Flexible conduit

Flexible cord approved for extra-hard service, flexible metal conduit, and liquidtight flexible conduit for limited flexibility. A suitable grounding conductor must be provided inside the flexible cord s outer jacket. Flexible conduit must be bonded with an external jumper or an approved internal system jumper external bonding jumpers are disallowed for flexible conduit exceeding six feet. Typical liquidtight and flexible cord connectors and an explosion-proof flexible connection are shown in Figure 17-23. 

TWo additional observations can be made. First, because a rigid pipe transmits almost all the load of the earth cover to the trench bed, someone will occasionally be heard to say that rigid pipe, such as concrete pipe, does not require side support. This is not true. Second, because of the difference in the ways rigid and flexible conduit distribute the load of their earth cover, flexible piping materials are often said to require less bedding bearing strength, because they impose less of a load... 

Given these differences between rigid and flexible conduit, let us examine the differences between steel and RTR pipe, both of which are, of course, flexible conduits. First, steel pipe is by definition constructed from a material, steel, that for our purposes is a homogeneous isotropic substance. Therefore, steel pipe can be considered to have the same material properties in all directions that is, it is equally strong in both the hoop and longitudinal directions [Fig. 4-2(b)]. 

By now it should be apparent that, while both steel and RTR pipe are by definition flexible conduit, they are also quite different and therefore require different design approaches, even though initially at least then-design considerations are identical. As with steel pipe, the RTR pipe designer must concern oneself with both pipe deflection and buckling analysis. Unlike the steel pipe designer, however, the RTR pipe designer must also examine a third area of concern. 

In terms of dead loads, the shape of the trench in which the pipe will be buried is also a factor. Generally speaking, a narrow trench with vertical sidewalls will impose less of a load on the pipe than will a wider trench with sloping side walls. It is necessary also to know the modulus of soil reaction (E), which is dependent on the type or classification of the native soil, the backfill material that is contemplated, and the desired consolidation of the backfill material. Soil consolidation is important, because it contributes to the strength of a flexible conduit in a buried pipe system. 

Flexible conduit manufactured to BS 731-1 1993 is made of interlinked metal spirals often covered with a PVC sleeving. The tubing must not be relied upon to provide a continuous earth path and, consequently, a separate CPC must be run either inside or outside the flexible tube (lET Regulation 543.2.7). 

Flexible conduit is used for the final connection to motors so that the vibrations of the motor are not transmitted throughout the electrical installation and to allow for modifications to be made to the final motor position and drive belt adjustments. 

Flexible conduit manufactured to BS 731 -1 1993 is made of interlinked metal spirals often covered with a PVC sleeving. 

Connection of Ml cables can be made directly to motors, but to absorb the vibrations a 360° loop should be made in the cable just before the termination. If excessive vibration is expected, the Ml cable should be terminated in a conduit through box and the final connection made by flexible conduit. 

Let us start by examining a rigid pipe. Because of its rigid, inflexible characteristics, surface load intensifies at the crown of a rigid pipe and is transmitted through the pipe directly to the bed of the trench in which the pipe rests. This is not true with flexible conduit. Because a flexible conduit deflects under covering load of earth, this deflection transfers portions of the load to the surrounding envelope of soil. This is true of both... 

Steel and RTR pipe. The result is that the support of the surrounding earth actually increases the strength of the flexible conduit. Therefore, analyzing the type and consolidation of backfill materials must be considered an integral part of the design process. 

Flexible metallic conduit should have a non-metallic inner sheath or be suitably designed so that movement cannot cause abrasion of the cables. Flexible metallic conduit is not a suitable protective conductor. A flexible cable conductor should be used for this purpose to ensure earth continuity between the fixed conduit and apparatus, and the flexible conduit joints should be earth bonded at both ends. 

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