143 AYUDA RÁPIDA
E. Llave desmonta-rueda
The cross-arm configuration strength to withstand imposed loads due to conductor stresses is evaluated after considering the following parameters:
• electrically sound • weight
• reliability/durability
• flexibility (utilisation of standard components).
The most common cross-arm configuration currently used in the UK is the standard intermediate horizontal type typical of BS 1320 and ENATS 43-10.
The ENATS 43-20 intermediate cross-arm is wider (at an overall length of 2.5 m) and weighs one third more (42 kg) than the earlier ENATS 43-10 design (28 kg).
Making further reference to the BS 1320/ENATS 43-10 designs, these overhead line specifications standardised the cross-arm design for conductors up to and includ- ing 32 mm2 HDCu or 50 mm2 ACSR and for voltages up to and including 11 kV. Overhead lines with larger conductor sizes have adopted the wider cross-arm referred to in ENATS 43-20 and ENATS 43-40.
Cross-arms to the specifications mentioned to date have been designed with a factor of safety of 2.5 applied on the ultimate strength of the material, and it has been noted that in general terms these cross-arms have performed satisfactorily in service for many years.
5.4
Designing horizontal cross-arms for single supports
Cross-arms for single supports can generally be treated as two cantilevers fixed at the support. The cross-arm at the support is therefore subjected to the following bending moments (BM):
1 BM due to the weight of a span of ice-coated conductor acting in a vertical direction at a prescribed distance from the pole centre.
2 BM due to span of wind-loaded conductors acting at a distance above the centre of the cross-arm (assuming pin or post type insulators).
3 BM due to alterations of profile. This may be either a positive or negative com- ponent of line tension, depending upon whether the profile imposes a downpull or an uplift on the conductors.
Overhead line design 69
5.5
Vertical and strut loadings
Where section angle or terminal structures are considered, then the forces acting on the pole top are far greater. The horizontal loading being applied to the top of the structure must therefore be counteracted in some form. This is generally achieved by a stay wire formation.
Due to the combination of both horizontal loading and stay strut loading, a vertical strut load is applied to the structure. The forces acting on the pole are therefore dependent on the stay formation and the slope or angle it falls from the pole. The more acute the angle of stay slope the greater the vertical loadings imposed on the structure. It is therefore important to try and achieve the greatest stay angle possible and this is typically 45◦where the resultant line tension will be equal to the strut load due to conductor load.
It is also important to account for the following additional loads when calculating the absolute load on the pole:
1 the weight of the conductors for the span length (three conductors× windspan × weight/m)
2 the weight of the pole top fittings (insulators and cross-arm steel work)
3 the additional loadings if any for downpull on one or either side of the structure in question.
Once all of these issues have been considered it is then possible to consider matching a structure to the loads calculated. The crippling forces obviously must fall below the maximum permissible strut load that the wood pole structure can accommodate.
5.6
Support design
BS 1990 provides details of typical sizes for Scots Pine (Pinus Sylvestris), the pre- ferred wood pole support of the electricity supply companies in northern Europe. This specification, however, merely provides guidance in relation to the typical strut strengths of the typical poles provided relative to grade and diameter.
Reference tables are usually provided for the line design engineer to determine which pole should be chosen. If the maximum pole strength cannot be accommodated in the spreadsheet then it may be necessary to consider using a different support type such as an ‘H’ pole configuration. The crippling load imposed on one structure can now be shared accordingly between the two poles. This may, however, not be an equal share depending on how the stays are arranged.
5.7
Windspan and foundation
Three factors can effect whether a pole is suitable for use, one we have just discussed being its crippling load, the two others are its windspan capability and its foundation strength.
5.7.1
Windspan
There is a maximum strength in a pole based on its ability to withstand horizontal loads. The wood pole can only withstand a degree of pressure applied to the top of the structure in a horizontal plane based on the span of conductor and pressure applied to the conductor length and area. The values given for typical windspans are a function of its:
• modulus of elasticity • diameter at the groundline
• diameter at the point of application of the load
• distance from the groundline to the point of application of the load.
5.7.2
Foundation
The foundation capabilities are based on the horizontal forces applied at the top of the structure and the foundation’s ability to withstand this force. The foundation’s resistance to withstand this applied force is a function of its stability (condition of the soil) and the resistance area of the pole and any associated blocks below the ground line against the soil. The resistance can be improved by changing the ground conditions (imported backfill or concrete) or increasing the area of resistance with the addition of baulks or increased depth by auguring.
5.8
UK line design for the future
5.8.1
General
For existing lines and any modifications or extensions to them, the appropriate line design specification as described in chapter 4 can be used. In terms of new lines, BS EN 50423 and BS EN 50341 will progressively be introduced and employed. In essence, the design approach in these standards is close to reverting to the deter- ministic ENATS 43-20 situation for lines at normal altitude and then using the higher loads at higher altitudes. This chapter is not intended to be a line design specification as this is beyond the scope of this book, however, the basics of line design on the basis of the new deterministic approach will now be given.