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In this article, we try to clarify the type of circuit breakers that can be used, in order to have an idea before replacing, installing or choosing them.
We remind you that the differential switch is classified as a safety device, so we thought it important to dwell on it with more than a few articles.
RCDs are classified according to the detectable waveformat the differential sensitivityto the intervention timeat the presence or less than protections against overload or short circuit. In the latter case we have not only the pure differential, but a magneto-thermal differential, which performs three important functions in a single electromechanical component. The use of single blocks is not always convenient, it depends on the type of installation, flow rates, heat dissipation, the space available on the electrical panel/switchboard.
AC type differentials (for alternating current only): they are still very popular, cheaper, generalist. In dated systems it is easy to find this type of differential. It is not said that they are not good, in some cases they are more than enough. They are suitable when devices are generally envisaged in the system where the differential fault current will be predominantly sinusoidal, because they are sensitive to this type of current. So no inverters present, “few” “electronic” appliances constantly in use.
Type A differentials (alternating + button): they are the substitutes of the AC in the majority of modern systems, as they are sensitive not only to sinusoidal currents, but also to pulsating, unidirectional currents. They are useful when there may be brief surges, due to electronic equipment, voltage variations induced by nearby thunderstorms. The main disadvantage is that they cost more.
Type B differentials (alternating + push button + continuous): they have the characteristics of type A, in addition they are sensitive to currents with direct components, for example where there are photovoltaic systems or inverters. The cost is higher than type A.
So let’s see a summary table of what has been said so far. It can be seen that the category A residual current device includes the functions of the AC, and so does type B which includes the other two. In the column on the right, the symbols that are stamped on the differential, so that its type can be recognized.
Tripping time (rapid or selective differentials)
General type differentials: given the fault, they intervene more rapidly the more serious the extent of the fault. The CEI 64-8 standard describes a minimum trip time of 0.3s, i.e. safety tripping must occur within 3 tenths of a second at the most, from the moment the circuit-breaker detects the fault.
Selective differentials: compared to the previous ones, they have a fixed trip time but with a delay, e.g. 1 second. This is because they can be installed upstream of other residual current devices in more complex systems, when there is a fault in one area the power is cut off in safety but only in that area, without causing inconvenience to the rest of the system. We talk about selectivity.
Adjustable/programmable selective differentials: here the delay time is adjustable, again to ensure selectivity. This kind of switches can only be used in the industrial sector, because the adjustment must be performed by expert persons (PES). In industry, as in the commercial sector, it is unthinkable that a single residual current device can cut power to the entire building.
Low Sensitivity Differentials: when the current that causes tripping is greater than 0.03Ampere (30mA). In this case there must be coordination with the earth system, therefore suitable and mandatory. Typical use destinations in the commercial and industrial sector.
High sensitivity differentials: when the current that causes tripping is less than 0.03Ampere (30mA). They are also called “life-saving” since the current that would pass through a human body, in the event of a fault, causes the switch to trip at current thresholds that are not dangerous for the person.
Which Differential is right for me?
For what has been explained so far, you need to know the type of current and future differential fault currents (common sense and experience) in your plant, the equipment, the machinery, the technologies, the type of use. The choice is therefore dictated by an analysis of the specific case, simple in the civil sector, articulated up to complex in the other sectors. However, we can give some indications.
In the civil sectortype A differentials are typically installed. Often the preference is for differentials with fault current control and self-reset, to have safety and periodic verification which is almost never done, in which case we invite you to read our article on the matter. Only type A is not mandatory, in many cases AC may be sufficient, especially for simple systems. Bear in mind that nowadays new technologies have brought a massive use of electronics into our homes, as well as switching power supplies scattered around the house: for this reason, an extra precaution would be needed, with regard to some cases of fire in dated systems, where no adjustments had been made, let alone a check of the protections and insulation, including the earth system. In this case, due to the nature of the pulsating components, type A differentials are recommended.
In the industrial sector normally there is a plant project and coordination, the choice is almost obligatory if not with a margin of discretion in the distribution sub-boards and/or for the end users. If the business is “small”, type A and selective/adjustable differentials are typically installed, depending on the manufacturing.
In the commercial sector it depends on the size, on the power involved. It falls back as in the industry if with a mandatory project, otherwise the same considerations of the civil sector are considered valid, with some more coordination.
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