Secondary networks to serve industrial plants

The design of distribution systems for industrial plants will always constitute individualized problems. There is little hope that any particular form of distribution will be applicable to all industrial plants because the processes of manufacture and the arrangement of buildings differ widely depending on circumstances. There are, however, certain fundamental advantages to the network system of distribution which justify a much wider use than it has found to date. By far the greater part of factory distribution is still of the radial type and the network system must compete with the radial system in economy, reliability, and flexibility. The purpose of this paper is to show that it can do so successfully. Assuming that the product of the factory is such that a relatively large number of small and medium-sized motors are required rather than a few large ones, it will be found that the physical location in the factory of the various machines is not of such importance in the layout of the distribution scheme as the total connected load, the diversity factor and the over-all power factor, and it is on its ability to take advantage of diversity that some of the economy of the network system rests. The information available on the load density of factories is quite fragmentary. For the more common products of manufacture it varies from 5 to 30 kw per 1,000 square feet, 10 to 20 being the value mostly encountered, although in exceptional cases it may go as high as 100 kw. Load density should not be confused with the rating of the installed motors and utilization devices. The diversity between the various loads will considerably reduce this figure, and the “demand-factor,” that is, the ratio of the maximum demand over a period of time to the sum of the utilization device ratings, which is of significance in determining the transformer capacity to be installed, may be 50 per cent or less.