The crossarm is an important component of pole and tower. It is used to install insulators and hardware fittings, support conductors and lightning wires, and maintain a specified safe distance for them.
1. Functions of Crossarm
The crossarm refers to angle steel horizontally fixed on the top of electric poles, equipped with porcelain insulators to support overhead wires.
2. Definition of Crossarm
The crossarm is angle steel horizontally fixed on the top of electric poles with porcelain insulators mounted on it for supporting overhead wires.
As a key part of pole and tower, the crossarm is applied to install insulators and hardware fittings, bear conductors and lightning wires, and keep them at the required safe distance.
3. Classification of Crossarm
3.1 Classified by Purpose
- Tangent Crossarm
- Angle Crossarm
- Tension Crossarm
Tangent Crossarm:Under normal operation and wire non-breaking conditions, it bears the vertical and horizontal loads of conductors.
Angle Crossarm: Besides bearing the vertical and horizontal loads of conductors, it also withstands large unilateral conductor tension.
Tension Crossarm: In addition to vertical and horizontal conductor loads, it bears the tension difference of conductors.
3.2 Classified by Material
- Iron Crossarm
- Porcelain Crossarm
- Composite Insulated Crossarm
Iron Crossarm: Made of angle steel, it features high mechanical strength and durability, and requires hot-dip galvanizing for anti-corrosion treatment. Poor anti-corrosion treatment will cause rusting, peeling and pitting, shortening its service life, with weak insulation and lightning resistance.
Porcelain Crossarm: It has high insulation and lightning resistance levels, good natural cleaning effect and low accident rate, and is more reliable than pin insulators in polluted areas. It is light in weight, convenient for construction, maintenance and live-line operation. The porcelain crossarm can deflect automatically by a certain angle; in case of wire breakage, it can adjust conductor tightness independently to effectively prevent secondary accidents. With low mechanical strength, it is easy to damage or crack during transportation and construction, so it is mostly used in open areas. Integrating crossarm and insulator functions with low cost, it simplifies line pole and tower structure and delivers significant economic benefits.
Composite Insulated Crossarm: Mainly made of glass fiber epoxy resin and silicone rubber, it owns high mechanical strength, excellent dielectric properties, hydrophobicity, aging resistance and corrosion resistance, with a long service life. Its creepage distance is more than 750mm and dry arc distance is more than 600mm, facilitating live-line work. The application of composite insulated crossarms on 10kV lines can reduce the use of lightning arresters. Meanwhile, one set of drainage protection devices or lightning protection devices such as lightning arresters shall be installed for every 10 poles to drain lightning overvoltage, further cutting operation and maintenance costs.
4. Installation of Crossarm
- All line crossarms and their accessories shall be hot-dip galvanized.
- Crossarms shall be installed flat and neat, with installation deviations not exceeding the following specified values:
- The vertical deflection at the end of crossarm shall not exceed 20mm;
- The horizontal torsion at the end of crossarm shall not exceed 20mm;
- For crossarms of double poles, the height difference at the connection between crossarm and cement pole shall not exceed 5/1000 of the connection distance;
- The horizontal torsion shall not exceed 1/100 of the total length of the crossarm.
- Installation of crossarms on tangent poles:The center line of the upper hoop on single-top bracket is 15cm away from the pole top; the center line of the upper hoop on double-top bracket is 35cm away from the pole top. For horizontal arrangement, the horizontal center line of 10kV line crossarm is 15cm away from the pole top, and the single crossarm shall be installed on the load side. When high and low voltage insulated wires are erected on the same pole, the minimum vertical distance between crossarms is 1.2m.
- Installation of crossarms on angle poles and tension poles:Poles and towers with an angle below 45° shall adopt single-layer double crossarm horizontal layout; poles and towers with an angle of 45° and above shall adopt double-layer double crossarm horizontal layout.
4.1 For 0°-45° tension angle poles, the horizontal center plane of 10kV and low-voltage line crossarms is 15cm away from the pole top, and the crossarms shall be perpendicular to the bisector direction of line angle.
4.2 For 45°-90° tension angle poles, the horizontal center plane of upper crossarms for 10kV and low-voltage lines is 15cm away from the pole top. The upper and lower crossarms shall be perpendicular to the directions of upper and lower lines respectively. The horizontal center plane of lower crossarms of 10kV lines is 60cm away from that of upper crossarms, and the horizontal center plane of lower crossarms of low-voltage lines is 30cm away from that of upper crossarms.
- Installation of crossarms on terminal poles:The horizontal center plane of 10kV and low-voltage line crossarms is 15cm away from the pole top, and the crossarms are installed perpendicular to the line direction.
In conclusion, crossarms are widely used on electric poles and iron towers to support and protect power lines. There are various materials for crossarms, and they can be divided into multiple types according to purposes, including tangent crossarms, angle crossarms and tension crossarms. In view of the current development of the power industry, composite insulated crossarms have become the mainstream trend, and fiberglass crossarms are widely used at present.
