The electricity pylon - more correctly called a suspension or transmission tower - carries overhead three-phase electricity between substations in all weathers. Terminal towers are located at each end of the route, whilst deviation towers enable the wire route to be realigned.
These "lattice" towers are significantly more economical to construct and repair than attempting to bury high voltage insulated cables underground, and the ambient air also acts as a natural cooling system to help with heat dissipation on the wires. Larger towers provide a greater span, needing fewer towers to suspend cables over a distance, but variations in design are used depending on local conditions (e.g. aircraft or natural landscape considerations).
The largest Super Grid towers support wires operating at 400,000V. Fibre optical cables are wrapped around many cables to carry Internet traffic: the light signals are unaffected by the high voltages. The smaller towers seen in the countryside or near towns and villages are usually owned by the Regional Electricity Companies (RECs) rather than the National Grid.
Power cables are uninsulated and usually made of aluminium alloy, which is lighter than comparable conductors so that slimmer, smaller towers can be used. The towers are inherently earthed, and an individual earth wire can often be seen connecting the tops of towers together.
|A shot of a 'terminal tower' - the starting or finishing point of a countryside march of pylons||Electricity pylon showing insulators, protective "guard ring" loops and multiple conductors connected in parallel to handle the load.|
To ensure that the high voltage cable and the earthed tower are separated from each other, chains of porcelain or toughened glass insulators are used. A 132kV wire might use just nine insulators, whilst a 400kV Super Grid power line may demand twin chains of 24 insulators. Atmospheric pollution is another factor which determines how many insulators are needed, because fall-out from industry and salts in the atmosphere can degrade the insulating effect.
Each insulator is actually a capacitor having metal end caps separated by the dielectric material of the insulator. This produces a series of air-spaced capacitors between the metal caps and the tower, resulting in an uneven distribution of voltages across the insulators. Hence the power line voltage will be unevenly dropped across the capacitances, but the one nearest the power cable could operate near its maximum voltage breakdown limit.
The addition of a guard ring helps relieve the stress on the insulator dielectrics nearest the power cable by shunting their capacitance, and it also ensures that any possible flashover is diverted away from the insulator surfaces to prevent damage.
Insulators have a undulating cross section to increase their surface area, useful in wet weather.