JPH0371514A - In-conduit power cable - Google Patents

Abstract

PURPOSE:To drastically facilitate snake formation during thermal expansion of a power cable without winding a linear material by selecting an optimal winding pitch univocally determined by the height of a spiralled recess formed on the circumference of the power cable, flexural rigidity of the power cable, and by the weight of the cable. CONSTITUTION:A value of pitch L univocally determined by the height (h) of a spiralled recess formed on the circumference of a power cable main body, flexural rigidity BI of the power cable, and by the weight (w) of the cable so as to minimize the snake axial tension F of the power cable determined by an expression (in the equation, A is a conductor cross section of the power cable, E is a Young's ratio of the power cable, beta is coefficient of thermal expansion of the power cable, and T is the change in temperature of the conductor of the power cable).

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electrical cable installed in a conduit, and more particularly to a power cable that absorbs thermal expansion and contraction within a conduit by its own snake.

[Conventional technology]

In recent years, power cables have been drawn in and laid using existing conduits, but in this case, there is often insufficient space for manholes, and restrictions are placed on the off-top dimensions that can be set there. be. Therefore, thermal expansion and contraction of the offset portion in the manhole is suppressed and the power cable is snaked within the conduit to absorb the thermal expansion and contraction of the power cable itself.

As a means of giving a snake shape to the power cable itself in a conduit, generally a linear body is wound spirally on the same upper part of the power cable body at a predetermined pitch, and when the power cable undergoes thermal expansion and contraction, the linear body is A well-known method is to help the wound caple form its own snake state by winding it.

Fig. 3 shows an outline of such a power cable installed in a conduit, in which a power cable C takes a snake shape in a conduit P between manholes M H+ and M Ht. The terminal of power cable C is
The manhole protruding from the pipe mouth MH,,MH! The terminals are connected to the junction boxes J, . . . , L in an offset shape.

The offset portions of the power cable C inside the manholes M H+ and M H are provided with a thermal expansion and contraction device to suppress movement of the power cable C due to thermal expansion and contraction.

S is a linear body spirally wound around the power cable C in the conduit P at a predetermined pitch, and its existence causes a snake to occur when the power cable C is expanded and contracted due to heat. It's Toshide.

By the way, if such a linear body S is simply wound around the power cable C, it is difficult for the power cable C to form a snake shape, and for this reason, various modifications have been made to the linear body S and the like.

For example, as disclosed in Japanese Patent Publication No. 39-20972, weights are connected to both ends of a linear body wrapped around a power cable, and tension is applied to the linear body to generate snakes when the cable thermally expands and contracts. How to make it easier to cause
As disclosed in Japanese Patent No. 27641, a method has been proposed in which a wire-shaped body that is thickened in some places in the longitudinal direction is spirally wound around a power cable, and the cable is supported by the thickened portions of the wire-shaped body.

[Problem to be solved by the invention]

According to the method disclosed in Japanese Patent Publication No. 39-20972, ■ A tensioned linear body is attached to the surface of a power cable (P
It may bite into the outer covering (such as the VC anticorrosion layer) and cause damage to the surface of the cable, leading to water intrusion into the cable, which may significantly shorten the life of the cable.

■ In order to apply tension to the linear body, a weight connected to the end of the linear body is placed inside the manhole, but it is difficult to secure such an installation space in the narrow manhole due to lack of space. and is not a reality expert.

According to the method disclosed in Japanese Patent Publication No. 57-27641, ■ it is difficult to manufacture linear bodies that are thick at regular intervals;
This method requires special manufacturing equipment and is therefore not viable to provide as an economical product.

■ Wrapping a linear body around a power cable increases the outer diameter of the power cable including the filamentous body, and when installing using an existing empty conduit, it can be applied depending on the diameter of the conduit. In some cases, it may disappear, and its range of use is limited.

Furthermore, in any of the above methods, the linear body is wound around the circumference of the power cable main body, so even if the linear body is wound around the power cable main body at a determined pitch, When such a power cable is drawn into a conduit and laid, there is a risk that the linear body will rub strongly against the inner surface of the conduit and the pitch of the winding will be disordered, causing regular snakes of the power cable within the conduit. There were times when I couldn't expect any transformation. In such a case, thermal expansion and contraction of the power cable may not be sufficiently absorbed or bending stress may be applied to local parts of the power cable, impairing the long-term reliability of the power cable.

SUMMARY OF THE INVENTION In view of the problems of the prior art described in Salt 1, the present invention provides an electric cable installed in a conduit that can extremely easily form a snake during thermal expansion and contraction of the electric power cable without wrapping a linear body around it. has a purpose.

[Means to solve the problem]

According to the present invention, the power cable has a spiral convex portion that is integrally formed so as to wrap around the same upper part of the power cable body at a predetermined pitch, and the height of the six portions is The objective is to select the optimum pitch L that is uniquely determined by h, the bending rigidity El of the power cable, and the weight i1w of the cable.

More specifically, regarding the linear body, the winding pitch L is uniquely determined by the height h of the linear body, the bending rigidity El of the power cable, and the weight W of the cable. The purpose is to select a value that minimizes the snake axial force F of the power cable, which is determined by the following formula.

βLT 3 lk5 EI Feg2a' 3 1 EA α+-tan-' 4 (a --) In the above formula, A is the conductor cross-sectional area of the power cable, E is the Young's modulus of the power cable, and β is the wire of the power cable. expansion coefficient, T
is the change in temperature of the conductor of the power cable.

In a preferred embodiment, the spiral convex portion is made of the same material as the plastic sheath forming the outer sheath of the power cable body, and is extruded at the same time as the sheath is extruded. However, a plastic material different from the plastic sheath may be extruded in the form of a string onto the same top of the plastic sheath, and then spirally swirled and integrated.

It should be noted that the in-pipe electrical cable according to the present invention can be effectively employed when the pipe inner diameter D' falls within the following range.

D'<2.160+α (D: outside diameter of the ii cable pull, α: gap between the electrical cable pull and the conduit; margin (usually 20 to 50 degrees), that is, the inner diameter of the conduit D
If ' is greater than or equal to (2,16D+α), a triplex type cable can be formed, and it is not necessary to wrap a linear body around a single-core power cable as in the present invention.

〔Example〕

A detailed explanation will be given below based on FIGS. 1 and 2.

FIG. 1 shows a power cable 3 in which spiral convex portions 1 are integrally formed on the outer periphery of a power cable main body 2 at a predetermined pitch.
As is clear from Figure 0, which shows the state in which the power cable is drawn into the conduit 4 in a straight line and laid, the power cable main body 2 immediately after being drawn in and laid is in a straight state,
The lower part of the protrusion 1 that protrudes on the lower surface of the power cable main body 2 at a constant pitch L contacts the inner bottom surface of the conduit 3, and the lower surface of the sheath of the power cable main body 2 is separated from the inner bottom surface of the conduit between them. ·are doing.

This initial state remains the same regardless of the value of the pitch ■ of the convex portions 1, but when the pitch L of the convex portions 1 exceeds the value, there is a gap between the lower raised portions of the convex portions 1. The straight part of the power cable main body 2 that is supported bends downward in an arch shape over time due to its own weight, and eventually the second
It will bend to the snake laying state as shown in the figure. In other words, the power cable main body 2 is given a snake shape depending on the pitch L of the convex portions l and its own h and d.

In such a state, when the power cable main body 2 is about to undergo thermal expansion and contraction due to energization, the power cable main body 2
However, the initial snake shape obtained as described above is changed within the conduit 3 and undergoes expansion and contraction deformation.

However, if the pitch L of the convex portions 1 is too large,
Since the initial snake shape of the power cable main body 2 itself approaches a straight state, it becomes difficult to transition to snake deformation after installation due to the initial snake shape, and the axial force during thermal expansion and contraction in the power cable main body 2 becomes relatively large. The stress burden on the thermal expansion/contraction suppression device (see Figure 3) attached to the offset portion of the end of the power cable body 2 within the manhole becomes significant, and there is even a possibility that the expansion/contraction suppression effect of the thermal expansion/contraction suppression device will be lost. Ta.

From the above, in the present invention, the pitch L of the convex portion 1 is
and its own height h, the optimum values are selected according to the bending rigidity E and the weight W of the power cable main body 2 itself.

Hereinafter, a method for determining the height h of the convex portion 1 and the pitch L with respect to the power cable body 2 will be described.

■ How to determine the size d of the convex part 1 The higher the convex part 1 is, the better, but the condition is that it can be drawn into the conduit 3 of a predetermined size, so the outer diameter of the power cable body 1 and the conduit 3 In relation to the inner diameter D' of
1) Select an outer diameter that satisfies the conditions of the formula.

p+2h+α<D' (1) (α is 6 parts i
The remaining gap between the outer diameter of the power cable including the conduit 3,
In anticipation of snake deformation after installation, usually 20~50++
Let it be n. ) ■ How to determine the pitch L of the convex portion l Once the convex portion 1 with the height h selected by the above formula (11) is selected, the pitch L of the power cable body 2 determined by the following formula (2) is determined accordingly. The pitch L may be determined so that the snake axial force F is minimized.

βLT a’　Lk.

1 EA αTen - 4 (a --) (In the above formula, A is the area of one rest area of the power cable body, E is the Young's modulus of the cable body, β is the coefficient of linear expansion of the cable, and T is the cable ) [Function/Effect] As is clear from the above explanation, according to the in-duct cable of the present invention, the power cable is integrally wrapped around the outer periphery of the power cable body. The power cable has a spiral convex portion formed in the convex portion, and the pitch L and the height h of the convex portion are selected to be optimum values according to the bending rigidity El and the weight W of the power cable body itself. As a result, the straight part of the power cable main body, which is supported on the inner bottom surface of the conduit by the lower raised part of the convex part, bends downward in an arch shape over time due to its own weight, eventually leading to snake installation. The power cable body is inevitably given a snake shape depending on the pitch L of the protrusions and its own height h.

For these reasons, ■ Even if the inner diameter of the conduit is not sufficient, the height of the protrusion should be selected within a range that allows the cable to be drawn in, and the pitch should be adjusted so that the snake axial force of the power cable is minimized. By calculating it, it becomes possible to easily realize the occurrence of snakes in the power cable conduit.

■ Even if the axial force associated with thermal expansion and contraction is large, such as in the case of large single-core cable cables, the initial snake is reliably applied, so thermal expansion and contraction after installation can be carried out smoothly and reliably. can be set.

■ If expansion and contraction is forcibly suppressed by a thermal expansion/contraction R control device at the offset section, local buckling will occur in the power cable extending inside the conduit, and conventional wire wrapping However, according to the present invention, a spiral convex portion integrally formed on the cable is provided with a specific pinch. Therefore, large lateral pressure or distortion does not occur locally due to the convex part around which the cable body is wrapped, and the bending deformation caused by thermal expansion and contraction is dispersed over the entire length of the power cable in the conduit by integrally forming the convex part. This makes it ideal for maintaining the reliability of power cables.

■ Since the spiral convex part is integrally formed on the outer periphery of the cable (sheath), there is no fear that the pitch will change even if the cable is subjected to strong frictional force when it is drawn into the conduit, and the above It can be carried out reliably.

Therefore, it is possible to enjoy the benefits such as this, and therefore, it is possible to extremely easily form a snake when the power cable is thermally expanded and contracted without having to make any special additions such as wrapping a wire body around the power cable body. It can be said that the intended purpose of providing a cable has been fully achieved and the practical effects are great.

[BRIEF DESCRIPTION OF THE DRAWINGS] Figures 1+a+ and (bl) are a cross-sectional explanatory view and a vertical cross-sectional explanatory view illustrating an embodiment of the in-pipe electrical cable of the present invention, and Figure 2 is an explanatory view of the same cable. Fig. 3 is a longitudinal cross-sectional explanatory view showing a situation with thermal behavior after installation, and Fig. 3 is an explanatory view showing an outline of an electric cable installed in a pipeline by snake installation.In the figure, 1 indicates a spiral convex part. , 2 is the power cable body, and 3 is the conduit.

Claims (1)

[Claims] (1) A power cable that is installed in a conduit so that a snake can be attached thereto has a spiral convex portion that is integrally formed so as to wrap around the circumference of the power cable body at a predetermined pitch. Regarding the spiral convex part, the pitch L is uniquely determined by the height h of the convex part, the bending rigidity EI of the power cable, and the weight w of the cable, and the snake axial force F of the power cable is determined by the following formula. A power cable installed in a conduit, characterized in that the value is selected to be the minimum value. ▲There are mathematical formulas, chemical formulas, tables, etc.▼ However, ▲There are mathematical formulas, chemical formulas, tables, etc.▼ k_3=α+α/2cos2α−3/4sin2αk_
5=1/4+3/4cos2α+α/2sin2α-c
osα α=tan^-^1L/4(ah/2) In the above formula, A is the conductor cross-sectional area of the power cable, E is the Young's modulus of the power cable, β is the coefficient of linear expansion of the power cable, and T
is the change in temperature of the conductor of the power cable.