPS, must be installed correctly and mainly well dimensioned, this must also be done in the input standard.

The electric current that passes through the circuit breaker of the input pattern is always greater than the other currents that pass in the installation circuits because when it passes through the QDC, the current that comes from the input pattern is distributed to these circuits. Thus, it is natural that the input standard circuit breaker is always greater than or equal to the QDC circuit breaker.

To find out whether or not the switchboard circuit breaker can withstand a higher rated current than the input standard circuit breaker, the answer is basically to understand what selectivity is, as this is a determining factor for the safety and perfect functioning of the installation electrical, where all these installations must have selectivity.

Suppose in a home the 50A standard circuit breaker and the 70A QDC circuit breaker, if an electrical fault occurs in the home that exceeds 50A, and the partial circuit breakers fail, the standard circuit breaker will trip first then the QDC circuit breaker, in this way, to rewire the circuit, the person will have to move to the input pattern.

QDC circuit breaker is larger than the meter breaker.

The example we mentioned above is a clear example of a lack of selectivity in the circuits. Selectivity serves to ensure that the circuit breaker closest to the electrical fault always acts in such a way that the smallest possible number of circuits and loads are affected, that is, in this case, the circuit breaker closest to the fault is the QDC circuit breaker, which will not be triggered in case of circuit failures.

It is important to highlight that the NBR 5410 standard addresses in item the selectivity between overcurrent protection devices, which says the following:

“When reasons dictated by safety and/or the use of the electrical installation require that the continuity of service is not affected except by the slightest occurrence of a fault, devices located in series must have their performance characteristics selected, in order to ensure that only the device responsible for protecting the circuit where the fault occurs will act (selectivity). ”
In addition to the circuit breakers, the standard NBR 5410 also speaks in item about the selectivity among the DR protection devices.

So to give a very precise and summarized answer, the QDC circuit breaker cannot be bigger than the input standard circuit breaker, because as we just saw, this type of installation does not guarantee selectivity in the circuit.

Doing the correct sizing of the entire installation ensures its selectivity, making the installation safe, thus preventing malfunction of the circuit and possible accidents.

The following video is from the Mundo da Elétrica channel, it explains in detail what is selectivity and the importance of selectivity in the circuits, in addition to showing some examples that will help you to understand even more about this subject.