Electrical distribution board: choosing modular protective devices

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Critical nodes of any electrical network are distribution boards. Modular hardware and its correct layout are key safety indicators. Today we will introduce you to DIN standard modular protective devices and their application..

Electrical distribution board: choosing modular protective devices

Devices of DIN standard have a protective plastic casing, grooves on the back of which are intended for mounting on a 35 mm wide-profile rail. The standard width of one module is 17.5 mm, i.e. the installation pitch is 18 mm. The distance between the curved sides of the rail and the back side of the front protective panel is not less than 43 mm. If, according to the requirements for dust and moisture protection, it is not necessary to firmly press the protective lamellas to the shelves of the devices, the latter may be of smaller depth. In any case, it is better to equip the shield with devices of the same format, except for cases when special electrical equipment is located in the general shield – PLC, relay protection or network transformers.

Electrical distribution board: choosing modular protective devices

Circuit breakers

The basis of any electrical network and the most extensive class of modular equipment. The main purpose is to protect cable lines and equipment from unacceptably high currents. However, the mechanism of their work is somewhat more complicated than it seems..

The circuit breaker carries two devices on board that monitor the current in the network: thermal and electromagnetic releases. As an exception, there are MA type circuit breakers (without thermal protection) and load breakers (modular switches). In the remaining circuit breakers (types A, B, C, D, K, Z), the thermal release limits the continuous overload current (TP), and the electromagnetic release limits the short-circuit current (TKZ).

Electrical distribution board: choosing modular protective devices Circuit breaker device: 1 – control lever; 2 – electromagnetic release; 3 – bottom terminal; 4 – arc chute; 5 – thermal release (bimetallic plate); 6 – upper terminal

The type of machine is determined by the characteristics of the thermal release. If the electromagnetic one works extremely simply (it instantly breaks the circuit with a sharp jump in current), then the thermal one is able to withstand the exceeded currents for a certain time. More accurate data can be derived from the graphs of current-time characteristics, which any more or less experienced electrician saw at least once in his life.

It is not easy to choose a circuit breaker in accordance with the type of equipment used, cable brand, line length and other factors, but the durability of the entire electrical network and the correct operation of protective devices depend on it.

Electrical distribution board: choosing modular protective devices

By the way, the machines still have a lot of small parts in addition to releases, and their durability and reliability also depend on them. The quality of the drive mechanism, frame, spark arresters and clamps, as well as the difference between their variations, can be easily determined even visually, many well-known brands have sketches of their products or samples in transparent cases, and the network has enough photos of disassembled machines.

Differential devices

One of the most important breakthroughs in modern wiring is compact devices operating on the principle of a leakage relay. Without going into the details of the mechanism of operation, we can say that the protective component compares the quantitative value of the current entering the circuit and leaving it. If the difference between the values ​​is large enough (10 mA to 1 A), the device breaks the circuit. There are two classes of such devices: residual current devices (RCDs) and differential circuit breakers (DAV).

Electrical distribution board: choosing modular protective devices

Most often, diffusers are used to protect people from electric shock. The human body can safely flow from 0.05 to 0.1 A AC, depending on the exposure time. The leak is detected instantly, and the shutdown lasts no more than one half-period, so it can be argued that the impact of a current of 0.01 amperes for 0.02 seconds will be safe even for a child. But leaks in the power grid are a common thing, they can appear on their own due to high humidity and broken insulation (this is one of the arguments against HB tape). In practice, the most sensitive 10 and 30 mA RCDs only work with more or less updated wiring. In a network of apartments aged 30-40 years, they give one or two false positives per week.

For this and other reasons, the sensitivity of the RCD is deliberately underestimated. It is believed that most people can tolerate a short shock of 0.1 A normally, therefore all devices up to 100 mA are designed specifically for human protection. But after all, the leakage current can pass not only through a person, insulation breakdowns on a wall or a protective conductor are also possible, which leads to a fire hazardous situation. The circuit breaker will not trip in this case: the current is too low. RCDs and DAV over 100 mA are conventionally called fire protection.

Electrical distribution board: choosing modular protective devices

Installation of an RCD “for people” is mandatory on each line to which stationary household appliances with a metal case, water heaters, dishwashers and washing machines, sockets in the bathroom and on the kitchen apron are connected. The rest of the outlet groups can be connected through a common difavtomat, although more often they are also grouped in 4-5 pieces. A fire-fighting RCD is installed at the input to the shield, and its sensitivity is selected based on the duration of the cable lines and their total leakage.

Devices can be mechanical or electronic. The former are preferable due to their lower sensitivity to voltage fluctuations. On the other hand, some of the electronic RCDs have the ability to self-reset when the accident disappears..

Relay protection devices

The abundance of electronic household appliances always raises the risk of mass damage due to overvoltage, switching overloads or inconsistencies in the current frequency with the industry standard. Unfortunately, the poor quality of the mains voltage still occurs in remote areas, even in new buildings accidents happen..

The complexity and cost of protective measures should be directly proportional to the cost of the equipment connected to the network. At a minimum, any modern apartment that has at least one digital TV should have a voltage relay at the input..

Electrical distribution board: choosing modular protective devices

The most primitive devices have a purely relay principle of operation. The settings for the upper and lower shutdown thresholds are set by two potentiometers. Three-phase voltage relays can have built-in protection against load asymmetry, that is, they monitor the difference between line voltages.

Devices controlled by integrated microcircuits stand apart among relay protection devices. The electronic principle of operation allows not only to emergency shutdown of the consumer, but also to diagnose the network, report the reason for the shutdown and even monitor not only the voltage, but also other indicators.

Electrical distribution board: choosing modular protective devices

The highest class of devices is called UKZ – complex protection devices (UBZ 302, for example). They not only monitor the voltage, but also have built-in current protection, sometimes they even carry a digital computer on board, which makes it possible to take into account the power consumption in its three components in operation or track the values ​​of leakage, inductance and circuit capacitance. As a rule, such devices are characterized by the presence of a user interface and a flexible system of settings, optionally the presence of lines with controlled switching.

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