Air itself is a good insulator . Whilst it cannot of course be wrapped around cables in the ordinary sense of wrapping insulation around , it does form the insulation when bare conductors are used

Bare conductors are used principally on rising mains i high buildings . These are the main distributing electric power from the main intake of a building to distrivution boards at different levels . The scheme of distribution is discussed in Chapter 5 and here we are concerned olny with the construction of the rising-main bars . Bare conductors used for rising mains must be correctly spaced from each other to give the necessary air gap for adequate insulation , be not subject to flexing under fault conditions , and should have a protective casing . They must be made inaccessible to unauthorized persons and must have feedom to expand and contract . There are several proprietary system of bare rising mains , and a typical one is illustrated in Figure 2.4 . it is frequently used for the vertical distribution in blocks of flats .
  The conductors are held in porcelain or sometimes plastic cleats . Apart from supporting the conductors , the cleats keep them the correct distance awy from each other for air gap to have sufficient insulation for the working voltage of the system . The cleats are fixed to the back of a metal trunking which completely encloses the conductors . The dimensions are , of course , such that the air gaps between the conductors and the trunking gives enough insulation . The front of the casing is hinged and can be opened for maintenance . it is possible to put a solid insulating plate acrpss the inside  of the casing atevery floor level to form a barrier to air and smoke moving up and down the casing . In many places this has to be done to satisfy fire prevention regulations , and also BS 7671 .Banks of fuses can be fixed within the casing to form a distribution boards as part of the vertical distributor .
  A similar system can be used for horizontal distribution  . here again there are seeral proprietary system consisting of horizontal conductors supported on insulators inside metal trunking . THy are used particularly in factories .