In the thirties also, the use of cold- rolled steel led to an increase in flux density, which was from thirteen or fourteen thousand gauss to sixteen five hundred or seventeen thousand gauss. The results of this reduced core weight and the insulation clearances due to vacuum filling led to marked changes in the ratings of transformers versus their size and weight, and incidentally cost. The change in construction of the core from butt and lap to mitre or to wound cores for smaller units was necessary to obtain the full properties of the material. The transformer is a part of the system. The effect the system has on the transformer can be illustrated by the following comments.
Transformers are subject to short-circuit, and very early it was considered in the design.
A rather crude calculation was devised and design changes made. The service records then became satisfactory, and the systems did not deliver currents sufficient to make short-circuit failures a serious consideration. Very few tests were made to see whether the design short-circuit strength was met. Those that were were made on distribution transformers. In the late 1960s, power systems became stiffer and tests had been devised to cover intermediate sizes, but they are still not commonly applied.