Cable temperature under load. And lived heating cables

Wires and cables, being conductors, are heated by the load current. The value of the permissible heating temperature for insulated conductors is determined by the characteristics of the insulation, for bare (bare) wires - by the reliability of the contact connections. The values ​​of the long-term permissible heating temperature of wires and cable cores at an ambient temperature of + 25ºС and an earth or water temperature of + 15ºС are indicated in the electrical installation rules (PUE).

The amount of current corresponding to the long-term allowable temperature of a given wire or cable core is called the long-term allowable load current ( I additional). The values ​​of long-term permissible current for various cross-sections of wires and cable cores, as well as various conditions for their laying, are given in the PUE and reference literature. Thus, determining the cross section of wires and cable cores by heating is reduced to comparing the maximum operating current of the line with the tabular value of the long-term permissible load current:

according to which the corresponding standard section of wires and cable cores is selected from the tables. If the ambient temperature differs from the tabular values, then the value of the long-term permissible current is corrected by multiplying by the correction factor, the values ​​​​of which are taken according to the PUE and reference literature.

The section of wires and cable cores selected according to the heating condition must be consistent with the protection, so that when a current flows through the conductor that heats it above the permissible temperature, the conductor is disconnected by a protective device (fuse, circuit breaker, etc.).

Calculation and selection of cross-sections of wires and cable cores is performed in the following sequence:

1) the type of protective device is selected - a fuse or a circuit breaker;

2) if a fuse is selected, then the rated current of its fuse is determined, which must satisfy two conditions:

where is the maximum load current when starting an asynchronous squirrel-cage motor (its starting current);

Coefficient characterizing the operating conditions of the engine; for normal operating conditions = 2.5; for severe conditions = 1.6 ... 2.0.

According to the larger calculated value of the rated current of the fuse-link, the standard value of the rated current of the fuse-link is selected;

3) the long-term permissible load current is determined, corresponding to the selected rated current of the fuse fuse:

For paper insulated cables,

For all other cables and wires;

these ratios are taken for the case when the network wires are protected from overloads. According to the PUE, such networks include lighting networks in residential and public buildings, commercial and service premises of industrial enterprises, as well as in fire and explosion hazardous areas; for cases in which it is necessary to protect the wires only from short circuits, the ratio is selected:

The calculated value of the long-term allowable load current obtained is rounded up to the nearest tabular value of the long-term allowable load current and the corresponding standard cross-section of wires or cable cores;

4) if a circuit breaker is selected as a protective device and it protects the network wires from overloads, then all the above ratios are valid, in which instead of the rated current of the fuse link, the rated current of the circuit breaker release must be indicated;

The maximum permissible heating temperature of the cable is of great importance, since the load capacity, service life and reliability of the cable depend on it.

Each type of cable insulation is designed for a certain long-term permissible temperature, at which the aging of the insulation is slow. Exceeding the heating temperature of the cable above the allowable speeds up the aging process of the insulation and reduces the service life of the cable.

When the cable is heated, paper insulation undergoes the most rapid aging, the mechanical strength and elasticity of which decrease. Long-term permissible temperatures for power cables of a stationary laying are given in Table. 17.

Table 17
Long-term permissible heating temperature of cable cores

When the cable is turned on under load, its cores are first heated, and then the insulation and sheath. Experimental measurements have established that the temperature difference between the core and sheath of a 6 kV cable is approximately 15 ° C, and for 10 kV cables - 20 ° C. Therefore, in practical conditions, they are usually limited to measuring the temperature of the sheath, given that the temperature of the cable core is 15-20 °C higher.

The heating temperature of the conductors can also be determined by calculation using the formula

where t о6 is the temperature on the cable sheath, °С; I - long-term maximum cable load, A; n is the number of cable cores; ρ - specific resistance of copper or aluminum at a temperature close to the core temperature, Ohm.mm 2 /m; S K - the sum of thermal resistances of insulation and protective covers of the cable, Ohm (determined from the reference book); q - cross-section of the cable core, mm 2.

Control over the heating of cables during operation is carried out by measuring the temperature of the lead or aluminum sheath, or armor in those places of the cable route where, presumably, the cable line can overheat against permissible temperatures. Such places can be gaskets near heat pipelines, in an environment with high thermal resistance (slag, pipes, etc.), where unfavorable conditions are created for cooling the cable line.

It is recommended to measure the temperature on the surface of cables laid in the ground with thermocouples. To install thermocouples on the cable route, a pit measuring 900x900 mm is torn off with a recess of 150-200 mm in one of the walls of the pit along the cable axis. After removing the outer cover, cleaning the armor from corrosion, a reliable contact is created (with low-melting solder or foil) with the thermocouple wire.

Rice. 113. Temperature measurement on the surface of a working cable:
1 - cable, 2 - building, 3 - thermocouple shields, 4 - metal pipe, 5 - heat pipe

Measuring wires are taken out through a gas pipe and connected to special boxes, after which the pit is covered with earth. The scheme of temperature measurement on the cable surface is shown in fig. 113. Temperature measurement on the surface of controlled cables with simultaneous measurement of current loads is carried out within a day after 2-3 hours. to improve cooling conditions. In some cases, it is advisable to replace the overheated section of the line with a large-section cable. Measurement of the temperature of cables laid openly in cable structures can be carried out with a conventional laboratory thermometer, fixing it on the cable sheaths. It is necessary to carefully monitor the ambient temperature and the operation of ventilation in cable structures. Cable heating is monitored as needed.

When choosing a cable, a lot of various parameters are taken into account, ranging from the cross section of the cores to the insulation material. Why is it important to know details such as shell material? After all, its main function is to protect against electric shock. If the insulation is up to the task, then more attention needs to be paid to the more important characteristics of the cable. Unfortunately, many make this mistake, in fact, the permissible heating temperature of the cable and the insulation material are unusually related. Each type of protective sheath is designed for a certain temperature, if it exceeds certain values, then the aging process of the insulation is accelerated. This seriously affects the life of the cable, and not rarely the equipment connected with it. The permissible heating temperature of the cable is the parameter on which not only the load capacity of the cable depends, but also the reliability of its operation. Permissible heating temperature of a cable with different types of insulation All types of materials used as insulation for conductive conductors have their own physical characteristics. They have different density, heat capacity, thermal conductivity. As a result, this affects their ability to withstand heat, so vulcanizing polyethylene can maintain its performance characteristics up to 90 ° C. On the other hand, rubber insulation is able to withstand a significantly lower temperature load - only 65ºС. The permissible temperature for heating a cable with PVC is 70 degrees and this is one of the most optimal indicators. One of the most important indicators is the permissible heating temperature of the cable c. This type of cable is used extremely widely and is designed to work with different voltages. That is why you should be careful in this characteristic, it changes as follows:

• for a voltage of 1-2 kV, the maximum allowable temperature for cables with lean and viscous impregnation is 80ºС;
• for a voltage of 6 kV, insulation with viscous impregnation withstands 65ºС, with depleted impregnation 75ºС;
• for a voltage of 10 kV, the allowable temperature is 60ºС;
• for a voltage of 20 kV, the allowable temperature is 55ºС;
• for a voltage of 35 kV, the allowable temperature is 50ºС.

All this requires increased attention to the long-term maximum load of the cable, operating conditions. Another of the insulation materials in demand today in the electrical industry is cross-linked polyethylene. It has a complex structure that provides unique performance characteristics. The permissible heating temperature of the cable and XLPE insulation is 70ºС. One of the leaders in this parameter is silicone rubber, which can withstand 180ºС. What can cable overheating lead to Exceeding the permissible heating temperature of the cable leads to the fact that the properties of the insulation change dramatically. It begins to crack, crumble, resulting in a risk of a short circuit. The service life of the cable with each exceeded degree is seriously reduced. This requires more frequent repairs, costs, so it is better to initially use the cable that is designed to solve certain problems. But even this is not enough, it is necessary to regularly monitor the temperature of the shell, especially in those places where overheating can be assumed. These may be places near heat pipes or create unfavorable conditions for cooling.

To select a heating cable, you need to understand what technical characteristics you need to pay attention to, as well as understand what your heating needs are. This article will discuss the main characteristics of heating cables for the needs of heating water pipes.

Heating cable power

The first characteristic that you need to pay attention to is the power of the heating cable. It is measured in watts per linear meter and, depending on the models, can be from 5 to 150 W / m. The greater the power, the greater the consumption of electricity and the greater the heat output.

Low power cables are used to heat the water supply - from 5 to 25 W / m, depending on how the heating cable is installed and where the water supply passes, you can focus on the following power:

• the water supply is laid in the ground, the cable inside the pipe is enough 5 W / m
• the water supply is laid in the ground, the cable is outside the pipe - power from 10 W / m
• water supply is laid through the air - from 20 W / m

The pipe and the heating cable in all cases must be insulated with a layer of insulation of at least 3-5 mm.

In the case of a resistive heating cable, the power remains constant throughout its entire length and regardless of the temperature of the pipe, but the self-regulating cable reduces the power consumption and its temperature if the pipe is already heated. This saves a significant amount of electricity, and the greater the working power of the self-regulating cable, the greater the savings.

The dependence of heating power on temperature is shown in the graph.

The graph shows power versus temperature for five different self-regulating cables with different power ratings from 15 W/m to 45 W/m. The greatest efficiency from the use of such cables is obtained when used in conditions of an extended water supply system, which runs in very different temperature conditions. The greater the temperature difference, the greater the savings.

However, when heating a small section of the water supply, it is not so noticeable. If water is supplied from a well, then its temperature, regardless of the time of year, ranges from 2 to 6 degrees, and the task of the heating cable is simply to prevent it from freezing, that is, to maintain it at a level of about +5 degrees Celsius. This means that the heating cable will work in the temperature range from 0 to 5 degrees, while the difference in power is only a few watts (from 2 W for a low-power cable, up to 5 W for a 45-watt cable).

Heating cable temperature

The second important characteristic is the operating temperature. According to this indicator, all heating cables are divided into three categories:

1. Low temperature with operating temperature up to 65 degrees
2. Medium temperature - 120 degrees
3. High temperature - up to 240 degrees

Only low-temperature cables are used for heating the water supply, moreover, they never work at temperatures even close to their maximum 65 degrees.

Application area

According to the field of application, cables are divided into two types:

1. Food - only it can be used for installation inside a pipe when heating a water supply system, which is used for domestic needs, supplying drinking water.
2. Technical - it is used for mounting outside the pipe in any case, it can be mounted inside the pipe only when water is not used for food (for example, in irrigation, washing or heating systems).

Read also:

• Heating cables are used for heating plumbing, roofing, cornices and other elements where water freezing in winter is undesirable. The simplest option is resistive heating cables, they are single-core and two-core.
• Self-regulating heating cables are used to heat plumbing in places where it is laid above the freezing level of the soil - for example, at the point where the pipeline enters the house. A self-regulating cable has the ability to independently change the intensity of heating in different areas depending on the need: the lower the temperature of the heated object, the more the cable heats up.
• The self-regulating heating cable can be installed in different ways: inside the pipe and outside, placed along the pipe or in a spiral.
• The thermostat is an electrical circuit switching device that is used to turn on and off heating devices such as radiators, heating cables in a floor heating system or in anti-icing systems. In principle, the connection diagram is the same for all thermostats.

A correctly calculated and properly executed electrical network does not guarantee the exclusion of the possibility of emergency situations leading to unacceptable overheating of electrical circuits in the event of a short circuit.

For example, a similar situation, as noted in the work, occurs when the load is connected to the outlet network through an extension cord. Starting from a certain length of the extension wire added to the group line, the resistance of the phase-zero circuit increases to a value at which the short-circuit current will be less than the operating threshold of the electromagnetic release of the circuit breaker. Therefore, when electrical installations, it is desirable to take into account the possibility of abnormal operating conditions of electrical wiring.

In accordance with the "Limiting temperatures of electric cables for a rated voltage of 1 kV under short circuit conditions", the temperature of the cable cores (up to 300 mm 2 inclusive) with PVC insulation during a short circuit should not exceed 160 degrees. Achieving this temperature is allowed with a short circuit duration of up to 5 seconds. With such a short circuit duration, the cable insulation does not have time to heat up to the same temperature. For longer short circuits, the maximum heating temperature of the cores must be reduced.

Let us consider the occurrence of a similar situation using the example of using the automatic switch of group "C". Time - the circuit breaker current characteristic is shown in Fig. 1. In the given characteristics, zone "a" - a thermal release and zone "b" - an electromagnetic release are distinguished. The graph shows two curves 1 and 2 of the circuit breaker operation time versus current, which show the limits of the technological spread of the circuit breaker parameters during its manufacture. For circuit breakers of group "C" within the technological spread, the ratio of the operating current of the electromagnetic release to the rated current of the thermal release is in the range from 5 to 10. We are only interested in curve 2 for alternating current (AC), showing the maximum time of operation of the switch. As can be seen from the graph in Fig. 1, with a slight decrease in the short-circuit current below the operating threshold of the electromagnetic release, the operating time of the circuit breaker is determined by the thermal release and reaches a value of the order 6 seconds.

Rice. 1 Time - current characteristic of automata of group C.

Let's try to find out what happens to the cables during the period of time during which the thermal release trips. To do this, it is necessary to calculate the dependence of the temperature of the cable cores on the time of passage through them of currents close to the threshold of operation of the electromagnetic release.

Table 1 gives the calculated values ​​​​of the temperatures of the cable cores depending on the duration of the short circuit (at different currents) for a cable with copper cores with a cross section of 1.5 sq. mm. The cable of this section is widely used in lighting residential and public buildings.

To calculate the temperatures of cable cores, the calculation method from "Calculation of thermally permissible short-circuit currents taking into account non-adiabatic heating" was used.

The temperature of the cable cores is determined by the formula:

Θ f = (Θ i +β)∙exp(I AD 2 ∙t/K 2 ∙S 2) - β (1)

where, Θ f is the final temperature of the cable cores about C;

Θ i - initial temperature of the cable cores about C;

β is the reciprocal of the temperature coefficient of resistance at 0 °C, K, for copper β=234.5;

K is a constant depending on the material of the conductive element, A s 1/2 /mm 2, for copper K=226;

t - short circuit duration, s;

S - cross-sectional area of ​​the conductive core, mm 2;

I SC - known maximum short circuit current (rms value), A;

I AD =I SC /ε - short-circuit current, determined on the basis of adiabatic heating (rms value), A;

ε - coefficient taking into account heat removal to neighboring elements;

X, Y - constants used in the simplified formula for cores and wire screens, (mm 2 / s) 1/2; mm 2 /s, for cables with copper conductors and PVC insulation X=0.29 and Y=0.06;

The calculations are made for the temperature of the cable before the short circuit is 55 degrees. This temperature corresponds to the operating current passing through the cable before the occurrence of a short circuit of the order of 0.5 - 0.7 of the maximum allowable long-term current at an ambient temperature of 30 - 35 degrees. Depending on the expected operating conditions of the electrical installation, the temperature of the cable cores before a short circuit can be changed when designing the electrical network.

Table 1

From Table 1 it can be seen that the maximum short-circuit current (if the electromagnetic release does not operate), which does not cause heating of the conductors above 160 degrees in 6 seconds, is approximately 100 A. That is, a cable with a cross section of 1.5 mm 2 can be protected by an automatic switch of the group " C" with a rated current of not more than 10A.

In the manufacture of cables, the cross section of the cores is often underestimated. Understating the cross section by 10% is a common occurrence. In the markets it is not difficult to find cables with a large underestimation of the cross section.

Table 2 gives the calculated values ​​of the temperatures of the cable cores when the cross section is underestimated by 10%. As can be seen from the table, the C10 circuit breaker does not protect such a cable with 100 percent reliability.

For the most critical facilities, especially those with building structures made of combustible materials, it is advisable to choose a circuit breaker when designing an electrical installation according to Table 3, in which the cross-sections of the conductors are given with a 20% understatement. The protection of such cables will be provided by the automatic switch C6 or B10, in which the ratio of the operating current of the electromagnetic release to the rated operating current of the thermal release is in the range from 3 to 5. This will significantly increase the reliability of the electrical wiring.

table 2

Table 3