Lightning surges transferred from one circuit to another through transformers

IT is generally recognized that through the medium of transformers surges can be transferred from circuits exposed to lightning to circuits considered otherwise nonexposed. The latter may connect to rotating machines or to industrial apparatus of inherently low-impulse insulation level. On these seemingly nonexposed circuits may be found apparatus of old design which as a rule are even more vulnerable to surges. In these problems the question is to determine 1. The extent apparatus may be endangered. 2. Whether the relative risk incurred weighed against the importance of the installation and the cost of damage, warrants protective equipment.