Savings are always relevant. Savings are always relevant Wrong choice of safety factor extreme cases

There are two main approaches to determining the safety factor: statistical and economic.

Statistical methods based on the required level of service:

· Probability of inventory shortages during one inventory turnover cycle (or during the period between two reorders),

· Probability of meeting demand,

· Level of readiness – characterized by the period during which reserves should be “positive”,

· Optimal frequency of inventory shortages during the reporting period.

Economic methods based on cost optimization:

· Acceptable level of losses due to lack of stocks in the warehouse,

· Optimal ratio of storage costs and losses due to lack of stocks in the warehouse.

Let's take a closer look at the standing order method in a simplified form.

It is necessary to determine the value of the reserve stock for which the optimal ratio of storage costs and losses due to stock shortages will be.

Let's consider solving this problem when using an inventory management system based on the standing order method. The size of the safety stock will determine the reorder point. The solution to this problem will not affect the optimal order size, but will only affect the change in the reorder point. Therefore, we optimize two types of costs:

Costs of holding safety stock, which are part of the total storage costs and which will be equal to:

TC = C h 1 *R, (9.32)

where C h 1 is the cost of storing 1 unit of inventory for the reporting period, R is the amount of reserve stock.

Losses due to inventory shortages, which are equal:

U = C d 1 *S*r, (9.33)

where C d 1 is the loss due to a shortage of 1 unit of inventory in the warehouse, S is the probable number of times there is a shortage of inventory during the reporting period, r is the average volume of inventory shortage in units.

In this problem, we consider losses due to inventory shortages, which do not depend on the duration of the shortage, but depend on the volume of the shortage and the number of shortages during the reporting period. A model in which these losses depend on the duration of the shortage requires more complex calculations.

The solution algorithm is based on the technique of marginal or limit analysis. In this technique, we add (or subtract) one unit from the parameter under study and analyze the impact of this change on the optimized value. If this influence is positive, then we continue to change this parameter in the same direction until it decreases to zero. If the influence is negative, then we change the parameter in a different direction and move again to zero influence. With zero influence, the parameter value is optimal. The calculation algorithm is shown in Fig. 9.14. This technique is often used to find optimal solutions in economic analysis.

Rice. 9.14. Algorithm for calculating the safety factor

The positive contribution (gain - savings in storage costs) from each additional unit will remain constant as the safety stock decreases.

The negative contribution (losses - losses due to inventory shortages) from each additional unit will increase as the safety stock decreases, since the probability of inventory shortages (S) will increase.

The gain is greater than the loss, then when we decrease the reserve stock by each unit, we receive additional profit as long as the gain is greater than the loss.

Losses are greater than gains, then an increase in the reserve stock leads to a decrease in losses.

The optimal size of the reserve stock is obtained under the condition:

S*C d 1 = C h 1 , (9.33)

Under this condition (9.33.) the gain is equal to the loss.

The complete algorithm for calculating cost optimization can be interpreted in Figure 9.15.

Rice. 9.15. An example of calculating the safety factor using the cost optimization method

· If we know storage costs (C h1) and losses due to inventory shortages (C d 1), we can calculate the optimal frequency of inventory shortages for the reporting period, at which the total costs will be minimal according to formula (9.33).

S = C h 1 / C d 1 – formula for calculating the optimal frequency of inventory shortages(9.34)

· Knowing the optimal frequency of inventory shortages for the reporting period (S) and the frequency of orders (N), we can calculate the probability of inventory shortages (P) for one inventory turnover cycle (or between two reorders):

P = S/N – formula for calculating the probability of inventory shortage for one inventory turnover period (9.35.)

· The value (P) is directly related to the safety factor (k) based on the rule of normal probability distribution. The safety factor is determined based on special tables that can be found in any literature on inventory management.

Safety factor

Safety factor

f - is used when determining the design loads on Pp based on the values ​​of the maximum operational loads Re and is equal to:
f = Pр/Ре.
K. b. is introduced to ensure a high level of aircraft reliability in terms of static strength, taking into account possible variations in external loads and the strength characteristics of the aircraft structure. Values ​​of K. b. are specified in the Strength Standards; in the aircraft industry, typical values ​​of f are from 1.5 to 2.

Aviation: Encyclopedia. - M.: Great Russian Encyclopedia. Editor-in-Chief G.P. Svishchev. 1994 .

See what “Safety factor” is in other dictionaries:

Safety factor- C – coefficient that determines the degree of increase in the control load in relation to the load on the product corresponding to its calculated load-bearing capacity. [GOST 8829 94] Term heading: Theory and calculation of structures Encyclopedia headings: ... ... Encyclopedia of terms, definitions and explanations of building materials

A correction factor to the experimental or calculated value of explosion hazard, which determines the maximum permissible value of this parameter (concentration, temperature, pressure, etc.) for a given production process. EdwART. Dictionary… … Dictionary of emergency situations

safety factor- - [Ya.N.Luginsky, M.S.Fezi Zhilinskaya, Yu.S.Kabirov. English-Russian dictionary of electrical engineering and power engineering, Moscow, 1999] Topics of electrical engineering, basic concepts EN safety coefficient of safetyf/s ... Technical Translator's Guide

safety factor- 3.99 safety factor (safety class resistance factor): Correction factor to the value of the load or other parameter (pressure, temperature, concentration, etc.), determining the degree of increase or decrease in the control value by ... ...

safety factor C- 3.6 safety factor C: Coefficient that determines the degree of increase in the control load in relation to the load on the product corresponding to its design load-bearing capacity. Source: GOST R 54271 2010: Anchors for overhead contact networks... ... Dictionary-reference book of terms of normative and technical documentation

safety factor- saugos laipsnis statusas T sritis radioelektronika atitikmenys: engl. degree of safety vok. Sicherheit, f; Sicherheitsfaktor, m; Sicherheitsgrad, m rus. safety factor, m; degree of safety, f pranc. coefficient de securité, m; degré...

safety factor- saugos faktorius statusas T sritis radioelektronika atitikmenys: engl. safety factor vok. Sicherheitsfaktor, m; Sicherheitsgrad, m rus. safety factor, m pranc. coefficient de securité, m; facteur de securité, m... Radioelektronikos terminų žodynas

An indicator characterizing traffic conditions on a specific section of the road (for example, in a populated area or on a curve in the plan) and the approach to it. Used to identify dangerous sections of roads.

When calculating individual components of the total heat load, it is necessary to reliably know all the above operating conditions of refrigeration equipment and product storage modes. However, often during calculations some of these parameters remain unknown. In this case, it is necessary to set some average parameters for a given operating mode and enter a coefficient for this component. In other words, this is a measure of our ignorance of any conditions or operating modes of the camera.

The safety factor value is usually in the range from 1.0 to 1.1.

Calculation example

Returning to the example, we note that when calculating the daily cargo turnover of the product, we used its estimated value of 10% of the full chamber load. Therefore, for this component of the thermal load we will introduce a safety factor of 1.1. As a result, for the magnitude of the heat load from the product we have:

Q" cont = Q cont * K without = 4.936 * 1.1 = 5.43 kW.

In addition, when calculating the heat load due to door opening, we also used the estimated daily value of cargo turnover, and therefore for this component of the load we will introduce a safety factor of 1.05:

Q" inf = Q inf * K without = 2120 * 1.05 = 2226 W.

The mechanical properties of the metal are tested in metallurgical plants using random tests, so it is more likely that material with the following properties, established by GOST, will enter the structure.

Control of the mechanical properties of the metal occurs on small samples during uniaxial tension, but in fact the metal works in large-sized structures during a complex stress state.

Safety factor on materials takes into account the effects of all these factors on reducing the load-bearing capacity of the structure.

It is possible to establish a drop in mechanical properties against standard values ​​as a result of processing statistical data from factory tests of steel, and the performance of steel in structures as a result of research.

Based on the results of analyzes of distribution curves of steel tests, it is possible to determine the safety factor for assigning the design strength of steel according to the yield strength.

As a result of establishing the calculated resistance according to the yield strength, the value of the coefficient k m = 1.1 - 1.2 for steel grades From 38/23 - From 60/45.

Safety factor for the material is accepted as increased if the design resistance is assigned based on the temporary strength.

Suppose that unforeseen circumstances occurred, after which the stresses in the structure reached the yield strength value, as a result of this, the stretched and bent elements began to receive increased deformations, but they will not become unusable, but if the stresses are equal to the temporary resistance, then the element will rupture, which is not possible cannot be allowed. Due to this safety factor by material for design resistance for steel grades S 46/33 and S 52/40 equals 1,5, For 60/45 - C 85/75= 1.6, and for From 38/23 - From 44/29 = 1,45.

9) Work and stability calculations of centrally compressed rods.

The behavior of a rod under load is characterized by a graph (Fig. 2.4, b), where initially, with increasing load, the rod retains a rectilinear shape, with a further increase in load, when the rod loses its stability and begins to bulge. A subsequent (small) increase in external load is accompanied by a rapid increase in transverse deflection f. After reaching the maximum load - the second critical force - the rod loses its load-bearing capacity (unstable state).

Steady state may be at and (points 1 and 2). However, when the rod can be in a stable state (point 2) and unstable (point 3) with the same compressive force.

A critical condition can be at and at (dots and ).

Fig.2.4. Operation of a centrally compressed rod:

a – design diagram; b – relationship between load and deflection of the rod

In practice, the flexibility of centrally compressed rods (columns, truss elements, frames, etc.) is approximately half of the specified limits.

In the above classical scheme, in which it is assumed that at the moment of loss of stability the load remains constant, then unloading occurs on the convex side of the rod and the material begins to work according to the elastic law. However, if the compressive strain during longitudinal bending increases or remains constant at each point of the cross section of the rod, i.e. unloading does not occur, then the entire section is in a plastic state, characterized by a tangential deformation modulus.

In this case, the critical stress in the plastic region will be In building structures, both schemes of operation of compressed rods are found. For example, compressed elements of statically indeterminate systems (trusses, frames) lose stability according to the classical scheme - with unloading. At the moment of loss of stability, a redistribution of forces occurs between the elements. In columns operating according to a statically determined scheme, the second scheme will be implemented - without unloading.

Until now, we have considered an ideally straight rod with a load applied strictly along the axis. However, in practice this does not exist. The design of the ends of the compressed rods does not provide ideal alignment, therefore these factors are taken into account by introducing the equivalent eccentricity of the compressive force “ ” into the calculation. It depends on flexibility and increases with its growth. In practical calculations they use, i.e. with random eccentricity. Then where - stability factor or it is also called the limiting bending coefficient under central compression.

The design standards provide formulas and corresponding tables for determining.

10) Work and stability calculations of eccentrically compressed and compressively bent rods.

When a rod is simultaneously subjected to an axial force and a bending moment (caused by an eccentric application of load), its load-bearing capacity is determined by the cross-sectional dimensions and the ultimate strength of the material.

In the elastic stage of the material's work, the stresses in the cross section of the rod can be represented as the sum of stresses from central compression and bending.

Fundamentals of stability calculations for eccentrically compressed and compressively bent rods.

The loss of bearing capacity of long flexible rods under the simultaneous action of compressive force and bending moment occurs due to loss of stability. In this case, the corresponding equilibrium state can be defined in the same way as for central compression, namely, a stable state; - unstable condition; - critical state (where and is the increment in the work of external and internal forces).

Eccentrically compressed rods of real metal structures lose stability when plastic deformations develop.

Critical force depends on eccentricity "e". In practice, it is more convenient to use dimensionless relative eccentricity m=e/ρ, Where ρ=W/A- core distance from the side of the most compressed fiber of the rod.

The formula for checking the stability of an eccentrically compressed rod will be

N / (Aφ e) R y γ c

To ensure the stability of eccentrically compressed (compressed-bent) rods, it is advisable to develop a cross section in the direction of eccentricity in order to save metal. For example, as shown in Fig. 2.6. In this case, the risk of loss of stability of the rod in the perpendicular direction increases – relative to the axis "y". In this regard, the formula for checking stability relative to the axis “ y” a reduced coefficient is introduced With.

N / cφ y A γ c R y

Where с =N cr .M/N cr =φ y .M/φ y ; φ y .N cr-respectively, the stability coefficient and the critical force under central compression; Ncr.M. φ y .M– critical force and corresponding stability coefficient of central compression relative to the axis "y" in the presence of a moment in the perpendicular plane. The coefficient “c” depends on the relative eccentricity m x =e/ρ x.rod cross-sectional shape and flexibility λy.

Fig.2.6. The most rational position of the I-section with eccentric compression of the rods

12) Work and calculation of butt welded joints.

When designing welded joints, it is necessary to take into account their heterogeneity, determined by the stress concentration, changes in the mechanical characteristics of the metal and the presence of a residual and stress-strain state.

Well-welded butt joints have a low stress concentration from external forces, so the strength of such joints in tension or compression depends on the strength characteristics of the base metal and the weld metal. The cutting of the edges of the elements being connected does not affect the static strength of the connection and may not be taken into account.

The weld seam at the beginning and end is saturated with defects (due to the unsteady thermal conditions of welding), therefore the beginning and end of the seam should be placed on technological strips; after welding is completed and the seam has cooled, these strips are removed. If it is impossible to bring the end sections of the seam onto the technological strips, the estimated length of the seam will be less than its actual length.

With the help of fillet welds, various types of connections are made in metal structures: T-joints, in a corner, overlapping.

Lap joints are made using fillet welds; they can be either flank or frontal.

In accordance with the nature of force transfer flank seams work simultaneously for shearing and bending. The fracture of the weld begins from the end and occurs both along the weld metal and the metal of the fusion boundary, especially if the deposited metal is stronger than the base metal.

Front seams transmit forces fairly evenly across the width of the element, but extremely unevenly across the thickness of the seam, due to the sharp curvature of the force flow when the force passes from one element to another, especially the stresses are high at the root of the seam. The destruction of frontal seams occurs in the same way as flank seams along one of the two sections ( weld metal or fusion metal).

Calculation:

When calculating welded joints, it is necessary to take into account the type of joint, welding method (automatic, semi-automatic, manual) and welding materials corresponding to the main material of the structure.

Calculation of butt welded joints under the action of an axial force passing through the center of gravity of the joint is performed according to the formula. From here

where is the smallest thickness of the elements being connected; - the estimated length of the seam, equal to its full length reduced by , or its full length if the ends of the seam are extended beyond the joint (for example, onto technological strips); - calculated resistance of butt welded joints according to the yield strength (see SNiP II-23-81*, appendix 5); - working condition coefficient.

In the absence of physical control methods, the calculated resistance of the metal of the welded joint according to the standards is .

Under shear force Q on the butt seam, shear stresses arise in the seam.

Design shear resistance of the connection, where is the design shear resistance of the base metal.

If the calculated resistance of the weld metal in a butt joint is less than the calculated resistance of the base metal, the test is performed along the cross-section of the weld metal.

Welded butt joints made without the use of physical quality control methods, with simultaneous action in the same section of the seam of normal stresses and acting in mutually perpendicular directions “X” and “Y” and tangential stresses, should be checked using the formula:

The calculated cross-sectional area of ​​the weld when fractured along the weld metal is equal to , upon destruction of the metal fusion boundary A wz = z k f l w

The calculated cross section for the metal of the fusion boundary is. In this case, the estimated seam length .

If , then the design cross-section is the cross-section of the weld metal and the stress

If , then checking the strength of the connection is carried out on the metal of the fusion boundary, then: ,

where is the force passing through the center of gravity of the connection; - the estimated length of the seam in the welded joint, equal to the total length of all its sections minus 1 cm; and - coefficients adopted according to Table 4.3 and taking into account metal penetration during welding.

14) Composite beams. Layout and selection of sections.

Composite section beams are used in cases where rolled beams do not satisfy the conditions of strength, rigidity, and general stability, i.e., for large spans and large bending moments, and also if they are more economical. The main types of sections of composite beams are shown in Fig. 4, c, d.

Rice. 5. Beam sections

a - rolled, b - pressed, c - welded, d - riveted and bolted

Composite beams are usually used welded. Welded beams are more economical than riveted ones. Their cross-section usually consists of three sheets: a vertical one - the wall and two horizontal ones - the shelves, which are welded at the factory using automatic welding. For beams under heavy moving loads (large crane beams), riveted beams are sometimes used, consisting of a vertical wall, belt corners and one to three horizontal sheets. Riveted beams are heavier than welded beams and more labor-intensive to manufacture, but their use is justified by favorable operation under high dynamic and vibration loads, as well as the relative ease of forming powerful chords.

To save material in composite beams, the sections along the length are changed in accordance with the diagram of bending moments. Elastoplastic work of the material in such beams is allowed with the same restrictions as for rolled beams.

The task of arranging the sections of composite beams is a variant, and the efficiency and manufacturability of the beams largely depend on its correct solution. The layout of the section must begin by determining the height of the beam, on which all other parameters of the beams depend.

13) Work and calculation of bolted connections.

Shear work is the main type of work of most joints, and it has its own characteristics in different joints.

In bolted connections with an uncontrolled tightening force of the nut of rough, normal and high precision, the force of tightening the package with bolts, and therefore the developing frictional forces between the connected elements under the action of shear forces on the connection, are uncertain and in most cases insufficient to fully perceive these shear forces. The operation of such a connection can be divided into four stages. At the 1st stage, until the friction forces between the connected elements are overcome, the bolts themselves do not experience shearing forces and work only in tension, the entire connection works elastically. This is how shear-resistant connections with high-strength bolts work. With an increase in the external shear force, the internal friction forces are overcome and the 2nd stage begins - the shift of the entire connection by the amount of the gap between the surface of the hole and the bolt rod. In the 3rd stage, the shear force is mainly transferred by the pressure of the hole surface to the bolt shaft; the bolt shaft and the edges of the hole are gradually crushed; the bolt bends and stretches, since the head and nut prevent the free bending of the rod. Gradually, the density of the connection is degraded, the friction forces are reduced and the connection goes into the 4th stage of work, characterized by its elastoplastic work. The destruction of the connection occurs from the shearing of the bolt, crushing and puncturing of one of the elements being connected, or tearing off the bolt head.

This work is greatly complicated by the irregular shape of the bolt and the wall of the hole, so the calculation of the connection is conditional.

A distinction is also made between the operation of single-bolt and multi-bolt connections. In a multi-bolt connection, the same irregularities in the shape of the bolt and hole, as well as possible gaps between the bolt and the hole, inevitably lead to uneven operation of the individual bolts of the connection, which is taken into account by appropriately assigning the connection operating conditions coefficient.

The calculation is carried out based on the possible type of failure of the connection by the shear of the bolt with thick sheets being joined or by the collapse of the surface of the hole with thin sheets:

a) calculated shear force perceived by one bolt:

(6.1)

The number of bolts n in a connection under the action of a shear force N applied to the center of gravity of the connection is determined assuming the operation of all bolts is the same

Strength calculations of the connected elements themselves are carried out taking into account the weakening of the section by holes over the net area Lit, but with the assumption of elastoplastic work of the material of the connected elements, taken into account by the operating conditions coefficient. It is accepted: for solid beams, columns and butt plates 1.1, for rod structures of roofs and floors 1.05 and is taken into account simultaneously with the coefficient of operating conditions of the entire structure;

c) in connections on high-strength bolts with a controlled tension force of the bolt (shear-resistant, frictional), the tightening forces of the connected elements with bolts are so great that under the action of shear forces, the friction forces arising in the connection completely perceive these shear forces and the entire connection works elastically.

The tension force of the bolt (equal to the design tensile force of the bolt) and the quality of the friction surfaces are of decisive importance in the operation of such a connection. The design shear force that can be absorbed in the connection of elements fastened with one high-strength bolt can be determined by the formula:

Similar to formula (6.2), the number n of bolts in a connection required to transmit shear force is found by considering the force distribution between the bolts to be uniform:

We are talking today with the deputy director of one of the leading domestic enterprises - manufacturers of instrument transformers, Electroshield-Co LLC, Viktor Vladimirovich Legostov.

– Viktor Vladimirovich, Electroshield-Co LLC this year celebrates 10 years since the commissioning of the first transformer of its own production. Due to what aspects did you manage to become one of the industry leaders in such a relatively short period of time?

– In short, this is a properly constructed production system, the most important property of which is the exact fulfillment of the customer’s technical requirements.

Using European technology and equipment, as well as high-quality imported materials, we create custom-made products that have no analogues in our country.

– Why foreign technologies, imported materials and equipment? Don't want to support a Russian manufacturer?

– Our production has two key specializations: production of transformers according to technical specifications specified by the customer; production of transformers for systems with increased safety requirements.

Unfortunately, today the equipment and materials necessary for production of this level are not produced in Russia. At the same time, we constantly work with domestic suppliers, trying to stimulate improvement in the quality of their products. I am convinced that this is the best support from the manufacturer.

– Viktor Vladimirovich, tell us what features distinguish your transformers from their analogues.

– The use of our technology and imported materials allows us to saturate a small device to the maximum, unlike analogues from other manufacturers who, using Russian materials, can invest much less in the same size.

Now many manufacturers have learned to make transformers with high accuracy classes, but it is often not possible to create a device with a set of specific non-standard parameters required by the customer. Some Russian manufacturers themselves order complex transformers from us.

The transformer calculation program we use allows us to calculate any transformer within 10–15 minutes. By changing and substituting various options for parameters, we obtain a physical model of the transformer. All realistically calculated options can be manufactured. Most manufacturers produce devices on a conveyor belt and, having made measurements at the output, record the parameters, send the device to the warehouse and then, when such a request appears, offer it to the customer. We initially proceed from the request and make the device that was ordered.

Moreover, today we are the only ones in Russia who complete orders with magnetic cores with identical magnetization parameters, which allows us to produce transformers with identical electrical characteristics.
In addition, we were the first in Russia to conduct insulation tests according to class “A” with measuring the level of partial discharges.

The use of our transformers at nuclear power facilities, such as Novovoronezh NPP, Kalinin NPP, Beloyarsk NPP, Kola NPP, confirms the high level of reliability and safety.

– Is it relevant today to produce transformers according to specified operational parameters?

– Savings are always relevant. The use of transformers with parameters that do not meet the actual requirements of metering and protection systems leads to more significant financial losses due to an increase in the current error and the transformers falling outside the declared accuracy class.

The need for automation and separation of metering and measurement circuits has given rise to new developments, the main principles of which are small dimensions, an increased number of windings, information protection, manufacturability, reliability, and multivariate characteristics.

In this regard, Electroshield-Co LLC is a trendsetter in the development of transformer manufacturing in Russia.

For the first time in Russia, we began to mass-produce current transformers with accuracy classes 0.2S and 0.5S in combination with high loads, with specified specific values ​​of device safety factors and maximum multiplicity, with high thermal resistance current at low rated currents, transformers with different coefficients transformation of measuring and protective circuits, switching of primary currents to reduce or increase the transformation ratio.

– How is the high accuracy class achieved in your transformers?

– For transformers with a high accuracy class we use permalloy cores. This material makes it possible to provide a specified accuracy class; its physical properties make it possible to convert the signal with minimal losses. Few people use permalloy; it is quite difficult to use and is not produced in Russia. It is easier to use amorphous alloys, but they do not have mechanical strength; a core made of such material must be placed in a special box, which increases the size of the transformer.

– Transformers with switching. Tell us in what cases they are needed.

– These are dual-use transformers. The first area of ​​their application is when production is built on old facilities. For example, previously all settings were made at 600 A for the primary current, but in reality the circuits are already 250-300 A.

A switching transformer is a device that can operate as both 300/ 5 and 600/ 5. One transformer can provide metering and protection to both lower and higher levels, with the possibility of increasing network capacity in the future.

The second area of ​​application is when it is necessary to preserve the old system of technical metering and relay protection, and make commercial metering at lower capacities. To solve this problem, it is possible to use a transformer with different transformation ratios, i.e. for commercial metering, the winding will be 300/ 5, and the protective winding and technical metering will be 600/ 5. All this can be done in one housing. In this case, the secondary winding for commercial metering is designed for a long operating time at 600 A.

– Is the transformation ratio maintained strictly at 1:2?

– The proportions can be different, for example, 500 A for 600 A, 600 A for 1000 A, 1500 A, 600 A for 800 A. It also happens 1:3, but this is difficult to implement. It is always necessary to consider specific tasks and calculate any device individually.

– How to correctly set secondary loads?

– This is a very important point. The program we use allows us to calculate loads with an error as close as possible to the zero mark in the corridor of current angular errors.

For example, consider the dependence of the absolute error of a current transformer with a transformation ratio of 100 / 5, accuracy class 0.5, with a rated load of 10 VA (Fig. 1). From this dependence it is clear that a decrease or increase in the applied load on the current transformer leads to a significant increase in the absolute value measurement errors. The graph shows possible options for leaving the class due to underload or overload, if in reality the transformer was designed for 10 VA.

– What is a safety factor and is it necessary to set it?

– This is a coefficient that shows how many times the secondary current on the measuring winding will increase if the primary circuit current increases sharply. The measuring winding is constructed in such a way that when a short circuit occurs, the core quickly becomes saturated and the current in it stops growing. For example, the secondary current is 5 A, and the coefficient is 10, then the maximum possible current that will arise in the secondary winding will be 50 A.

The graph (see Fig. 2) shows the difference in the safety factor of devices when using different grades of electrical steel. The graph shows that even for the TLO-10 transformer, when the load on the measuring winding decreases, the safety factor of the devices increases sharply and can no longer provide protection for the measuring devices at the moment of a short circuit in the primary circuit. When designing an metering and protection system, it is necessary to take into account the actual secondary load in the secondary circuit of the measuring winding and the safety factor of the devices, which must be indicated in the accompanying documentation for a specific transformer. In metering circuits already in operation, these parameters can be measured with sufficient accuracy and the system can be brought into compliance.

Using transformers with a correctly selected device safety factor in existing networks, there is no need to apply additional protection measures for old-style meters.

– What is the range of the safety factor of devices and what does it depend on? If the customer specifies a specific coefficient, is it possible to make it?

– There is no range of the coefficient; it is always a finite number and depends almost exclusively on the materials used, their quality and characteristics, manufacturing technology, and the customer can choose the safety coefficient at his own discretion.

– Tell us about another important parameter - the coefficient of the nominal maximum multiplicity of the protection windings. How important is it to specify this when ordering a transformer?

– Very often consumers or design organizations request a limit factor curve. One of the main parameters that is entered in the device passport is the magnetizing voltage, the point at which the curved section turns into a linear one. No matter how much the current grows in the primary winding, the current stops growing in the secondary winding. If we consider the safety factor of devices and the maximum multiplicity, the physical essence of these parameters is the same.

The limiting factor indicates to what value the current will increase during a short circuit in the primary winding, to what limit we must power the relay protection for it to work. The limiting factor is 10, which means that in the event of a short circuit in the primary circuit, the current in the secondary winding will be up to 50 A, no more. If, suppose, the relay protection is designed to operate at a current of 75 A, then a factor of 10 will not be enough, i.e. the protection “will not see” a short circuit, so the customer sets the maximum factor, for example 15, but this is the limit value, and it is necessary to take 16 so that the relay protection reacts and turns off all devices before the core begins to saturate.

The limiting factor curve is necessary to calculate the operation of the automation when using a standard device. At our enterprise, the consumer can order a transformer with any ratio for the required load.

– Viktor Vladimirovich, foreign specialists work at Electroshield-Co LLC. What functions do they perform?

– Foreign specialists work at the enterprise in the field of ensuring product quality and developing new products. In addition, they are consultants on process improvement, production ergonomics, and planning of new production facilities. Without false modesty, I would like to point out that the production process at Electroshield-Co LLC is no worse and even better than some foreign similar industries. When developing our production, we reviewed and took into account the mistakes of other manufacturers.

– What exactly was this negative experience embodied in?

– No production in the world has three-stage metrological control along the entire technological chain.

A system of route cards for each device, control of previous technological operations by subsequent ones, motivation of personnel in the field of control and quality assurance make it possible to completely eliminate the production of defective devices. The percentage of production defects today does not rise above 0.1 percent.

– Viktor Vladimirovich, today you talked about the intricacies of the correct selection of current transformer parameters. If you are interested in clarification, can your specialists provide local advice on these issues?

– Our company’s technical center conducts seminars for specialists from design and operational organizations on the following topics:
optimal selection of instrument transformer parameters, maximally adapted to specific metering systems;
combination of relay protection and automation systems with technical accounting;
calculation and production of relay windings with the required maximum multiplicity.

In the near future, we invite everyone to the exhibition “Energy and Electrical Engineering” in St. Petersburg on May 22-25 (Lenexpo exhibition complex, pavilion 7, stand No. F24) and to the exhibition “Electro-2012” in Moscow on June 13-16.

– Thank you for such detailed and interesting information. We hope that many technical specialists will be interested in the data provided. We look forward to new publications from you.