Causes of errors in electrical circuits

When designing, and when making changes, sometimes they make mistakes, and they are typical. Therefore, you should pay attention to them, which is done below.

The readers' attention was also drawn to the fact that the scheme (drawing) and electrical installation are far from the same thing and that in the book, when it is said about the scheme, it is always meant not the actual image, but what is depicted.

It is certainly obvious that the electrical installation must work, i.e., fulfill the functions assigned to it, be durable and safe. If these requirements are met, it is usually said that the scheme is good, and if not, the scheme is considered bad.

Are such estimates true? From the standpoint of people who only use electrical installations (and identify them with schemes), such estimates are quite natural. But an expert can not reason like that. There are cases, for example, when two electrical installations are mounted on the same circuit, but one of them works well and the other one malfunctions. But the point here is not at all in the scheme, but in the fact that one electrical installation is well-assembled, adjusted, regulated and systematically maintained, that is, the requirements of the EMS are strictly observed. The other one is mounted somehow, is not adjusted, and the operation is entrusted to insufficiently qualified personnel.

Let's consider one more example: according to the same scheme two electrical installations are carefully assembled. One of them works well, but the other can not be adjusted. Why is the circuit for one electrical installation good, but for another unsatisfactory?

The matter is that electrical installations are executed on different hardware base one, for example, on telephone relay, and another - on code. The circuit is not made at all, for all cases of life, but for specific conditions, in this case, based on the properties of telephone relays, but not code. So the reason for the failure was not that the scheme was bad, but that it was used in conditions for which it was not intended. Hence follows the conclusion: the scheme can be used only in those conditions that were adopted when compiling it.

In operation cases when the correctly applied scheme starts to give failures, for example, after repair of any its element are quite often in operation. So, in Example 8.6 (see the book), a disruption of the disconnecting motor drive is described, due to the fact that the terminals of one of the two field windings were crossed during the repair. As a result, the engine for which the circuit was designed was not the same one.

Similar phenomena can occur when replacing electrical equipment with a new one. For example, when the life of the oil circuit breaker has expired, the drive has been replaced with a new one, suitable in all respects but with different kinematics, and it required a different adjustment of the auxiliary contacts. The drive worked well. Therefore, the operating personnel did not pay attention to the fact that after disconnecting the switch, the disconnecting electromagnet of its drive remains under current. We noticed this error (and corrected the scheme, taking into account the features of the new drive), only after the electromagnet was burnt.

The same kind of malfunction is often the consequence of making any additional elements (contacts, resistors, capacitors) in the circuit or excluding elements that are mistaken for extra ones. These changes are made to solve particular problems (for example, an additional signal was needed, or they want to simplify the scheme), and as a result, the basic dependencies can be violated. The simplest example of 10.7 is given in the same chapter.

The considered examples are sufficient to draw the following conclusion: with any replacement of equipment, as well as when introducing or removing from the circuit any element not provided for by the compiler of the scheme, it must be carefully checked, based on the new conditions. A common mistake is drawing up a scheme without taking into account the real operating conditions of the electrical installation. As a result, practically impossible solutions appear; Typical examples are given and discussed in detail in § 8.2. Here we draw attention to especially dangerous mistakes, as a result of which the electrical protection provided by the PUE was made only formally, i.e., depicted in the diagram, but it was not checked by calculation whether it would work (see Example 8.16). The essence of the case in this example is that the fault current value is limited by the transformer resistance, because of which the current passing through the protective current coil at the fault (88 A) is less than the set value of relay 100 A) set by the circuit designer. In other cases, the fact that the coordination of the parameters of the interacting devices is not verified, faults are possible, because there is not enough time to perform the specified switching operations (see Example 8.10). It is also possible that there are unnecessary switches, which are by no means always safe. Thus, in exercise 9.6, the case of several false inclusions on faults is considered, because the length of the switching pulse is not matched with the time of the relay of the current relay. Adverse combinations of circuit parameters - inductance and capacitance - can lead to multiple false positives, see example 7.22. Hence follows the conclusion: the scheme can be considered complete only if it is verified with the help of calculating or constructing the interaction diagram (see § 5.2) in its operability.

Let's consider one more typical error, often allowed by the compilers of the schemes. It consists in the fact that the real operating conditions of the equipment are not taken into account. As a result, the equipment fails without performing the work for which it is intended. So, in example 8.18, the contacts of the AC voltage monitoring relay, due to vibrations that last for a long time, are sintered and, therefore, can not open when the bus voltage disappears. Another example: providing for the protection of the electric motor from overloading by means of electrothermal relays, the circuit designer does not take into account that a thermal current passes through the thermal elements (heating elements), which can melt the cells if their circuit is not sufficiently sensitive with fuses or circuit breakers. Conclusion: the scheme should ensure the unconditional safety of electrical equipment. Scheme makers often underestimate the importance of providing reliable power supply to operational circuits, calculating them only for normal operation. The consequences of such an unacceptable approach to the case are illustrated in Example 8.16: due to the inadequate section of the line feeding the actuating electromagnet of the drive (the cross-section is selected only by the permissible current, but without taking into account the line length), the emergency mode was abrogated by the ATS. Often mistaken when sectioning the buses of operational current, and also allow the connection of operational circuits of different lines. Dangerous consequences (and the way to eliminate them) of the ill-considered power system of the relay protection circuits is illustrated in Example 10.1. Conclusion: operational chains, sometimes called secondary (to distinguish them from primary, power networks) are by no means secondary. A great danger is represented by false chains. Typical cases and reasons for their occurrence, as well as ways to identify and eliminate such a situation are discussed in § 10.6. A few words about working drawings. In the book they are not considered (with the exception of a few examples in Chapter 3), since they are carried out according to industry normative documents. However, the basis for the implementation of the working documentation are the principal schemes. Consequently, the originator of the schematic diagram is obliged, if necessary, to draw the attention of the executors of the working documentation to those particulars that must be taken into account.

In other words, when drawing up the conceptual scheme, one must always think about what means and how it will be implemented. To understand the importance of this not quite obvious requirement, it is sufficient to turn to Fig. 7.17 and compare the contact groups, which, despite the same number of contacts in each of them, have completely different performance properties. It is to these properties that the schema compiler calculates, applying not any but a definite group and using every contact in a strictly defined chain in it.

Conclusion: in similar cases, the question of applying this or that product and the procedure for connecting wires to it is decided not by the installer and the executor of the working documentation, but by the one who makes the schematic diagram.

The schema compiler should:

A) Think carefully and provide everything necessary to link the new electrical installation with the one already in operation;

B) give instructions on how to check the new electrical installation;

C) clearly articulate what it is designed for and how it should be used. In a word, drawing up a scheme is much more complicated than it sometimes seems to people without sufficient experience.

Drawing up schemes is a creative process. It can not be enclosed in a predetermined framework. And at the same time, when designing, it is necessary to take into account a number of limitations that can be divided into several groups.

1. Restrictions arising from the requirements of the PUE, SNiP, standards and other normative and guiding documents (see §6.1).

2. Restrictions due to the properties and features of electrical products (§ 6.2).

3. Construction restrictions. For example, the normalized size of the passages and spans, the location of water, heating and electrical networks, the strength of the overlaps, etc. These limitations are generally not influenced by the drawing up of schematic diagrams, but the executors of the working documentation must take them into account.

4. Binding restrictions. They are caused by the fact that the new electrical installation must inevitably be associated with the operating one, for example, to receive power from it, to join the existing signaling system, and so on.

5. Constraints arising during the design process. So, for example, having stopped on a certain electrical circuit, having chosen equipment for it, the schematist is already connected. His further decisions are themselves limited.