The New Zealand Electricity Department has
 purchased a number of high voltage circuit
 breakers with prestressed porcelain columns on 
which the manufacturers have carried out shock
and vibration tests to determine strength and
damping characteristics and to make some
 estimate of earthquake resistance. This paper 
presents a detailed study of the behaviour of
 such circuit breakers when subjected to earthquakes and steady-state sinusoidal forging
functions. The system, assumed to be single-degree of freedom, has a bilinear force-deflection relationship. The system parameters
that varied in this study were: natural period, damping, yield strength and elastic characteristics. All investigations were conducted with
 the aid of an IBM 360/40 and an IBM 1130 digital computer using Jenning's Pseudo-Earthquakes.