Performance of electromagnetic shields in high voltage laboratories

The paper constitutes a systematic investigation into the calculation and measurement of electromagnetic shielding effectiveness of high voltage laboratories. Both electromagnetic field and interference measurements were carried out to assess the shielding effectiveness of the UHV hall of Hydro-Quebec Institute of Research. While satisfactory shielding was obtained below 1 MHz, the measurements emphasized the deteriorating influence of seams and corners and revealed a considerable decline in shielding quality above a few MHz. A circuit approach is used to deduce new formulae for computing shielding effectiveness. Furthermore, new expressions are deduced for the increase in the equivalent resistance of a shielded enclosure due to non-uniform distribution of the circulating currents caused by intermittent bonding. A separate analysis is given for the shielding effectiveness above 1 MHz where in an UHV laboratory the dimensions of the enclosure approach the wave-length of the disturbing field. Considering the complex nature of the problem, agreement between theory and measurements is satisfactory.